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1988 Floral Ontogeny and Histogenesis in Leguminosae. Kittie Sue Derstine Louisiana State University and Agricultural & Mechanical College
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Floral ontogeny and histogenesis in leguminosae
Derstine, Kittie Sue, Ph.D.
The Louisiana State University and Agricultural and Mechanical Col., 1988
Copyright ©1989 by Derstine, Kittie Sue. All rights reserved.
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UMI
Floral Ontogeny and Histogenesis in Leguminosae
A Dissertation
Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Botany
by Kittie S. Derstine B.S., Texas A & M University, 1970 M.S. Texas A & M University, 1978 May 1988 ©1989
KITTIE SUE DERSTINE
All Rights Reserved ACKNOWLEDGMENTS
It is with great pleasure that I thank my major professor,
Shirley C. Tucker, for her continual support and her careful editing of the dissertation. Each of my committee members deserves individual notes of appreciation. To William J. Blackmon, thanks for offering sound advice and summer support. To Marion D.
Socolofsky, thank you especially for your role in my EM training.
To Russell L. Chapman, thank you for your honesty and fairness. To
Lowell E. Urbatsch, thanks for your endurance, patience, untold hours of help in my learning word processing—without the luxury of using the computer the fin al stages would certainly have been exhausting. To Jose I. Rami rez-Domenech heartfelt appreciation for your photographic work in preparation of the plates. Deep appreciation and gratitude is expressed to the following individuals for their friendship: Patricia B. Cox, Alice Merrill, Mary Ann
Dickey, Olga Odiott, Alan J. Rebertus, Berthal D. Reynolds, Robert
Swartzwalder, Elizabeth Harris, Sue Marra, Alan Lievens, Paul
McKenzie, and Marie Standifer. To Dr. James S. Ice thanks for always being there. 1 would like to acknowledge the following associates that aided in my learning necessary technical skills: Russell H.
Goddard, Michael T. Postek, Kris Postek, and Amanda Lawrence. Thanks to the faculty and Graduate students of the Department of Biology at
University of Southwestern Louisiana for their encouragement.
Special notes of gratitude are reserved for the following members of my family for their love: Christopher W. Bennett, my husband; David and Kari Derstine, my children; Ann Bennett, my mother-in-law-friend; Camille S. Babb, my sister; my brother,
Russell S. Stanger, III and last, but by no means least, my parents,
Dr. and Mrs. Russell S. Stanger, Jr.
The research presented in this dissertation was supported in part by the following National Science Foundation grants to Dr.
Tucker: DEB-82-04132 and BSR-84-18922.
iii TABLE OF CONTENTS
Acknowledgments ...... i i
List of Figures...... * ...... v iii
A bstract......
I. Organ initiation and development of inflorescences and
flowers in Acacia baileyana (Leguminosae:
Mimosoideae ) ...... 1
A. Abstract...... 2
B. Introduction...... 4
C. Terminology...... 5
D. Materials and Methods ...... 6
E. Results...... 7
1. Inflorescence ...... 7
2. Ontogeny of the first-order inflorescence ...... 7
3. Ontogeny of the head ...... 8
4. Floral organography ...... 8
5. Floral organogeny ...... 9
a. Sepals ...... 9
b. P etals...... 10
c. Stamens ...... 11
d. Carpel ...... 13
e. Enlargement and differentiation of organs....13
F. D iscussion...... ,...14
1. Shift in apical meristem configuration ...... 14
2. Order of organ initiation among whorls...... 1 5
iv 3. Order of organ initiation within perianth whorls.16
4. Order of initiation of stamens...... 16
5. Organ position ...... 17
6. Common stamen primordia ...... 17
7. Mul tis tame ny ...... 1 8
8. Solitary carpel position and fate of the floral
meristem...... 20
G. Literature Cited...... 23
H. Figures...... 28
II. Unidirectional Floral Organ Initiation in Lupinus
havardii
(Leguminosae : Papilionoideae) ...... 42
A. Abstract...... 43
B. Introduction...... 44
C. Materials and Methods...... 44
D. Terminology...... 45
E. R esults...... 46
1. Organography...... 46
2. Inflorescence ...... 46
3. Floral Meristem...... 46
4. Floral Organogeny ...... 47
a. Bracteoles ...... 47
b. Sepals...... 47
c. P etals...... 48
d. Antisepalous Stamens...... 49
v e. Carpel ...... 50
f. Antipetalous Stamens...... 51
g. Stamen Development...... 51
F. Discussion...... 52
1. Floral symmetry...... 52
2. Activity of the floral meristem...... 52
3. Vertical order of initiation ...... 53
4. Unidirectional order of organ initiation ...... 54
5. Correlation of SEM and histogenetic evidence....56
6. Terminality of carpel...... 56
7. Comparison of organ in itia tio n ...... 57
G. Literature Cited...... 58
H. Figures...... 60
III. Floral Ontogeny in Parkinsonia aculeata
(Leguminosae : Caesalpinioideae) ...... 74
A. Abstract...... 75
B. Introduction...... 76
C. Materials and methods...... 76
D. R esults...... 77
1. Organography ...... 77
2. Organogeny...... 77
a. Sepals ...... 77
b. P etals...... 78
c. Antisepalous stamens...... 78
d. Carpel ...... 79
vi e. Antipetalous stamens...... 7 9
f. Floral organarrangement at anthesis ...... 80
E. Discussion...... 80
1. Order of organ initiation ...... 80
2 . Symme tr y ...... 81
F. Literature Cited...... 83
G. Figures...... 83
IV. Summary ...... 95
V. V ita...... 99
vii LIST OF FIGURES
Chapter 1 Figure 1. Diagram of first-o rd e r inflorescence ...... 29
2. Diagram of second-order inflorescence ...... 29
3. Floral diagram ...... 29
4. Polar view of first-o rd e r inflorescence, 1 , first-order bracts and second-order inflorescence meristem, primordia. First-order bracts, B1...... 31
5. First-order inflorescence meristem with tunica-corpus apical configuration®...... 3 1
6. Head meristem in axil of first-order bract, B1...... 31
7. Near median sa g itta l section through second-order inflorescence meristem before organogenesis begins..31
8. Head meristem with cells resulting from periclinal divisions, at arrows, initiating second-order b racts...... 31
9. Head meristem with second-order bracts formed low on the flanks of the meristem. First-order bract, B1, to l e f t ...... 31
10. Adaxial view of head. Second-order bract primordia, B2...... 31
11. Near polar adaxial view of head in Fig. 10. Order of appearance of 2 , bracts not obvious from this view...... 31
12. Second-order bracts formed from base of head to near summit of meristem. No floral apices present. Bar = 50 urn...... 3 3
13. Polar view of head. All second-order bracts removed to reveal floral meristems, f. Bract scar at arrow subtends no flower. Bar = 50 pm...... 33
14. Polar view of head with flowers beginning organogenesis. Apical residuum at arrow. Two bracts lateral to residuum with no apparent floral meristems formed. Bar = 50 pm...... 33
viii 15. Head with anomalous bract. Bar = 500 pm ...... 33
16. Polar view of head illu s tra tin g near synchronous growth of flowers. Bar = 500 pm ...... 3 3
17. Abortive bract or anomalous upgrowth of apical residuum at arrow. Bar = 50 pm...... 33
18. Polar view of the second-order inflorescence. Bare floral meristems visible...... 35
19. Median sagittal section of bare floral meristem.....35
20. Third sepal formed. Sepals numbered in clockwise helical order of initiation ...... 35
21. All sepals present and numbered in order of their initiation. Different sizes reflect order of appearance. All petals have initiated, only one is lab elled...... 35
22. All sepals present, one labelled but order not obvious...... 35
23. Sepal initiation: wall resulting from periclinal division at arrow. Apex is non-median...... 35
24. Simultaneous petal initiation...... 35
25. Median sagittal section of petal primordium. Arrow indicates periclinal wall after initiatory d iv isio n...... 35
26. Initiation of five common stamen primordia in antipetalous positions. Three are indicated by arrows. Some sepals and some petals removed ...... 37
27. Initiation of common stamen primordium. Periclinal divisions in itia tin g common stamen primordium at arrows...... 37
28. Radial expansion beginning at center of summit at the time of common stamen primordium in itia tio n . Anticlinal divisions at arrows. Apex is not median for any primordium...... 37
29. Individual stamen primordia (at arrows) formed on common stamen primordium. Four sepals and two petals removed. Central mound is carpel. Files of tunica cells radiate on flanks of the apex...... 37
ix 30. Flower with slig h tly older individual stamen primordia mostly in antipetalous positions. Sepals and petals removed. Carpel, C...... 37
31. Median section with two individual stamen primordia (arrows) at left ...... 37
32. Apex is undergoing radial expansion central to stamens. Flower with anticlinal divisions of radial expansion (between arrows) in subsurface tunica layer above uppermost individual stamen primordium and carpel ...... 37
33. Carpel initiation corresponding to stage in Fig. 15 periclinal division at arrow. S, stamen...... 37
34. Stamen primordia all around carpel including in antipetalous positions and upon flanks or plateau to base of carpel (mound in center of flower)...... 39
35. Carpel through cleft, abaxial side to left, adaxial side to right ...... 39
36. Polar view of carpel with well-formed cleft and cylindrical stamens...... 39
37. Carpel at stage similar to Fig. 23. Stamens cylindrical, carpelwith well defined cleft...... 39
38. Polar view of flower, anther d iffe re n tia tio n ...... 39
39. Lateral view carpel and stamens with anthers differentiating...... 39
40. Sepals approaching valvate condition. Bar = 50 pm...41
41. Petals (P) at the time sepals reach near valvate position. Bar = 50 pm ...... 41
42. Petals fully valvate and fused. Distal half of petal margin with interlocking hairs at arrows. Bar = 50 pm...... 4 1
43. Two petals in la te ra l view showing d is ta l thickening of p etal. Other petals removed. Stamens with differentiated anthers. Carpel, C. Bar = 50 pm...... 41
44. Section of petal margins showing interlocking hairs. Bar = 10 pm...... 41
x 45. Transverse section of flower showing proximal half of petal margins (arrow) appressed with no interlocking h airs. Sepal, S. Bar = 20 pm ...... 41
Chapter 2 Figure 1. Diagram of the raceme of Lupinus havardii...... 61
2. Floral diagram of Lupinus havardii...... 61
3. Lateral view of the raceme apex showing the summit of the meristem and the la st formed bract (B). Older bract, B (right) and the single axillary flower, f. Bar = 50 pm ...... 63
4. Near median longitudinal section of the inflorescence tip. B, subtending bracts. Floral primordium, f. Bar = 100 pm...... 63
5. Inflorescence tip, near median, tunica-corpus configuration. B, bract initiation, arrow. Bar = 50 pm ...... 63
6. Periclines of flower initiation, arrow. Bar = 10 pm...... 63
7. Polar view of the bilaterally symmetrical floral primordium subtended by it s bract,...... B 65
8. Near median s a g itta l section of flo ra l meristem before organogenesis. B, bract. Bar = 50 pm ...... 65
9. Bilaterally symmetrical floral meristem with the lateral bracteoles, Bl. Subtending bract scar, B. Bar = 50 pm ...... 65
10. Flower with first three sepal primordia. Bract, B. Bracteole, Bl. Bar = 50 pm ...... 65
11. LS of flower, near median for the abaxial sepal. Tunica-corpus apical configuration. Bar = 50 pm 65
12. Median sagittal section of abaxial sepal primordium. B, bract. Bar = 20 pm...... 65
13. Polar view of flower with abaxial sepals,S, and abaxial petals, P. Subtending bract, B. Meristem in the center of the flower. Bar = 50 pm ...... 65
xi 14. Non-median s a g itta l section of flower at abaxial petal initiation, at arrows. B, bract. Second tunica layer over summit. Bar = 20 pm ...... 65
15. Three sepals, S, v isib le . Five petals present. Prospective sites of the first whorl stamens in antisepalous positions. Meristem at center of flower. Bract, B. Bar = 50 pm...... 67
16. Flower with median abaxial and la te ra l antisepalous stamen primordia, SI, present. Bar = 50 pm...... 67
17. S agittal section, median for bract, B, and for median abaxial sepal, S. Site of initiation of first formed stamen, SI, which is along median sa g itta l plane. Bar = 50 pm...... 67
18. Periclinal divisions of initiation of first formed stamen, SI, between arrows. Bract, B. Bar = 20 p m ...67
IS. Initials (at arrows) of calyx tube formation on adaxial side of the flower, between the adaxial sepal lobes. Bar = 50 p m ...... 67
20. Frontal section of flower. Bl, bracteole. Cx, calyx tube between lateral pairs of sepal lobes. P, petal primordium. Bar = 50 jim...... 67
21. Site of carpel in center of meristem. First whorl stamens, SI, present. Petals, P. Sepal, S ...... 69
22. Median sagittal section of flower at the time of carpel initiation. Periclinal divisions of carpel initiation, between arrows...... 6 9
23. Adaxial view of flower with all sepals, S, petals, P, five stamens, SI, and carpel, C. Bl, bracteole. Bract, B, behind...... 69
24. Flower at time of beginning of apical growth of the carpel along it s abaxial side. Organs present include bracteoles, Bl, sepals, S, petals, P, antisepalous stamens, SI, and carpel, C...... 69
25. Near polar view of flower with calyx,Cx, petals, P, antisepalous stamens, SI, and carpel, C...... 71
26. Median sagittal section of carpel, C, and site of vexillary stamen, arrow...... 71
xii 27. Abaxial antipetalous stamen primordia, arrows.Lateral s ite s and vexillary stamen site vacant...... 71
28. All antipetalous stamens present, arrows...... 71
29. Adaxial view of flower with all floral organs present ...... 71
30. Median sagittal section of flower. Initiatory 71periclinal divisions of vexillary stamen, arrows...... 71
31. Calyx tube formation, adaxial view showing two lobes of the calyx tube. Calyx, Cx. Petal, P. Stamen, S. Carpel, C...... 73
32. Descending cochlear petal aestivation...... 73
33. Transverse section, wing petal and keel petal, K, margins meet ...... 73
34. Polar view of young flower. All floral organs visible. Stamen dimorphism obvious during mid-development. Antisepalous stamens, SI. Antipetalous stamens, S2...... 73
35. Early filament tube formation, stamen dimorphism. Antisepalous stamens, SI. Antipetalous stamens, S2..73
36. Monadelphous filament tube near anthesis ...... 73
Chapter 3 Figure 1. Inflorescence diagram of Parkinsonia aculeata ...... 86
2. Floral diagram of Parkinsonia aculeata ...... 86
3. First sepal initiated on abaxial side of floral meristem...... 88
4. Sepal primordia numbered in helical order of in itia tio n ...... 88
5. Lateral view of flower from adaxial side. Sepals in itia te d in counterclockwise helix. Sequence determined by relative sizes of primordia...... 88
6. Sepals uniform in size ...... 88
xiii 7. First-formed sepal cucullate. Helical order determined by relative sizes of sepals...... 88
8. Prior to anthesis sepals imbricate...... 88
9. Unidirectional petal initiation beginning with two abaxial petal primordia, arrows. Four sepals removed...... 90
10. Lateral petal primordia (lower arrows) and abaxial antisepalous stamen primordium, St. Three sepals removed ...... 90
11. Five petals present, lateral petal obscured by sepal. Lateral antisepalous stamens, St. Carpel primordium, C...... 90
12. Median adaxial petal, upper arrow. Antipetalous stamen primordium, arrow at lower right, on abaxial side of floral m eristem ...... 90
13. Sepals removed. Five petals, five antisepalous stamens, and four antipetalous stamenss present...... 92
14. Sepals removed. All flo ra l organs present...... 92
15. Sepals removed. Slightly oblique abaxial view comparing similar petal and antisepalous stamen sizes. Antipetalous stamens present...... 92
16. Lateral view of flower with sepals removed. Petals and antisepalous stamens of equivalent heights, carpel slightly higher ...... 92
17. Polar view of flower, carpel c le ft slig h tly out of median sagittal plane ...... 94
18. Stage sim ilar to Fig. 17 with one petal and one antisepalous stamen removed to reveal antipetalous stamen primordium, still smaller than antisepalous stamen primordia ...... 94
19. Ascending cochlear petal aestivation. Sepals removed...... 94
20. Carpel, with hairs, relative to height of stamens. Sepals, petals and numerous stamens removed...... 94
21. Flower near anthesis. Sepals and petals removed, anther sacs of two whorls of stamens uniform ...... 94
xiv 22. Flower at anthesis with all floral organs removed to show disc surrounding the carpel base...... 94
xv Abstract
Inflorescence organogeny and floral initiation are
investigated topographically (using scanning electron microscopy)
and histogenetically in Acacia baileyana (Mimosoideae) and Lupinus
havardii (Papilionoideae). Floral ontogeny is investigated
topographically in Parkinsonia aculeata (Caesalpinioideae). The
paniculate racemes of A. baileyana are constructed of first- and
second-order inflorescence meristems. Each produces first-order
and second-order bracts, respectively, in acropetal order. Each of
the former bracts subtends one second-order inflorescence meristem
while the latter bract type subtends a single flower. The radially
symmetrical floral meristem of A_. baileyana produces calyx members
in helical order and corolla members simultaneously with a
precocious terminal carpel. Common stamen primordia are initiated
in antisepalous sites. Three to five individual stamen primordia
form on each of the common stamen primordia a fte r which additional
stamen primordia appear laterally to fill in antipetalous areas
between the common stamen primordia. Proliferation of the androecium is accomplished after radial expansion of the floral apex providing space upon which individual stamen primordia may encroach upon the expanded apex to the base of the carpel.
Terminal racemes of L. havardii produce bracts in acropetal order.
Each bract subtends one flower. The bilaterally symmetrical floral meristem of L. havardii produces all floral organs in a
xv i unidirectional sequence within each whorl with some members of each whorl appearing precociously to the rest. Flowers of P. aculeata
are initiated on racemes in acropetal order and each flower is
subtended by a bract. The floral meristem of P. aculeata produces
sepals in helical order and the other floral organs in unidirectional order. Precocious appearance of the carpel is
consistent in all three species investigated. Its terminality is documented h istologically only in the papilionoid and the mimosoid
taxa.
xvii Chapter 1
ORGAN INITIATION AND DEVELOPMENT OF INFLORESCENCES AND FLOWERS
OF ACACIA BAILEYANA F. MUELL. (LEGUMINOSAE: MIMOSOIDEAE)
1 2
Abstract
Inflorescence and flo ra l ontogeny have been studied in the
mimosoid species Acacia baileyana using scanning electron
microscopy (SEM) on buds in all stages of inflorescence and floral
development and light microscopy (LM) of sectioned buds and
inflorescences. The paniculate inflorescence is comprised of
first-order and second-order inflorescences. First-order bracts
are produced in acropetal order by the first-order inflorescence
meristem and subtend second-order inflorescence (head) meristems.
The head meristem gives rise to second-order bracts each of which
subtends a single floral meristem. Although the second-order
bracts appear in acropetal sequence the floral meristems initiate
in the bract axils simultaneously after bracts are formed. All
flowers in a head develop synchronously. The sepals and petals of
the radially symmetrical flowers are arranged in alternating
pentamerous whorls. There are 30-40 free stamens and a unicarpellate gynoecium. The sepals are initiated helically with
the first-formed in one of two abaxial positions. Petal primordia
are initiated simultaneously and are alternate to the sepals.
Stamen initiation begins with the formation of common stamen primordia alternate to the petals and therefore opposite the
sepals. Clusters of three to five individual stamen primordia appear upon each of these common stamen primordia. The individual 3
stamen primordia appear eith er along the median planes of the
common stamen primordia or la te ra l to the median plane. Individual
stamen primordia appear then between adjacent common primordia
followed by addition of stamen primordia centripetally upon the now
flattened, expanded flo ra l apex. Carpel in itia tio n occurs as the
first individual stamen primordia are formed and after radial
expansion of the apex has delimited the remaining summit.
Histological study documents initiation of primordia seen in SEM by
establishing relative times of organ initiation. The apical configuration shifts from tunica-corpus in the bare floral meristem
to mantle-core with sepal initiation. Initiation of all floral organs occurs consistently by means of periclinal divisions of the outer corpus layers. The expansion of the receptacle occurs at the summit and later concentrically around the carpel primordium. The numerous anticlinal divisions in the mantle are responsible for the broadening of the apex to accommodate the large stamen number.
Exhaustion of the floral meristem and position of the carpel are confirmed through histological study. Enlargement and development of floral organs is described only until the stage when petals reach a fully valvate condition. 4
INTRODUCTION—Leguminosae has three subfamilies:
Papilionoideae, Caesalpinioideae, and Mimosoideae. The floral
characters that separate the subfamilies taxonomically are also of
phylogenetic importance. The developmental pathways are just
beginning to be studied (Tucker, 1984a) for origin of symmetry,
order of organ initiation, position of organs, aestivation, degree
of fusion and loss of parts. Representatives of Caesalpinioideae,
Papilionoideae (Tucker, 1984b) and Mimosoideae (Tucker, 1987 and
1988) have been examined with scanning electron microscopy with
respect to order (direction) of organ initiation and floral
symmetry, but the histogenetic basis has not been studied for
either. Relatively few papers have dealt with floral ontogeny of
members of Mimosoideae (Acacieae). Important concepts to be
re-examined include the position of the carpel (lateral or
terminal, Newman, 1933, 1934) and the activ ity of the common primordia as reported in Acacia nerifolia and Lysiloma vogelianum
(Gemmeke, 1982).
This chapter is the first of three which will comprise a histogenetic and exomorphic comparative study of flo ra l development in representatives of the three legume subfamilies. The tribe
Acacieae (Mimosoideae) contains two genera, Faidherbia A. Chev. and Acacia Miller. The latter genus includes three
subgenera—Acacia, Heterophyllum Vassal, and Aculeiferum Vassal
(Vassal, 1981). The 1200 species of Acacia sensu lato are tropical or subtropical (E lias, 1981) with over 700 species found in 5
Australia (Bernhardt, 1982). Acacia baileyana F. Muell. is
representative of the Mimosoideae (Tribe Acacieae) in the
subfamilial comparisons because its floral structure is characteristic of the subfamily and its tribe. The number of
floral organs is stable; consequently, this species makes an excellent model for ontogenetic study. It also displays some features of interest such as some variability in the order of sepal initiation and the marked radial expansion of the floral apex during stamen initiation. Activity of the floral meristem is traced from the bare floral apex through carpel initiation.
Development of the flo ra l organs is followed u n til the petals are fully valvate.
TERMINOLOGY—The term abaxial is used to describe the side of the flower away from the inflorescence axis, and the term adaxial, toward the inflorescence axis (Esau, 1977). Using the subtending bract (Fig. 18, B2) for reference, the abaxial side of a flower lies nearest the subtending bract while the adaxial side of the flower lies away from the subtending bract (Tucker, 1984b). With respect to the complex inflorescence structure the two orders of inflorescence meristems are called first-order and second-order inflorescence meristems. The second-order inflorescence is referred to as a head although these globose structures are not heads as used in composite taxonomy where the receptacle is flat and broad, but are reduced branches of a panicle. The products of the first-order inflorescence meristem are first-order bracts (Fig. 6
4, Bl). One head meristem (Fig. 4, arrow) is formed in the axil of each first-order bract. The head meristem gives rise to second-order bracts (Fig. 10, B2). Each second-order bract (Fig.
13, B2) subtends a single floral meristem (Fig. 13, f). Bract scars labeled B2 (Fig. 18) are those of second-order bracts.
Common primordia are discrete portions of the floral or inflorescence meristem that produce multiple individual primordia.
The individual primordia produced may be primordia of the same organ (individual stamen primordia) or primordia of different organs (petal and stamen or bract and flower, Saururus cernuus,
Tucker, 1975 and Houttuynia cordata, Tucker, 1981).
MATERIALS AND METHODS—Floral buds of Acacia baileyana were collected by Dr. Shirley C. Tucker in the Spring of 1982 from a tree cultivated in a greenhouse at Kew Botanic Gardens, Kew,
Surrey, England. Older flowering material was collected by S.C.T. at the University of California Arboretum, Davis, California in
1986. All material was fixed with "Kew mixture", with glycerin substituted for acetic acid of standard FAA (formalin-acetic acid-
50% ethyl alcohol: 5: 5: 90), and stored in 70% ethanol-glycerin
(1: 1). The material was transferred to 95% ethanol and stained in a 1% stock solution of safranin before dissections were made.
Tissues in higher alcohols are brittle, but much easier to manipulate while dissecting than material stored in FAA.
Standard procedures of TBA (tertiary-butyl alcohol) dehydration and in f iltr a tio n and embedding in "Paraplast Plus" were 7
followed for all buds for histological study. Histological
sections were cut at 5 jim using an American Optical Rotary
Microtome. These sections were stained with safranin-fast green
series (Berlyn and Miksche, 1976) or toluidine blue 0 (Sakai,
1973). The light micrographs were taken on Panatomic-X 35mm film
using a Leitz Orthoplan microscope equipped with an Orthomat
camera.
Specimens for scanning electron microscopy were dehydrated in a graded series to absolute ethanol. A graded series from absolute ethanol to the intermediate fluid, acetone, was used. All
specimens were c ritic a l point dried in a Denton DCP-1 C ritical
Point Drying Apparatus using carbon dioxide as the transition flu id . Specimens were then mounted in Television Tube Koat on o Cambridge pin-type stubs and coated with 200 A of gold-palladium in a Hummer Sputter Coater II. All specimens were observed and photographed at 25 KeV on a Hitachi S-500 scanning electron microscope with Tri-X Ortho A" X 5" sheet film.
RESULTS—Inflorescence—The flowers of Acacia baileyana are borne in complex panicles (Fig. 1). The second-order branches
(Fig. 2) are reduced to the point that the flowers are sessile on a globular receptacle which is supported by an elongated peduncle.
There are up to 25 heads per inflorescence. Heads contain 16-20 flowers each of which is subtended by a second-order bract.
Ontogeny of the firs t-o rd e r inflorescence—The f i r s t order inflorescence meristem (Fig. A) produces first-order bracts in an 8 acropetal helix. This meristem above the last-formed bract primordium has a height of 32 pm and a diameter of 80 pm (Fig. 5).
The apical configuration is tunica-corpus in which there is either a two or three-layered tunica. Periclinal divisions have initiated the bract primordium (Bl, Fig. 5). This meristem is active throughout the growing season, producing up to 25 heads.
Ontogeny of the head—In the axil of each first-order bract
(Fig. 6, Bl) one head meristem arises (H, Figs. 4, 6). In polar view the head meristem (Fig. 6) appears elongate tangentially. The head (Fig. 7) is 68 pm in diameter, 58 pm in height, and has a uniseriate tunica over a uniformly meristematic corpus. This head produces from 16 to 20 second order bracts by p ericlin al divisions in the outer corpus layer (Fig. 8, arrow). The head meristem remains rather oblong during the earliest bract production (Fig.
9). Second-order bracts arise on the head meristem in acropetal helical order (Fig. 10, 11). All floral meristems are initiated synchronously on a head (Fig. 13). Production of floral primordia seems to be delayed until after all second-order bracts (Fig. 12) are formed. The head meristem produces 16-20 second order bracts subtending flowers and terminates in an apical residuum (Fig. 13,
14, r), an anomalous sterile bract (Fig. 15), a stunted flower
(Fig. 16), or an unidentified structure (Fig. 17).
Floral Organography—Each flower (Fig. 3) includes five sepals, five petals, 30-40 free stamens and a single carpel, with its cleft oriented adaxially. There are no bracteoles. 9
Development among a ll flowers of a head (Fig. 13, 16) is nearly
synchronous with slight delays in the uppermost and lowermost
flowers per head. Each flower is radially symmetrical with the
sepals and petals being uniform in size within each whorl (Fig. 3).
The median sa g itta l plane of the flower bisects the subtending bract, abaxial petal, adaxial sepal, and carpel (Fig. 3). The position of the median petal is determined by the position of the
two abaxial sepals which lie to either side of the median sag ittal plane. All flowers in each head have this arrangement with slight shifts in sepal and petal positions and/or the carpel cleft relative to the subtending bract. The slight deviations in the carpel alignment from the median plane are not reflected in the overall symmetry of the flower.
Floral organogeny—Second-order bracts are initiated by the second-order inflorescence meristem in an acropetal helix. In the axil of each second-order bract one flower primordium (Fig. 18, f) is formed. The bare floral meristems appear oblong transversely.
The bare floral meristem before sepal initiation measures about 75
;im in diameter, about 65 jam deep, and about 65 ^im in height. The floral meristem has a tunica-corpus configuration with a uniseriate tunica (Fig. 19). The entire floral primordium is uniformly meristematic throughout.
Sepals—Five sepal primordia are initiated helically beginning with one in either of the two abaxial sepal sites followed by the primordium in the median adaxial position (Fig. 20). The third 10 sepal primordium appears in the second abaxial position (Fig. 20).
The fourth and fifth primordia are then formed successively in the lateral positions (Fig. 21). The direction of the helix may be either clockwise as in Fig. 20 or counterclockwise. Sequence of sepal initiation may be determined by relative sizes of the primordia at a stage similar to Fig. 21. Sepals in other flowers may appear to be uniform in size (Fig. 22) suggesting simultaneous order of initiation. Occasionally the lateral sepals will appear followed by the abaxial sepal and then the adaxial pair resulting in a bidirectional sequence (not illustrated). With sepal initiation the floral meristem becomes radially organized.
Initiation of sepal primordia occurs as one or more periclinal divisions in the outer corpus layer of the mantle (Fig. 23, arrow).
With sepal initiation the apical configuration of the floral meristem has begun to s h ift from tunica-corpus to a mantle-core configuration (Fig. 23). Vacuolation of a few cells at the central base of the flo ra l meristem (Fig. 23) marks the inception of the core. The single tunica layer is retained and the mantle remains intensely meristematic in contrast to the core cells which enlarge and become vacuolated. The floral meristem has enlarged to a height of 65-70 um and has a diameter of 75-80 jim.
Petals—Five petal primordia arise simultaneously on the flanks of the floral meristem as a whorl and are alternate to the sepals (Fig. 24). The median petal is located in the abaxial position between the two abaxial sepals (Fig. 24, arrow). 11
After petals are initiated the floral meristem is 78-85 pm in
height above the axils of the sepals and is 65-70 pm wide above the
axils of the petals. At petal initiation the core of the floral
apex is enlarging as more cells appear increasingly vacuolate (Fig.
25). Petal initiation occurs by periclinal divisions (Fig. 25,
arrow) in the outer corpus layers.
Stamens—Five common stamen primordia (Fig. 26, arrows) are
initiated simultaneously in antisepalous positions on the flanks of
the floral meristem. At the start of stamen initiation the floral meristem is 47 pm in height and 93 pm wide (Fig. 27) measured from
axil of the petal vertically to the summit of the floral apical meristem. The core is well established under an evenly distributed mantle of meristematic cells. There are five planes of radial
symmetry through the sepals and common stamen primordia and five additional planes of symmetry through the petal primordia.
The common stamen primordia are initiated simultaneously by periclinal divisions in the outer mantle layer (Fig. 27, arrows).
During in itia tio n of individual stamen primordia, the flo ra l apex undergoes a marked two-step radial expansion. F irs t, numerous anticlinal divisions occur at the summit of the meristem in the tunica and outer corpus layers (Fig. 28, arrows). The "shoulder"
(Newman, 1936a) of the truncate apex is any one of five common stamen primordia (Fig. 29). Files of protodermal cells radiate from the central mound at the summit (Fig. 29). Three to five individual stamen primordia (Figs. 29, 30) are formed on each of 12
the five common stamen primordia. One individual stamen primordium
is initiated first on the shoulder of each of the common stamen
primordia. Other stamens are initiated above, below and somewhat
la te ra lly . These individual stamen primordia are formed by
periclinal divisions in the outer corpus layer (Fig. 31).
The second step in radial expansion of the apex is
accomplished by anticlinal divisions that continue concentrically
around the central mound and occur between the periphery of that
central mound and the limit of the uppermost individual stamen
primordia (Fig. 32, between arrows). The width of the area undergoing expansion between the arrows is about 30 pm. The diameter of the central mound (Fig. 33) is about 46 pm. All mantle
layers are involved in anticlinal divisions which spread the mantle
over the broad core (Fig. 32). Radial expansion is evidenced by
the broadening of the flower to 125 pm diameter between stamen primordia.
The radial expansion of the floral apex delimits the area of
the carpel and provides adequate space (Fig. 32) for the production of numerous additional stamen primordia. In itia tio n of stamen primordia continues next in antipetalous areas and then acropetally around the flattened apex to the base of the carpel (Fig. 34).
Stamens eventually surround the carpel base (Fig. 35, 36). Four to six individual stamen primordia occur in irregularly vertical rows
(Fig. 34) along radii of the shoulders while two or three individual stamens lie along radii between shoulders. 13
Carpel—The carpel appears as a circular mound in the center of the flower (Fig. 34). Carpel initiation (Fig. 33, arrow) occurs as the f ir s t stamen primordia are forming on the common stamen primordia, during the radial expansion of the summit of the flower
(Fig. 19). The carpel is initiated by a cluster of adjacent periclinal divisions of which one is seen in Fig. 33. These divisions occur in the center of the remaining meristem. The floral meristem is utilized completely and exhausted in the production of the single carpel.
The c le ft is formed when the carpel is 20-30 pm in height and results from differential apical growth of the carpel (Fig. 35).
The abaxial side of the carpel grows upward faster than the adaxial side forming an oblique bowl-shaped area in the adaxial lateral surface (Fig. 22). All cells at the summit are involved directly in the upward or apical growth of the carpel, leaving no meristem remaining lateral to the carpel (Fig. 35).
Enlargement and differentiation of organs—Organ expansion is followed here only until the petals are fused in the valvate position (Fig. 42). Sepals expand to reach a more or less valvate position (Fig. 40) when they are about 180 pm in height (Fig. 40,
41). Sepals undergo little additional expansion as the petals outgrow them in height (Fig. 42).
As the sepals reach the near valvate position, the petals
(Fig. 41) are about 80 pm in height. The distal half of the petal is thickened to form callus and becomes fringed with epidermal 14 cells that interlock (Fig. 42, 43, and 44). The petals in longitudinal section (Fig. 44) show the interlocking papillate epidermal cells. In transverse section the proximal petal margins are broadly appressed (Fig. 45). Petal expansion and development is synchronous, all petals being of equal height and width at any point in time.
The stamen primordia grow apically and are cylindrical in longitudinal section reaching about the same height as the carpel
(Fig. 35). At the stage when the carpel is about 46 jim in height the stamen primordia are about 68 ;um in height (Fig. 36). Anthers differentiate (Fig. 38, 39 and 43) when the petals first become valvate.
Carpel development beyond initiation is observed only until the ascidiate primordium has an adaxial cleft (Fig. 36, 38 and 39).
DISCUSSION—Shift in apical meristem configuration—The floral apex of Acacia baileyana shifts from a tunica-corpus configuration with one tunica layer to a mantle-core configuration at the time of sepal initiation, as in Drimys winteri (Tucker, 1959) and
Pseudowintera tra v e rsii (Sampson, 1963). The flo ral apex of Drimys lanceolata (Tucker and Gifford, 1966a) has a tunica-corpus configuration that shifts to a zonate pattern as petal initiation begins. The mantle-core configuration is generally found in larger apices although the meristem of Acacia baileyana is roughly half the size (sagittal diameter) of Drimys winteri and Pseudowintera traversii as organogenesis begins. Other flowers such as Saururus 15 cernuus (Tucker, 1975) are of equivalent size (30-80 pm in diameter) to Acacia baileyana but do not change in configuration.
There appears to be no obvious correlation between apical size at floral onset and a shift in type of configuration.
Order of organ initiation among whorls—In general floral organs initiate in an acropetal sequence beginning with sepals.
Sequence of organ in itia tio n in Acacia baileyana is acropetal beginning with sepals and continuing through the common stamen primordia. This acropetal order was reported for 19 mimosoid taxa which have been investigated (Tucker, 1987, Table 2).
Exception to the purely acropetal sequence in Acacia baileyana is the appearance of the carpel before all the individual stamen primordia are formed. Carpel precocity is recorded in the following other legume species: Acacia longifolia and Acacia suaveolens (Newman, 1936a); Erythrina crista-galli and Lupinus albus (van Heel, 1981); Pithecellobium dulce and Bauhinia purpurea
(van Heel, 1983) and in Pisum sativum, Psoralea macrostachya,
Dioclea aff. ucayalina, and Neptunia pubescens (Tucker, 1987).
Carpel precocity is noted among Leguminosae (Tucker, 1984a) with a unidirectional order of organ initiation. Early carpel initiation has been noted among the following non-legume species: Hydrocleis nymphoides (Singh and S a ttle r, 1973), Limnocharis flava (S attler and Singh, 1977), and in polyandrous palms (Uhl and Moore, 1980).
From a limited sampling precocious carpel initiation is a rather common occurrence. 16
Order of organ initiation within perianth whorls—In Acacia
baileyana order of initiation within each perianth whorl varies.
Sepals are whorled but are initiated in helical order or, less
frequently, in simultaneous order. The order of sepals among other
mimosoids varies more than for any other whorl, although helical
and simultaneous orders are the patterns most commonly encountered
(Tucker, 1987). The petals are whorled in A. baileyana and all
other mimosoids which have been studied (Rohrbach, 1870; Newman,
1933, 1934; S a ttle r, 1973; Gemmeke, 1982; Ramirez-Domenech &
Tucker, 1987, and in press; Tucker, 1987). Bauhinia fassoglensis,
a caesalpinioid, is helical in the calyx and unidirectional in the
petals and stamens (Tucker, 1984 b).
Order of initiation of stamens—Initiation of the five common
stamen primordia is simultaneous in A. baileyana and A. oxycedrus
(Rohrbach, 1970). A spiral sequence of initiation of all organs of
both Acacia suaveolens and Acacia longifolia is reported (Newman,
1936a), but this work was based only on sections and hence this
conclusion is open to question.
A pattern of stamen initiation similar to that described in
Acacia baileyana was reported for palm species (Uhl and Dransfield,
1984) with a trimerous flo ra l architecture. In Eugeissona 20-70
stamens arise in a sequence that is a slight variation from the
theme of common stamen primordia observed in Acacia baileyana.
Three antisepalous stamens arise as a whorl. A ridge develops between the stamens upon which numerous stamens are in itia ted 17
sequentially from the points toward the center. Activity of the common (ridge) primordium produces 21-24 stamens in a triangular unit.
The order of stamen initiation seems to be related to the direction of the expansion of the meristem. If radial expansion occurs concentric to the summit, stamen in itia tio n proceeds acropetally. If basal expansion occurs, stamen initiation proceeds basipetally. Only through detailed comparative histological study can this hypothesis be tested.
Organ position—In mimosoids such as Acacia the petal positions are basically different from those in Caesalpinioideae and Papilionoideae. In the latter two the median sepal is located on the abaxial side of the flower (adjacent to the subtending bract) and the median petal (standard) is located on the adaxial side of the flower (adjacent to the axis). The present study, as well as preliminary work (Derstine and Ramirez-Domenech, 1985) on representatives in four mimosoid tribes (Mimoseae, Ingeae, Acacieae and Adenantherae), shows that these mimosoids have a contrasting arrangement of sepals and petals in which the median sepal is on the adaxial side and the median petal is on the abaxial side of the flower.
Common stamen primordia—Common primordia are an unusual feature among legumes, having been reported only in Pisum (Tucker, in press) and may produce similar organs. In Acacia baileyana the common stamen primordia are isolated islands of activity producing 18 clusters of three to five stamens. In Limnocharis flava (Sattler and Singh,1977) each of the common stamen primordia give rise to three individual stamen primordia. The meristematic activity spreads between these "primary androecial primordia" to form a ring. The "ring" or "ringwall" seen by Gemmeke (1982) in Acacia neriifolia is not evident in Acacia baileyana yet the sequence of the lateral spread of the stamens toward the center is similar.
The common stamen primordia described here more closely approximate the "episepalous primary primordia" reported in Lysiloma vogelianum
(Gemmeke, 1982).
Common primordia may produce different organs. In inflorescence ontogeny of Saururus cernuus (Tucker, 1979), and
Houttuynia (Tucker, 1981) each common primordium forms a bract and a floral meristem. A single primordium in Ischnosiphon elegans
(Kirchoff, 1983) gives rise to two flowers in a cymule. One common petal-stamen primordium produces one petal primordium and two stamen primordia in Alisma triviale (Singh and Sattler, 1972).
Common stamen-petal primordia also are reported in many Primulales
(Sattler, 1967).
Multistameny—Among legumes, multistameny is a specialization occurring principally in two mimosoid tribes: Acacieae and Ingeae.
In Arecaceae (Uhl & Dransfield, 1984) i t also is a derived condition. However, the possession of numerous free stamens has tra d itio n a lly been considered a prim itive condition, for example in
Ranunculus (Tepfer, 1953), Drimys (Tucker, 1959; Tucker & Gifford, 19
1966a, b) and Pseudowintera (Sampson. 1963). In prim itive flowers,
all stamens arise singly in approximately helical order, while in
the mimosoid and arecoid examples, the multiple stamens frequently
arise from common primordia, ring meristems, or after late radial
expansion of the receptacle, as in Acacia baileyana. The expansion
of the floral meristem provides additional space for increased
numbers of organs. In A. baileyana the radial expansion provides a
broadened plateau upon which the numerous stamens are in itia te d
acropetally. The first divisions occur at the summit and delimit
the area of the carpel. The anticlinal divisions concentric to the
carpel buttress increase the surface for stamen primordia to fill in to the base of carpel. The overall result of the radial expansion is a substantial increase in flower diameter compared to the diameter of the flower at sepal and petal initiation. Other mimosoids p articularly in the trib es Acacieae and Ingeae also show p ro liferatio n of stamens (Gemmeke, 1982; Ramirez-Domenech and
Tucker, 1987). Bernhardt and Walker (1984) report in discussion on bee foraging greater than five hundred stamens in A> myrtifolia.
It is necessary to investigate taxa such as A. myrtifolia developmentally to determine sequence of stamen production by the floral apex. Within Mimosoideae, the multistaminate taxa such as
A. baileyana are considered derived while the haplo- and diplostemonous taxa such as Mimosa ( Ramirez-Domen/ch, pers. comm.) and Neptunia (Tucker, 1988) are viewed as more prim itive. There is a trend toward u tiliz in g the expanded apex rather than the proposed 20 increase in sterilization of flowers through decreased use of the floral meristem. It cannot be determined if the expansion is the cause or the effect, nor can it be stated that the stamen numbers increase is due to the expanse.
There are other examples of an increase in the floral meristem to accommodate large numbers of flo ral organs. In the palm
Eugeissona (Uhl and Dransfield, 1984) an antepetalous trira d ia te ridge expands to provide space for the subsequent production of numerous stamens. In contrast the floral meristem in Limnocharis flava (S a ttle r and Singh, 1977) increases to produce numerous stamens. This increase occurs in height basally after which the individual stamens are initiated centrifugally. The floral apex of
Illicium floridanum (Robertson and Tucker, 1979) increases in height and width during initiation of floral organs and reaches maximum width before carpel initiation.
Solitary carpel position and fate of the floral meristem—Carpel position and fate of the floral meristem are important in interpreting the carpel according to the classical theory as a conduplicate leaflet or a peltate leaflet (van Heel,
1981). The carpel in Acacia baileyana appears to be terminal because it initiates at the summit of the meristem and leaves no apical residuum. Newman (1933) concluded that the carpel of Acacia baileyana lies in a lateral position with an apical residuum at the base, although his histological sections do not show a residuum.
To substantiate his argument, he refers to an anomalous flower that 21
produced a flower at the base of the carpel. In both Amherstia
nobilis (van Heel, 1983) and Albizzia lophanthe (Sattler, 1973), a
second non-functional carpel occasionally occurs in the purported
area of the residual apex. In view of these reports, Newman's
finding a second flower is not surprising.
In legumes Calliandra tetragona, Lupinus albus and Erythrina
crista-galli (van Heel, 1981) initiation of the single carpel
primordium exhausts the apical meristem. In the Cassia species
investigated by van Heel (1981), the carpel is formed on the side
of the flo ra l apex but la te r in development the whole flo ra l apex
has become carpel. In the same investigation, van Heel (1981)
states that the floral apex of an Albizzia species is initially
changed into the oblique bowl-shaped carpel primordium, but is
"well demarcated from the floral apex." The carpel primordia in
these species are obliquely bowl-shaped. The cleft becomes lateral
as the abaxial margins grow more rapidly than the adaxial margins
resulting in an ascidiate structure.
Newman (1936a & b) concluded that the carpel in Acacia
suaveolens and Acacia longifolia is lateral. His determination was based on a carpel after cleft formation at the time the carpel is
1-2 mm in height. He observed the carpel to be an inrolled foliar
structure leaving a residuum as the margins of the carpel began differential growth. Thompson (1936) re-interpreted Newman's evidence and asserted that legumes in both those species are
term inal• 22
Developmental evidence of terminal carpels is reported in various non-legume taxa: Drimys lanceolata (Tucker and Gifford,
1966a,b) and Pseudowintera traversii (Sampson and Kaplan, 1970).
In each species the entire apex is "transformed" into the carpel primordium. This finding agrees with my interpretation of the
Acacia carpel as terminal. 23
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Systematics. Part 1. R.M. Polhill and P.H. Raven (Eds.), Royal
Botanic Gardens, Kew. pp. 169-171. 28
Fig. 1-3. Diagrams of first-o rd e r and second-order inflorescence and of the flower in Acacia baileyana. 1. Diagram of first-o rd e r inflorescence, 1°, with heads, 2°. 2. Diagram of head. Second-order bract, B. Flower, f. 3. Floral diagram. Axis of head, a. Second- order bract, B. Sepal, S. Petals, P. Stamens, St. Carpel, C. 29 30
Fig. 4-11. First-order and second-order inflorescence development.
Fig. 4, 6, and 9-11 are SEM micrographs. For Fig. 5, 7, 8 are longisections. For Fig. 4-1 and 9-11, Bar = 50 yum. For Fig. 8, Bar =
30 /im. Polar view of first-order inflorescence, 1°, first-order bracts and second-order inflorescence meristem, 2°, primordia. First- order bracts, Bl. 5. First-order inflorescence meristem with tunica- corpus apical configuration. 6. Head meristem in axil of first-order bract, Bl. 7. Near median sagittal section through second-order inflorescence meristem before organogenesis begins. 8. Head meristem with cells resulting from periclinal divisions, at arrows, initiating second-order bracts. 9. Head meristem with second-order bracts formed low on the flanks of the meristem. First-order bract, Bl, to left. 10. Adaxial view of head. Second-order bract primordia, B2.
11. Near polar adaxial view of head in Fig. 10. Order of appearance of 2, bracts not obvious from this view. 31
ssife 32
Fig. 12-17. Development of head and fate of the head meristem after initiation of second-order bracts is complete. 12.
Second-order bracts formed from base of head to near summit of meristem. No floral apices present. Bar = 50 pm. 13. Polar view of head. All second-order bracts removed to reveal floral meristems, f. Bract scar at arrow subtends no flower. Bar =
50 pm. 14. Polar view of head with flowers beginning organogenesis. Apical residuum at arrow. Two bracts lateral to residuum with no apparent floral meristems formed. Bar = 50 pm.
15. Head with anomalous bract. Bar = 500 pm. 16. Polar view of head illustrating near synchronous growth of flowers. Bar =
50 pm. 17. Abortive bract or anomalous upgrowth of apical residuum at arrow. Bar = 50 pm.
34
Fig. 18-25. Floral organogenesis of sepals and petals of Acacia baileyana. Figs. 18, 20-22, and 24 are SEM micrographs. Fig.
19, 23 and 25 are longisections. Subtending bracts have been removed on a ll SEM specimens. All bars on SEM are 50 ^um. B, bract. S, sepal. P, p etal. St, stamen. 18. Polar view of the second-order inflorescence. Bare floral meristems visible. 19.
Median sagittal section of bare floral meristem. 20. Third sepal formed. Sepals numbered in clockwise helical order of initiation. 21. All sepals present and numbered in order of their initiation. Different sizes reflect order of appearance.
All petals have initiated, only one is labelled. 22. All sepals present, one labelled but order not obvious. 23. Sepal initiation: wall resulting from periclinal division at arrow.
Apex is non-median. 24. Simultaneous petal initiation. 25.
Median sagittal section of petal primordium. Arrow indicates periclinal wall after initiatory division.
36
Fig. 26-33. Stamen and carpel initiation. Fig. 26, 29, and 30 are SEM preparations; Fig. 27, 28, 31-33 are longisections of flowers. Organs and organ scars (from removal) are labeled as sepal (S), common stamen primordium, St, or petal (P). All bars
= 50 jim. 26. Initiation of five common stamen primordia in antipetalous positions. Three are indicated by arrows. Some sepals and some petals removed. 27. In itia tio n of common stamen primordium. Periclinal divisions initiating common stamen primordium at arrows. 28. Radial expansion beginning at center of summit at the time of common stamen primordium initiation. Anticlinal divisions at arrows. Apex is not median for any primordium. 29. Individual stamen primordia (at arrows) formed on common stamen primordium. Four sepals and two petals removed. Central mound is carpel. Files of tunica cells radiate on flanks of the apex. 30. Flower with slightly older individual stamen primordia mostly in antipetalous positions.
Sepals and petals removed. Carpel (C). 31. Median section with two individual stamen primordia (arrows) at left. 32. Apex is undergoing radial expansion central to stamens. Flower with anticlinal divisions of radial expansion (between arrows) in subsurface tunica layer above uppermost individual stamen primordium and carpel. 33. Carpel initiation corresponding to stage in Fig. 15 periclinal division at arrow. S, stamen. 37 38
Fig. 34-39. Expansion and differentiation of carpel and
anthers. Fig. 34, 36, 38 and 39 are SEM preparations. Fig. 35
and 37 are longisections. Bars = 50 pm. 34. Stamen primordia
all around carpel including in antipetalous positions and upon
flanks or plateau to base of carpel (mound in center of
flower). 35. Carpel through cleft, abaxial side to left, adaxial side to right. 36. Polar view of carpel with well-formed cleft and cylindrical stamens. 37. Carpel at stage
similar to Fig. 23. Stamens cylindrical, carpel with well defined cleft. 38. Polar view of flower, anther differentiation. 39. Lateral view carpel and stamens with anthers differentiating. 39 AO
Fig. AO-A5. Sepal and petal aestivation. 40. Sepals approaching valvate condition. Bar = 50 yum. 41. Petals (P) at the time sepals reach near valvate position. Bar = 50 pm. 42. Petals fully valvate and fused. Distal half of petal margin with interlocking hairs at arrows. Bar = 50 pm. 43. Two petals in lateral view showing distal thickening of petal. Other petals removed. Stamens with d ifferen tiated anthers. Carpel, C. Bar =
50 pm. 44. Section of petal margins showing interlocking hairs.
Bar = 10 pm. 45. Transverse section of flower showing proximal half of petal margins (arrow) appressed with no interlocking hairs. Sepal, S. Bar = 20 pm. 41
'•A't'ii-'- Chapter 2
UNIDIRECTIONAL FLORAL ORGAN INITIATION IN LUPINUS HAVARDII WATS.
(LEGUMINOSAE: PAPILIONOIDEAE)
42 43
Abstract
Inflorescence ontogeny, histology, and unidirectional floral
organ initiation has been studied in Lupinus havardii Wats.
(Leguminosae: Papilionoideae). The flowers, borne in a raceme, each have five sepals, five p etals, ten stamens in two whorls of
five and a unicarpellate gynoecium. The floral organs initiate unidirectionally within each whorl. Within a whorl the organs initiate first on the abaxial side of the floral meristem, followed by initiation of lateral organs and then adaxial organs. Between whorls organ initiation overlaps in the time of initiation. The abaxial sepal is followed immediately by lateral sepals which initiate before the first abaxial petals are formed. The keel petals are followed by the wing petals which initiate before adaxial sepals appear. Abaxial first-whorl stamens are initiated before the adaxial median petal (standard). Appearance of the adaxial sepals, adaxial petals, and adaxial first-whorl stamens is followed by the carpel primordium. The second-whorl stamens are the last floral organs to be formed and follow the unidirectional sequence from abaxial to adaxial side of the flower. The vexillary
(median adaxial antipetalous) stamen is the last product of the floral meristem. During mid-development the calyx tube is formed and the filament tube begins intercalary growth. Petal sculpturing occurs during petal expansion. 44
INTRODUCTION—It is the purpose of this work to study the ontogeny of the floral meristem during floral organ initiation in a papilionoid legume, Lupinus havardii Wats. (Leguminosae:
Genistae), in order to compare ontogeny with that of other subfam ilial representatives. Sequence of flo ra l organ in itia tio n has been studied in a variety of papilionoid legumes, but never using both SEM and histological evidence. The two subfamilies differ in that the papilionoids have unidirectional floral organ initiation in sepal, petal and stamen whorls, while caesalpinioids studied so far show variability in order of initiation. The sepals of caesalpinioids initiate helically or unidirectionally according to species.
MATERIALS AND METHODS—A voucher specimen and flo ra l buds were collected in April, 1982 by KSD in Big Bend National Park, Brewster
County, Texas by permission from the National Park Service. All material was fixed in the field and stored in FAA (formalin-ethyl alcohol-acetic acid). All tissue sectioned for anatomical study was dehydrated in standard tertiary-butyl alcohol (TBA) series and infiltrated and embedded in "Paraplast Plus". Buds were sectioned at 4 or 5 jum with an American Optical Rotary Microtome. The sections were mounted on microscope slides and stained with either safranin-fast green series or toluidine blue 0 and mounted in
Permount. Light micrographs were taken on a Leitz Ortholux compound microscope with an Orthomat 35 mm camera. 45
Before dissections were made the buds were moved to 95% ethanol and stained in a 1% safranin stock solution. The tissue stored in the higher alcohol becomes brittle and is easier to d issect.
Additional material was taken through the higher alcohol series into a graded series of ethanol: acetone to 100% acetone
(intermediate fluid). The specimens were then critical point dried in a Denton Critical Point Dryer using carbon dioxide as the tran sitio n flu id . The buds and inflorescences were mounted in o "Television Tube-Koat" on stubs and coated with 200 A of gold-palladium in a Hummer Sputter Coater II. All SEM micrographs were taken with Tri-X Ortho 4X 5 inch sheet film on a Hitachi
S-500 scanning electron microscope.
Terminology—Floral organs of the Papilionoideae are arranged in pentamerous whorls beginning with the sepals. Organs are positioned alternate to as well as acropetal to the previously formed set in the following sequence: sepals, petals, five antisepalous and five antipetalous stamens, and a solitary carpel
(Fig. 1). Antisepalous and antipetalous sites, respectively, refer to the episepalous and epipetalous sites used in Tucker (1984a & b,
1987) and Gemmeke, (1982). The median sepal lie s along the median sagittal plane on the abaxial side of the flower. Petals stand in positions which, as a consequence, places the median sagittal petal
(standard) on the adaxial side of the flower. Five antisepalous stamens occur in the positions alternate to and above the petals 46
and are also referred to as first-whorl or outer stamens. The
second-whorl or inner stamens that lie along radii common to the
petals are in antipetalous positions. Due to the pentamerous
nature of the flower and the alternating arrangement of the floral
organs, only one member of each whorl lies in the median sagittal
plane. The remaining four members of each whorl occur in two
pairs: a lateral pair and either an abaxial (distal) pair or an
adaxial (proximal) pair.
RESULTS—Organography—Lupinus havardii is a robust winter
annual that has terminal racemes (Fig. 1) that are 18-45
centimeters long (including peduncle) (Correll and Johnston, 1970).
The highly zygomorphic, fully expanded flower has a two-lipped
calyx tube with five lobes, five petals with descending cochlear
aestivation (Tucker, 1984 a,b ), ten monadelphous and dimorphic
stamens and a unicarpellate gynoecium (Fig. 2).
Inflorescence—The inflorescence meristem (Fig. 3) produces bracts in helical acropetal order. One flower (Figs. 3 and 4, f) is borne in the axil of each bract. The inflorescence has a domed apical meristem with a tunica-corpus configuration with two tunica
layers (Fig. 5). The inner tunica layer is well defined only at
the summit. Bract primordia are in itia te d on the flanks of the inflorescence meristem by periclinal divisions in second tunica layer and outer corpus layers (Fig. 5, arrow).
Floral meristem—The floral meristem is initiated on the inflorescence meristem by periclinal divisions in the outer corpus 47 layers in the bract axil (Fig. 6). The anticlinal divisions contributing to the heightening of the primordium are located in the tunica and subsurface layers (Fig. 6, arrow). In polar view the floral primordium is oblong (Fig. 7). The meristem is broader in the frontal plane than in the sagittal plane (Fig. 7) thereby establishing bilateral symmetry of the flower before organogenesis.
As seen in sagittal section the configuration of the floral primordium is tunica-corpus with two tunica layers (Fig. 8). The entire flo ra l apex is homogeneously meristematic. The flo ral meristem is approximately 54 /am in height and 68 ;um wide in the sagittal plane (Fig. 8) before any organs form. The anticlinal divisions (Fig. 8, arrow) provide for further expansion of the meristem.
Floral Organogeny—Bracteoles—The first products of the floral meristem are two bracteoles formed simultaneously on the lateral flanks of the floral meristem (Fig. 9). Initiation of these structures was not investigated exomorphologically any further or histologically.
Sepals—After the simultaneous production of bracteoles, the first floral organ to be produced by the floral meristem is the abaxial median sepal (Fig. 10) followed in close succession by a pair of lateral sepals. Due to the eccentric shape of the meristem at this time and the angle between its long axis and that of the inflorescence axis, the adaxial side of the meristem is exposed and no primordia are seen (Fig. 10). A sepal primordium is initiated 48
by numerous periclinal divisions (Figs. 11 and 12, S). The other
four sepal primordia are probably initiated in the same manner.
The periclines that initiated the primordium are seen in higher
magnification in Fig. 12. In contrast to the homogeneous
distribution of meristematic tissue throughout the bare floral
meristem (Fig. 8) the cells of the highest stainability appear to
be shifted toward the abaxial half of the summit of the meristem
above the median sepal primordium (Fig. 11). The s h ift may be
correlated with initiation sites of subsequent abaxial primordia in
the unidirectional sequence.
Calyx tube formation begins after the five sepal lobes have
been formed. The abaxial lip of the calyx tube forms (Fig. 10,
ridge) between the abaxial sepals as petal primordia become
recognisable (Fig. 10). The sepal primordia (Fig. 17 & 20) have
not enlarged much during petal initiation. Intercalary growth of
the calyx tube continues until a shallow cup-shaped calyx is formed
(Fig. 24, 29 & 31). The polar view of the flower in Fig. 34 shows
the two lips of the calyx: three abaxial lobes and two adaxial
lobes.
Petals—The prospective sites for the first-initiated petals
(keel petals) are in the abaxial positions to either side of the median sagittal plane of the flower. At the stage shown in Fig. 10 a ridge between and including the prospective petal sites is
labeled (between arrows) and is the putative site of the abaxial median antisepalous stamen. The flo ra l apex begins in itia tio n of 49
the petals (Fig. 13) before the adaxial pair of sepals is formed.
The slightly older flower in a non-median sagittal section (Fig.
17) possesses a two-layered tunica which is most uniform directly
over the summit of the meristem. Numerous periclinal divisions
have in itia te d an abaxial petal primordium (Figs. 14, between
arrows). Wing petal initiation is followed by that of the standard
petal. Five petal primordia are present as well as all sepal
primordia (Fig. 15, proximal pair of sepals, obscured by other
p arts).
The petal primordia (Figs. 24, P) tend to grow laterally in
contrast to upward growth of the sepal primordia (Fig. 24, S & 27,
Cx). As the flower proceeds through mid to late stages of development, calyx tube and petals grow in height. Pattern of petal aestivation (Fig. 32) is descending cochlear (Tucker, 1984a)
in which the margins of the standard petal overlap the upper margins of the wings which in turn overlap the upper margins of the keel petals. The margin of the keel petal is more tapered that of
the wing petal (Fig. 33) and curves inward. The keel petals are fused by appression of the lower margins and are not overlapping each other. Wing petals become sculptured abaxially near the base in the later stages of expansion.
Antisepalous Stamens—The flower in frontal view (Fig. 20) reveals a broad, flattened apex with a tunica corpus configuration maintained throughout. The antisepalous positions (Fig. 15 & 16) for the first whorl stamens are alternate to the petals and 50 opposite the sepals. The first stamen to be initiated lies in the median s a g itta l plane of the flower and is d irectly above the median sepal (Fig. 16, 17, SI & 18, SI). The median abaxial stamen is initiated first, followed in close succession by the two lateral stamen primordia (Fig. 21, SI). Numerous periclinal divisions in the second tunica and outer corpus layers have initiated the median abaxial stamen (Fig. 19, at arrows). The active cell divisions on the adaxial side of the flower in Fig. 17 are between two adaxial sepals. This is not a site for an individual sepal primordium but w ill contribute to the calyx tube that has begun to form (Fig. 20).
The densely staining meristematic tissue is uniformly distributed across the summit as seen in the frontal plane (Fig. 20).
Carpel—The carpel appears when the first-whorl stamens are present and before any second-whorl stamens are initiated. The carpel primordium is the slightly elevated central area (Fig. 23 &
24). Numerous periclinal divisions (Fig. 22, between arrows) which occur in the second tunica layer at the summit of the meristem initiate the carpel. Immediately to either side of the group of periclinal divisions (Fig. 22), the second tunica layer remains intact. To either side of the carpel (Fig. 22) are the putative sites for the lateral pair of antipetalous stamens. The carpel primordium is prominent as the adaxial pair of antisepalous stamen primordia (Fig. 27, at arrows) are formed. 51
At the time the carpel cleft is beginning to form, the calyx
tube is complete and the petal and first whorl stamen primordia are
approximately equal height (Fig. 24).
Antipetalous Stamens—The last set of organs initiated by the
floral meristem are the antipetalous stamens. The floral apex
retains a 2-layered tunica and a corpus (Fig. 22, to either side of
the carpel primordium). Part of the meristem, with two tunica
layers is present in the future site of the median antipetalous
stamen (Fig. 26, arrow). A lack of periclines confirms the delay
in the production of the antipetalous stamens (Fig. 26) especially
on the adaxial side of the flower. The antipetalous stamens are
initiated beginning with the two abaxial (distal) antipetalous
primordia (Fig. 27, arrows). The rest follow in unidirectional
order: the lateral pair, and then the median sagittal antipetalous
stamen primordium (Fig. 28, arrows & 29, in adaxial view). The
median sa g itta l antipetalous stamen, the la st organ formed by the
floral meristem, is initiated by periclinal divisions (Fig. 30,
arrow) in the outer corpus layers.
Stamen development—Stamen dimorphism is expressed by differing
sizes between the two whorls of stamen primordia (Fig. 27) which
continues through anthesis (Fig. 36). Not only is the stamen dimorphism obvious at in itia tio n (compare Fig. 27 & 28); i t remains
so through mid-development (Fig. 34), late-developmental stages
(Fig. 35) and anthesis (Fig. 36). The monadelphous filament tube grows by intercalary growth uniting the filament bases of all ten 52 stamen primordia (Fig. 35 & 36). Growth of the filament tube and growth of the individual filaments of the stamens above the filament tube continues (Fig. 36). The anther sacs of the antipetalous stamens remain smaller than the anther sacs of the antisepalous stamens (Figs. 35 & 36).
DISCUSSION—Floral symmetry—The oval flo ral meristem of _L. havardii is strongly zygomorphic from the time of in itia tio n in the axil of the subtending bract. The floral meristem remains zygomorphic throughout organogenesis due to the unidirectional order of in itia tio n of each whorl. After a ll organs have been initiated the bilateral symmetry is maintained on the basis of the pentamerous nature of each whorl. During the mid-stages of development (Tucker, 1984a) in which organ expansion occurs, organs within each whorl are very uniform somewhat masking the strongly zygomorphic nature of the flower. Expansion and d ifferen tiatio n of the petals marks the final stage in which the fully expanded flower is again obviously zygomorphic.
Activity of the floral meristem—The configuration of the floral meristem before organogenesis is tunica-corpus. Only during abaxial sepal and abaxial petal initiation does there appear to be a concentration of darkly staining densely cytoplasmic cells toward the abaxial portion of the apex (where initiation begins in each whorl) and across the summit. The adaxial flank at this time appears to contain a second tunica layer more uniform than the adaxial portion of the meristem. The cells on the adaxial flank in 53 general appear to be more vacuolate than cells along the abaxial flank. The second tunica layer appears consistently disrupted at the sites of organ initiation. Over the summit the second tunica layer has relatively lower meristematic activity. Localized periclinal divisions in the second tunica layer and underlying layers of the corpus account for the initiation of organs.
Vertical order of initiation—Abaxial members of each floral organ whorl are initiated in acropetal order with precocious overlap among other members of each whorl. The carpel precedes the antipetalous stamens. It is important to determine not only the horizontal order of organ appearance but also the vertical order.
Organs appear generally in acropetal order: sepal, petal and then stamen primordia. As the adaxial members of the f i r s t stamen whorl are initiated, the carpel primordium becomes evident as a mound.
Its appearance is documented histologically with numerous periclinal divisions in the second tunica and outer corpus layers.
It is not u n til a fte r the carpel has begun apical growth and c le ft formation that the members of the second whorl of five stamens appear first on the abaxial side of the carpel.
Order in Lupinus can be compared with that of Acacia baileyana in which sepals are initiated helically before petals and common stamen primordia are initiated simultaneously. The sequence of individual stamen primordia interrupts the strictly vertical order of initiation of floral organs. Sepals in Parkinsonia aculeata arise helically followed by unidirectional order of petals and two 54
sets of five stamens from abaxial to adaxial side of the flower.
In all three species the carpel arises precociously.
Unidirectional order of organ initiation within whorls—Unidirectional organ initiation has recently been described
in detail using SEM in a caesalpinioid, Bauhinia fassoglensis, and
two papilionoids, Genista tinctoria and Lupinus affinis (Tucker,
1984b). The sepals of the caesalpinioid initiate helically while
the other organs follow unidirectional order of initiation from abaxial to adaxial side of the flower. The latter two species initiate all floral organs from the abaxial to the adaxial side of
the floral meristem. Investigations of order of appendage in itia tio n (Tucker,1984b, 1987) to ta l 26 genera + , none of which has been documented histo lo g ically . Tucker (1987, Table 3) reports nine other caesalpinioids having a helically derived calyx and unidirectional whorls of petals and stamens. In the same paper, unidirectional initiation is reported in about 30 papilionoids, showing that this pattern, demonstrated histogenetically for
Lupinus havardii in the present paper, is extremely common among papilionoids. Deviations from the unidirectional pattern have been reported and need to be restudied in Astragalus (Hofraeister, 1868),
Galega (Tucker, 1987), Lathyrus (Payer, 1857), and Pisum (Sattler,
1973). Recent work by Tucker (in press) suggests that Pisum is also unidirectional. Rohrbach (1870, in Tucker, 1987) reported an abaxial to adaxial transverse order of sepal initiation and an adaxial to abaxial (or simultaneous) order of petal initiation in 55
Acacia oxycedrus. The exception to the simultaneous appearance of perianth organs of mimosoids should be verified with SEM and histologically. More reliable are the recent findings by Gemmeke
(1982) of adaxial to abaxial sequence of sepal appearance in
Lysiloma vogelianum, a mimosoid. The flo ral meristem of Lupinus havardii initiates its floral appendages unidirectionally in each whorl as in the papilionoid species studied by Tucker (1984b,
1987). Before all members of one whorl have been initiated, the
first members of the next whorl in acropetal order appear. An exception in this sequence occurs with the carpel appearing precociously before any of the antipetalous stamens are initiated.
In addition, precocity characterizes some members of each whorl.
The primordia of the antipetalous whorl are noticeably smaller that those of the antisepalous whorl. It has been proposed by
Heslop-Harrison (1964) that high auxin levels favor carpel development and can suppress stamen growth. The stamens however grow with filaments contributing to the monadelphous androecial tube, yet the anthers are much reduced. The precocious carpel in
Acacia baileyana is followed by initiation and development of the antipetalous stamens of which the primordia and the anther sacs are reduced. Heslop-Harrison (1964) rationalizes the condition under which both organs develop simultaneously as an "escape from the initial regulating influence." Pollen is produced but it was not determined either in what quantity or to what extent it is viable in Lupinus. 56
Correlation of SEM and histogenetic evidence—From consistent observations of periclinal divisions initiating primordia in the mimosoid and papilionoid taxa, it can be concluded that the protuberances observed with the scanning electron microscope are floral organ primordia. Scanning electron microscopy can be used confidently to determine sequence of initiation of floral primordia unless the primordia arise so rapidly in succession that it is difficult to determine the order. In taxa in which the unidirectional overlap is obscure, histological investigations should be used to verify topographic findings as suggested for
Parkinsonia aculeata (Chapter 3). Where the order of initiation of floral organs proceeds from the adaxial to abaxial sides of the floral meristem (Gemmeke, 1982), detailed histological investigations are useful. In situations in which large numbers of taxa are examined, SEM alone can be used to examine flo ral characters.
Terminality of the carpel—In Lupinus havardii and Acacia baileyana the carpels are initiated by numerous periclines in the outer corpus. In the mimosoid that undergoes the radial expansion the site of the carpel is delimited by the anticlinal divisions that accomplish the second step in the radial expansion. In contrast the lupine shows no radial expansion and initiates the carpel at the summit of the floral meristem by a cluster of initial divisions. Since the lupine carpel is initiated before the whorl of antipetalous stamens, its initiation utilizes the summit of the 57 meristem and leaves meristematic tissue in only the sites to be occupied by the antipetalous stamens. There is no apical residuum and the carpel is considered terminal.
Comparison of histogenetic organ in itia tio n —Organ in itia tio n histologically is similar between the mimosoid and papilionoid species on the basis of periclinal divisions. Timing of the events of organ initiation is clearly different between the two taxa. The shift in apical meristem activity of the lupine characterizes the strongly zygomorphic flo ra l meristem observed in SEM. The globose flower of the mimosoid is reflected in the uniform meristem during sepal, petal, and common stamen primordia initiation. Activity of the meristem in the radially symmetrical flower is confined to the sites of the evenly spaced primordia and not on one side of the flower. 58
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Fig. 1-2. Diagram of the inflorescence and flower of Lupinus havardii. 1. Diagram of raceme. Bract, B. Flower, f. 2. Floral diagram. Inflorescence axis, a. Bracteoles, Bl. Bract, B. Sepals,
S. Petals, P. Stamens, St, monadelphous arrangement. Carpel, C. 61
Q Q Q O Q O a 0a o B Cb o a Bl o ® 7
2 1 62
Fig. 3-6. Inflorescence and initiation of bract and floral meristem of Lupinus havardii. 3. Lateral view of the raceme apex showing the summit of the meristem and the last formed bract (B). Older bract, B (right) and the single axillary flower, f. Bar = 50 pm. 4. Near median longitudinal section of the inflorescence tip. B, subtending bracts. Floral primordium, f. Bar = 100 pm. 5. Inflorescence tip, near median, tunica-corpus configuration. B, bract initiation, arrow. Bar = 50 pm. 6. Periclines of flower initiation, arrow. Bar = 10 pm.
64
Fig. 7-14. Bare floral meristem and initiation of abaxial
sepals and abaxial petal primordia. Bract scar, B, to lower side of the SEM's and to lower left in Fig. 10. 7. Polar view of the bilaterally symmetrical floral primordium subtended by i t s bract, B. 8. Near median sa g itta l section of flo ral meristem before organogenesis. B, bract. Bar = 50 pm. 9.
Bilaterally symmetrical floral meristem with the lateral bracteoles, Bl. Subtending bract scar, B. Bar = 50 jam. 10.
Flower with first three sepal primordia. Bract, B. Bar = 50 pm. 11. LS of flower, near median for the abaxial sepal.
Tunica-corpus apical configuration. Bar = 50 jim. 12. Median sagittal section of abaxial sepal primordium. B, bract. Bar =
20 jam. 13. Polar view of flower with abaxial sepals, S, and abaxial petals, P. Subtending bract, B. Meristem in the center of the flower. Bar = 50 jam. 14. Non-median sa g itta l section of flower at abaxial petal initiation, at arrows. B, bract. Second tunica layer over summit. Bar = 20 jim. 65 66
Fig. 15-20. First whorl stamen initiation. 15. Three sepals,
S, visible. Five petals present. Prospective sites of the first whorl stamens in antisepalous positions. Meristem at center of flower. Bract, B. Bar = 50 pm. 16. Flower with median abaxial and la te ra l antisepalous stamen primordia, SI, present. Bar = 50 pm. 17. Sagittal section, median for bract, B, and for median abaxial sepal, S. Site of initiation of f i r s t formed stamen, SI, which is along median s a g itta l plane. Bar = 50 pm. 18. Periclinal divisions of initiation of f i r s t formed stamen, SI, between arrows. Bract, B. Bar = 20 pm.
19. Initials (at arrows) of calyx tube formation on adaxial side of the flower, between the adaxial sepal lobes. Bar = 50 pm. 20. Frontal section of flower. Bl, bracteole. Cx, calyx tube between lateral pairs of sepal lobes. P, petal primordium. Bar = 50 pm. 67 68
Fig. 21-24. Carpel initiation. Bars = 50 pm. 21. Site of carpel in center of meristem. First whorl stamens, SI, present. Petals, P. Sepal, S 22. Median sagittal section of flower at the time of carpel initiation. Periclinal divisions of carpel initiation, between arrows. 23. Adaxial view of flower with all sepals, S, petals, P, five stamens, SI, and carpel, C. B1, bracteole. Bract, B, behind. 24. Flower beginning apical growth of the carpel along its abaxial side.
Organs present include bracteoles, Bl, sepals, S, p etals, P, antisepalous stamens, SI, and carpel, C. 69 70
Fig. 25-30. Initiation of antipetalous stamens and early carpel growth. Bars for Fig. 25-29 = 50 pm. Bar for Fig. 30 = 20 pm.
25. Near polar view of flower with calyx, Cx, petals, P, antisepalous stamens, S, and carpel, C. 26. Median sagittal section of carpel, C, and site of vexillary stamen, arrow. 27.
Abaxial antipetalous stamen primordia, arrows. Lateral sites and vexillary stamen site vacant. Calyx, Cx. 28. All antipetalous stamens present, arrows. 29. Adaxial view of flower with all floral organs present. 30. Median sagittal section of flower. Initiatory periclinal divisions of vexillary stamen, arrows. 71 72
Fig. 31-36. Later stages in floral development of Lupinus havardii. Bars for Fig. 31-34 = 50 pm. Bars for Fig. 35 & 36 =
500 pm. 31. Calyx tube formation, adaxial view showing two lobes of the calyx tube. Calyx, Cx. Petal, P. Stamen, S.
Carpel, C. 32. Descending cochlear petal aestivation 33.
Transverse section, wing petal and keel petal, K, margins meet.
34. Polar view of young flower. All flo ra l organs v isib le.
Stamen dimorphism obvious during mid-development. Antisepalous stamens, SI. Antipetalous stamens, S2. 35. Early filament tube formation, stamen dimorphism. Antisepalous stamens, SI.
Antipetalous stamens, S2. 36. Monadelphous filament tube near anthesis. 73 Chapter 3
Floral Ontogeny in Parkinsonia aculeata L.
(Caesalpinioideae)
74 75
Abstract
Floral ontogeny has been investigated in Parkinsonia aculeata
L. using scanning electron microscopy (SEM) on buds in stages of
appearance of all organs. Single flowers arise in the axil of
bracts produced by the racemose inflorescence meristem. The
flowers have five sepals, five petals, ten free stamens and a
single carpel. Sepals appear in a helical order beginning with one in an abaxial position. The remaining organs appear unidirectionally from the abaxial to the adaxial side of the floral meristem. The carpel appears before all antipetalous stamens are formed. The antipetalous stamens are the last whorl of organs to form and the median adaxial antipetalous stamen is the la st product of the floral meristem. Shifts in symmetry of the calyx and of the flo ra l meristem proceed from radial to weakly zygomorphic. Calyx aestivation undergoes marked shifts from near valvate through cucullate to imbricate. Zygomorphy of the floral meristem is due to the unidirectional nature of meristem activity. Later stages of development include uniform free stamens and ascending cochlear petal aestivation. An adaxial petal becomes larger than the rest in the fully expanded flower. 76
INTRODUCTION—The purpose of this work is to study floral
ontogeny in Parkinsonia aculeata (Leguminosae: Caesalpinieae).
The Caesalpinia group of the Tribe Caesalpinieae (Leguminosae)
contains nineteen genera including Parkinsonia (Polhill & Vidal,
1981). Flowers of Caesalpinia group range from nearly radial
( S tah lia) to strongly zygomorphic (Cenostigma, Zuccagnia, and
Caesalpinia) (Polhill & Vidal, 1981). Parkinsonia is separated on
basis of vegetative characters, fruit structure and indehiscence.
Parkinsonia aculeata, with i t s weakly zygomorphic flowers is
intermediate in symmetry within the tribe and differs from the
strongly b ila te ra l lupine and the radially symmetrical mimosoid.
Floral organ in itia tio n of other caesalpinioid legumes has been
studied in our laboratory (Tucker, et al., 1985; Tucker, in press).
Scanning electron microscopy is used to examine flo ra l organ
initiation.
MATERIALS AND METHODS—Floral m aterial of JP. aculeata was
collected from the following locales: 1) Louisiana, East Baton
Rouge Parish, Louisiana State University Campus along East Stadium
Drive between Stanford and West Parker along Corporation Canal during the spring seasons of 1982 and 1983; 2) Texas, Brazoria
County, Stanger Estates, two miles West of Farm Road 521 along
Middle Bayou, May 1983; 3) Louisiana, East Baton Rouge Parish,
River Road on levee at West Grant Street, March 1986. Tissue was fixed and prepared as previously described (Chapters 1 & 2). 77
RESULTS—Organography—Flowers of Parkinsonia aculeata are
arranged helically in pendulous axillary racemes (Fig. 1). The
inflorescences arise in the axils of long pinnately compound leaves
which have stout stipular spines. The inflorescence meristem
produces acropetally arranged bracts each of which subtends a
single flower. The inflorescence meristems appear to be somewhat
indeterminate in that floral production continues throughout the
spring and summer in the southern coastal regions of the United
States (Odenwald and Turner, 1980). There are no bracteoles. The
slig h tly zygomorphic fully expanded flowers are typically
pentamerous with organs arranged acropetally in the following
order: sepals, petals, outer and inner whorls of free stamens
(forming a disk after initiaion and before anthesis) and a single
carpel. These fully expanded caesalpinioid flowers are characterized by five uniform sepals and five petals generally
similar in size and shape. The standard petal becomes slightly enlarged and develops orange-red nectar guides after anthesis
(Cowan, 1981). In the bud the ascending cochlear imbricate pattern of petals is consistent with other subfamily members. The inflorescence morphology and anatomy has not been investigated.
Organogeny—Sepals—Five sepals appear in helical order beginning with a sepal on the abaxial side of the flower (Fig. 3).
The order of appearance of sepals is counterclockwise (Fig. 4,5) or clockwise. Sepals expand at different rates and reach a point at which the sepals (Fig. 6) are uniform in size and shape. 78
Differentiation of trichoraes occurs on the oldest sepal (Fig. 6).
With further expansion the abaxial sepal (Fig. 7) becomes cucullate, overtopping the others. The imbricate arrangement of sepals (Fig. 8) is such that the order of initiation is not obvious. Later sepal expansion was not investigated.
Petals—The order of petal appearance is not helical but unidirectional from the abaxial to the adaxial side of the floral meristem. The two abaxial petals (Fig. 9, arrows) appear in positions alternate to the sepals. It should be noted that the median sagittal plane of these floral meristems is frequently out of alignment with the plane that bisects the subtending bract.
Next, a pair of lateral petals (Fig. 10, arrows) is followed in close succession by the median adaxial petal (standard) (Fig. 10, upper arrow). Petals expand apically (Figs. 14 and 15) and laterally (Figs. 16 & 17) until they overlap marginally and show an imbricate pattern of aestivation (Fig. 19).
Antisepalous stamens—The median antisepalous stamen (Fig. 10,
St) and the two la te ra l stamens (Fig. 11, St & 12) appear a fte r the standard petal (Fig. 11, arrow) is evident. The antisepalous stamen primordia are about the same size as the petal primordia
(Fig. 10, 11, & 12) at the time of initiation. The adaxial pair of stamen primordia (Fig. 12, one site obscured by sepal) are formed to either side of the median adaxial petal.
The antisepalous stamens increase in height until they are about the same height as the petals. The antisepalous stamens 79
after differentiation of the anthers are larger than the
antipetalous stamens; although all ten anther sacs are the same
size (Fig. 21).
Carpel—The carpel primordium (Fig. 11, C) appears
precociously during the initiation of the antisepalous stamens.
The carpel primordium begins differential abaxial growth (Fig. 12,
13, 14, 15, & 16). Cleft formation (Fig. 13) is noticeable during
the initiation of the the inner whorl of stamen primordia. The carpel continues to expand apically in the bud (Figs. 17-19) until it is overtopped by the outer whorl of stamens (Fig. 20, carpel with many hairs). The carpel assumes a central position on the floral meristem and is considered terminal •
Antipetalous stamens—With the succession of the antisepalous stamen primordia and with the precocious appearance of the carpel, the sites (Fig. 12, arrow at lower right) for the antipetalous stamens are predetermined by the pentamerous shape of the flower.
The abaxial pair of antipetalous stamen primordia appears first followed in rapid succession by the lateral pair (Fig. 13, four present) and the median adaxial primordium (Fig. 14). The antipetalous stamen primordia remain smaller than the antisepalous stamen primordia (Fig. 13) during subsequent early stages. During differentiation within the bud the anther sacs of the two sets of stamens (Fig. 20) are of equivalent sizes. The filaments of the outer whorl (Fig. 20 & 21) remain longer than the filaments of the inner whorl of stamens. 80
Floral organ arrangement at anthesis—The sepals approach an
imbricate arrangement (Fig. 8), petal aestivation remains in an
ascending cochlear pattern (Fig. 19), the stamens are free (Fig.
21). Around the base of the carpel is a disc surrounded by the
filament bases (Fig. 22). Radial expansion has occurred presumably
during stages of development after all organs were initiated.
DISCUSSION—Order of organ initiation—In Parkinsonia aculeata
sepals appear in helical order followed by the petals,
antisepalous stamens and antipetalous stamens in unidirectional
order within in each whorl. The order of initiation of floral
organs in P. aculeata is similar to the order described for L.
havardii (Chapter 1). Other caesalpinioids with helical calices
are listed in Tucker (1987; in press) and include about 13 species
in Brownea, Cassia, Ceratonia, C ercis, Saraca and Schotia. A unidirectional sequence of sepal initiation observed in Caesalpinia pulcherrima and C_, ve sic aria (Tucker, et a l ., 1985) have not been observed in flowers of Parkinsonia aculeata. Most caesalpinioid
taxa studied share Parkinsonia *s pattern of helical calyx and unidirectional order on other whorls. A whorled transverse order of initiation of petals and stamens has been observed in Brownea coccinea and B. grandiceps (Hartog, 1888). The petal primordia and the two sets of stamen primordia appear in such rapid succession within each whorl that without careful examination of many stages
the order could appear to be simultaneous. In taxa such as the
Brownea species, histological evidence of the activity of the 81
floral meristem should accompany topographic investigation.
Because the sequence of in itia tio n of petals and stamens in j>. aculeata is timed so closely, in order to confirm the unidirectional order of initiation of these organs, the activity of
the floral meristem should be determined histologically. Based on histological documentation of the other legume taxa in this investigation it is safe to assume that organ initiation is immediately followed by a protuberance on the meristem.
The carpel primordium (seen as a mound) appears during production of antisepalous stamen primordia. The acropetal order of organ initiation is interrupted by the initiation of the carpel in Lespedeza thunbergii and Cassia marilandica (Tucker, 1987).
Appearance of the carpel primordium before all stamen primordia are initiated is reported among the following papilionoids:
Astragalus asper (Hofmeister, 1868), Pisum sativum (Tucker, 1987 & in press), and Lupinus elegans (Frank, 1875). Carpel precocity in
II* aculeata is consistent with the papilionoid and the mimosoid taxa included in this investigation and is not an unusual occurence among the Legurainosae.
Symmetry—The floral meristem of P. aculeata tends to shift in symmetry from radial at the time of sepal in itia tio n to zygomorphic beginning unidirectional petal and stamen appearance. The outward appearance of the flower is radial during calyx initiation, but sh ifts to zygomorphic with the overgrowth of the cucullate
(abaxial) sepal. During mid-development stages (Tucker, 1984) the 82 flower exhibits bilateral symmetry on the basis of carpel and the five-merous nature of the flower. Before the flowers open at anthesis the sepals are once again uniform in size and imbricate in arrangement. The shifts in sepal size and uniformity do not obviously reflect the shifts in symmetry of the floral meristem.
The unidirectional order of initiation can be inferred but not proven without histological documentation. 83
Literature Cited
CORRELL, D. S., AND M. C. JOHNSTON. 1970. Manual of the Vascular
Plants of Texas. Texas Research Foundation, Renner, Texas, p.
793.
COWAN, R. S. 1981. Caesalpinioideae. Iji Advances in Legume
Systematics, Part 1. R.M. P olhill and P. H. Raven, (Eds.), pp.
57-63. Royal Botanic Gardens, Kew.
HARTOG, M. M. 1888. On the flo ral organogeny and anatomy of Brownea
and Saraca Ann. Bot. 2: 177-196.
ODENWALD, N. G., and J. R. TURNER. 1980. Plants of the South. A
guide for Landscape design. Claitor's Publishing Division,
Baton Rouge, p. 361.
POLHILL, R. M. and J. E. VIDAL. 1981. Tribe Caesalpinioideae. In
Advances in Legume Systematics, Part 1. R.M. P olhill and P.H.
Raven (Eds.) pp. 81-96. Royal Botanic Gardens, Kew.
TUCKER, S. C. 1984a. Unidirectional organ initiation in leguminous
flowers. Amer. J. Bot. 71: 1139-1148.
------. 1987. Floral initiation and development in legumes. In
Advances in legume systematics, Part 3. C. H. Striton (Ed.),
Royal Botanic Garden, Kew. pp. 183-239.
------. 1988. Floral heteromorphy through organ suppression in
Bauhinia (Leguminosae; Caesalpinioideae). Amer. J. Bot. 75:
(In press). 84
TUCKER, S.C., S.R. RUGENSTEIN and K.S. DERSTINE. 1984. Inflated
thrichomes in flowers of Bauhinia. Bot. J. Linn. Soc. 88:
291-301.
------, 0. L. STEIN, and K. S. DERSTINE. 1985. Floral development
in Caesalpinia (Leguminosae). Amer. J. Bot. 72: 1424-1434. 85
Fig. 1-2. Diagrams of raceme and flower of Parkinsonia aculeata. 1.
Inflorescence diagram of Parkinsonia aculeata. Bract, B. Flower, f.
2. Floral diagram of Parkinsonia aculeata. 2. Floral diagram.
Inflorescence axis, a. Bract, B. Sepals, S. Petals, P. Stamens,
St. Carpel, C. 86
Sto
B 87
Fig. 3-8. Sepal initiation and development. Bar = 50 ^um in Fig.
3-6 and 500 in Fig. 7 & 8. Sepal, S, bract, B. 3. First sepal initiated on abaxial side of floral meristem. 4. Sepal primordia numbered in helical order of initiation. 5. Lateral view of flower from adaxial side. Sepals initiated in counterclockwise helix. Sequence determined by relative sizes of primordia. 6.
Sepals uniform in size 7. First-formed sepal cucullate. Helical order determined by relative sizes of sepals. 8. Prior to anthesis sepals imbricate. 88 8 9
Fig. 9-12. Petal and antisepalous stamen initiation. Bar = 50 ^m.
Bract, B. Sepal, S. Carpel, C. 9. Unidirectional petal initiation beginning with two abaxial petal primordia, arrows.
Four sepals removed. 10. Lateral petal primordia (lower arrows) and abaxial antisepalous stamen primordium, St. Three sepals removed. 11. Five petals present, la te ra l petal obscured by sepal. Lateral antisepalous stamens, St. Carpel primordium, C.
12. Median adaxial p etal, upper arrow. Antipetalous stamen primordium, arrow at lower right, on abaxial side of floral meristem. 90 91
Fig. 13-16. Antipetalous stamen initiation and petal antisepalous and carpel expansion. Bar = 50 ^im. Carpel, C. 13. Sepals removed. Five p etals, five antisepalous stamens, and four antipetalous stamens present. 14. Sepals removed. All flo ra l organs present. 15. Sepals removed. Slightly oblique abaxial view comparing sim ilar petal and antisepalous stamen sizes.
Antipetalous stamens present. 16. Lateral view of flower with sepals removed. Petals and antisepalous stamens of equivalent heights, carpel slightly higher. 92 93
Fig. 17-22. Petal, stamen and carpel development. Fig. 17-19, Bar
= 50 p m. Fig. 20-22, Bar = 500 ^pm. 17. Polar view of flower, carpel c le ft slightly out of median sa g itta l plane. 18. Stage similar to Fig. 17 with one petal and one antisepalous stamen removed to reveal antipetalous stamen primordium, still smaller than antisepalous stamen primordia. 19. Ascending cochlear petal aestivation. Sepals removed. 20. Carpel, with hairs, relative to height of stamens. Sepals, petals and numerous stamens removed.
21. Flower near anthesis. Sepals and petals removed, anther sacs of two whorls of stamens uniform. 22. Flower at anthesis with all floral organs removed to show disc surrounding the carpel base. 94 Chapter 4
Summary
95 96
Sumraa ry
Symmetry, order of organ initiation, organ position, and fate of
the floral meristem are the taxonomically important ontogenetic characters examined in the following legume species: Acacia baileyana (Mimosoideae), Lupinus havardii (Papilionoideae), and
Parkinsonia aculeata (Caesalpinioideae). Topographic information and histogenetic evidence are closely correlated in A. baileyana and L. havardii. P. aculeata is examined topographically with SEM. histological evidence is implied in P_. aculeata.
Symmetry and order of organ initiation are closely related developmentally. The activ ity of the flo ra l meristem is documented histologically throughout unidirectional organogenesis confirming the symmetry observed topographically in Lupinus. Bilateral symmetry is expressed in flowers of Lupinus havardii in the floral meristem before organogenesis and is maintained throughout unidirectional organogenesis. Subtle expression of zygomorphy in the lupine occurs during mid—developmental stages by the two—lipped calyx and the carpel primordium compared to the strong bilateral symmetry that is evident in the fully expanded typically papilionoid flower.
Floral meristems of P. aculeata and A. baileyana begin initiation of floral organs on radially symmetrical apices. Sepals of both species are initiated helically. Beginning with petal 97
initiation the flowers of the caesalpinioid and the mimosoid taxa diverge from one another in developmental pathways. P. aculeata
initiates the remaining floral organs unidirectionally thereby
expressing b ila te ra l symmetry. During mid-developmental stages
bilateral symmetry is expressed by the median sagittal plane
through the carpel primordium. The fully expanded flower of P. aculeata shows weak zygomorphy.
The mimosoid retains radial symmetry throughout simultaneous organogenesis of petals and of common stamen primordia as
simultaneous whorls. Radial symmetry is consistent throughout the complex sequence of stamen in itia tio n in A. baileyana.
Histological evidence confirms the topographic evidence from SEM in the mimosoid. Calyx and corolla members are uniform within each whorl and although a plane of b ila te ra l symmetry may be drawn through the carpel, the flower is radially symmetrical.
The floral meristem shifts its configuration from tunica-corpus to mantle-core, a common event at floral onset. The double stepwise radial expansion of the floral meristem is the means by which the flo ra l meristem accommodates for in itia tio n of large numbers of stamens present in Acacia baileyana, but absent generally in Papillonoideae and Caesalpinioideae. Anatomical sections showing the meristematic activity on the abaxial side of the flower confirmed the findings of concentrations of primordia seen abaxially in L. havardii with SEM. Organs are consistently initiated by periclinal divisions in the subsurface tunica layer, 98 and in no instances were periclines observed in sites other than organ sites. The carpel is determined histologically to be terminal in L. havardii and A. baileyana and appears similarly terminal in P. aculeata. Chapter 5
Vita
99 1 0 0
Vita
Kittie Sue Derstine
Born in West Columbia, Texas, on June 25, 1947, Attended
Texas A & I University from September 1965-May 1966. Received a B.
S. in Animal Science from Texas A & M University in December, 1970.
Received M. S. in Botany from Texas A & M University in August,
1978. Currently employed as General Biology Laboratory
Coordinator, Department of Biology, University of Southwestern
Louisiana, Lafayette. DOCTORAL EXAMINATION AND DISSERTATION REPORT
Candidate: K ittie S. Derstine
Major Field: B otany
Title of Dissertation: Floral Ontogeny and Histogenesis in Leguminosae
Approved:
Major Professor and Chairman
%an of the Graduate
EXAMINING COMMITTEE:
im
V\ . vv-vVJtJ ^
Date of Examination:
A p ril 29. 1988