American Journal of Botany 96(1): 207–215. 2009.

X YLEM OF EARLY ANGIOSPERMS: () HAS NOVEL TRACHEID MICROSTRUCTURE 1

Sherwin Carlquist, 2,4 Edward L. Schneider, 2 and C. Barre Hellquist3

2 Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, California 93105 USA; and 3 Biology Department, Massachusetts College of Liberal Arts, North Adams, Massachusetts 01220 USA

SEM studies of xylem of stems of Nuphar reveal a novel feature, not previously reported for any angiosperm. Pit membranes of tracheid end walls are composed of coarse fi brils, densest on the distal (outside surface, facing the pit of an adjacent cell) surface of the pit membrane of a tracheid, thinner, and disposed at various levels on the lumen side of a pit membrane. The fi brils tend to be randomly oriented on the distal face of the pit membrane; the innermost fi brils facing the lumen take the form of longitudinally oriented strands. Where most abundantly present, the fi brils tend to be disposed in a spongiform, three-dimensional pattern. Pores that interconnect tracheids are present within the fi brillar meshwork. Pit membranes on lateral walls of stem tracheids bear vari- ously diminished versions of this pattern. Pits of root tracheids are unlike those of stems in that the lumen side of pit membranes bears a reticulum revealed on the outer surface of the tracheid after most of the thickness of a pit membrane is shaved away by the sectioning process. No fi brillar texturing is visible on the root tracheid pits when they are viewed from the inside of a tracheid. Tracheid end walls of roots do contain pores of various sizes in pit membranes. These root and stem patterns were seen in six spe- cies representing the two sections of Nuphar , plus one intersectional hybrid, as well as in one collection of Nymphaea , included for purposes of comparison. Differences between root and stem tracheids with respect to microstructure are consistent in all spe- cies studied. Microstructural patterns reported here for stem tracheid pits of Nymphaeaceae are not like those of Chloranthaceae, Illiciaceae, or other basal angiosperms. They are not referable to any of the patterns reported for early vascular . The adap- tational nature of the pit membrane structure in these tracheids is not apparent; microstructure of pit membranes in basal angio- sperms is more diverse than thought prior to study with SEM.

Key words: basal angiosperms; fi brils; Nuphar ; Nymphaeaceae; microstructure; pit membranes; tracheids; xylem.

Nuphar is a genus of great potential interest due to its phylo- 1987 ). Various degrees of pit membrane presence also occur genetic position in angiosperms. Nuphar is sister to all other in end walls of vessel elements of Illicium (Carlquist and genera of Nymphaeaceae (Les et al., 1999). In turn, Nymphae- Schneider, 2002). In the genera of these two families, pit mem- ales, comprising Cabombaceae, Nymphaeaceae, and Hydatel- brane remnants may be intact in end walls of some otherwise laceae (Saarela et al., 2007), is sister to all angiosperms except vessel-like tracheary elements, while other vessel elements re- Amborellaceae based on DNA data ( Soltis et al., 2000 ) tain extensive fl akes, webs, or porose sheets of primary wall ma- Amborella was found to be vesselless by Bailey and Swamy terial, and yet others vessel elements have few pit membrane (1948). Results based on SEM studies confi rm the exclusive remnants in end walls. Primary xylem and earlier-formed sec- presence of tracheids in the wood ( Carlquist and Schneider, ondary xylem in Chloranthaceae and Illiciaceae show greater 2001). End walls of Amborella tracheids have pit membranes degrees of retention of pit membranes in end walls of tracheary intact, but with small circular porosities larger than plasmodes- elements. Thus, Swamy and Bailey (1950) reported Sarcandra mata (Carlquist and Schneider, 2001). Similar results were ob- to be vesselless. Had they examined material with a greater ac- tained in Bubbia (Carlquist, 1983) and Tetracentron ( Carlquist, cumulation of secondary xylem, they might have found vessels. 1988). Porose pit membranes were reported in end walls of tra- The aforementioned genera of angiosperms represent expres- cheids in Nuphar and other Nymphaeaceae ( Schneider et al., sions intermediate between what are tracheids and vessel ele- 1995). All the above taxa qualify as vesselless on the basis of ments as generally defi ned. The textbook defi nitions of these SEM study. The pores in end walls of those tracheids represent cell types do not take into account such intermediacy, because some tendency toward vessel-like structure, but in physiological the terminology is still structured on the basis of light micros- terms, pit membranes, although porose, are relatively intact, and copy. Woods of a number of woody angiosperms have vessels thus one should term them tracheids. Similar considerations ap- that characteristically retain pit membrane remnants in perfora- ply to the xylem of most ferns ( Carlquist and Schneider, 2007 ). tions of cells can readily be termed vessel elements ( Carlquist, The distinction between tracheids and vessel elements does 1992c; Carlquist and Schneider, 2004). Thus, with the instances blur in secondary xylem of Chloranthaceae and Illiciaceae, how- mentioned, one can demonstrate a zone of intermediacy be- ever. End walls of presumptive vessel elements retain extensive tween tracheids and vessel elements, illustrating “ non-missing pit membrane remnants in the four genera of Chloranthaceae: links” that one would, in fact, expect to fi nd in angiosperms that Ascarina (Carlquist, 1990), Chloranthus ( Carlquist, 1992a ), possess a preponderance of primitive character states. Hedyosmum ( Carlquist, 1992b ), and Sarcandra ( Carlquist, The phylogenetic interest of Nuphar and the availability of excellent living materials of the majority of species of the genus 1 Manuscript received 17 January 2008; revision accepted 21 May 2008. have induced us to re-examine Nuphar xylem with SEM. Our 4 Author for correspondence (e-mail: [email protected]) earlier work was performed with an SEM of limited resolution capacity. In addition, new technical procedures for specimen doi:10.3732/ajb.0800348 preparation (Carlquist and Schneider, 2007) offer more reliable 207 208 American Journal of Botany [Vol. 96 results than hitherto available for showing the nature of pit RESULTS membranes in tracheary elements and thus for examining xy- lem evolution in early angiosperms. One might expect that mi- Roots— End walls of N. advena subsp. ozarkana root tracheids crostructure of xylem of Nuphar would represent ancestral have porose pit membranes. The photograph of end wall pits conditions for angiosperm xylem. However, xylem structure ( Fig. 1 ) reveals that the pit membranes are thin, with pores of evolves sensitively with respect to ecology and habit (Carlquist, various sizes. No membrane portions are shaved away in this 1975 ). Nuphar’ s phylogenetic position does not guarantee that view: such action would show in the surfaces of the secondary the xylem of Nuphar is an archive of primitive features. We wall. Lateral walls of N. advena subsp. ozarkana ( Fig. 2 ) have must be prepared to realize that each group among primitive smooth membranes, as seen from the inside of a tracheid. angiosperms has its own patterns, and its survival to the present The tracheid pits of N. polysepala (Fig. 3) are clearly porose. has included stories of divergence, rather than retention of an- The pit membranes, seen from an outer surface of a tracheid, cestral conditions. may have experienced some removal of an adjacent wall, but the reticulate appearance is valid for at least that wall layer. The same is true for N. japonica ( Fig. 4 ), in which portions of the pit membranes near lateral ends of pits (upper right) are less porose, MATERIALS AND METHODS suggesting the reticulate layer is paired with a nonporose layer. In Nymphaea , similar conditions are evident. Both of the The following collections form the basis for the current study. We follow the taxonomic system of Padgett (2007) . views shown here ( Figs. 5, 6 ) are from outer surfaces of root Nuphar section Astyla Padgett: N. advena (Ait.) Ait. subsp. advena — Fresh- tracheids, and both are probably lateral walls. The occurrence of water tidal area of Hudson River, north of Rodgers Island, Greenport, Columbia a nonporose wall portion (Fig. 5, left) suggests that the porose Co., New York, USA. 3 September 2001, C. B. Hellquist 16722 (GH). N. ad- portion of the wall represents shaving away of a wall layer. A vena subsp. advena — Backwater on west side of Sideling Hill Creek at Varner similar appearance is evident in Fig. 6. The nonporose pit mem- Gap, Route 454, 7 miles WSW of Burke Valley, Fulton Co., Pennsylvania, brane at right may represent a parenchyma– tracheid interface. USA. 21 June 1987. Hellquist 17082 (GH). N. advena subsp. ozarkana (G. S. Mill. and Standl.) Padgett— Cultivated on W side of West Road, Adams, Berk- shire Co., Massachusetts, USA from material collected by Donald Padgett. Stems— The pits in end walls of stem tracheids of Nuphar Hellquist 17083 (MASS). N. polysepala Engelm.— Cultivated at Santa Barbara differ markedly from those of roots. This is shown dramati- Botanic Garden from material cultivated at Black Lake, Nipomo Mesa, along cally by the views of N. advena subsp. advena ( Hellquist Highway 1, Santa Barbara Co., California, , USA (SBBG). N. variegata En- 17802 ). From the inside of the tracheid, one can see, closest to gelm. ex Durand: Cultivated on W side of West Road, Adams, Berkshire Co., the lumen, major thick strands of coarse fi brils that run in an Massachusetts. Hellquist 17084 (MASS). axial direction in the tracheid ( Fig. 7 ). These fi brils have thick- Nuphar section Nuphar : N. japonica DC.— Cultivated on W side of West Road, Adams, Berkshire Co., Massachusetts, USA. Hellquist 17085 (MASS). ness on a different order of magnitude from those seen in pri- N. microphylla (Pers.) Fernald — In Rice Lac (Lac-du-Bois), south side of Route mary cell walls with transmission electron microscopy. Exterior 313, east of Lac du Bonnet, 50° 17 ′ N, 95° 41 ′ W, Manitoba, Canada. 28 July to the major strands is a network is a three-dimensional network 1996, C. B. Hellquist & J. Wiersema 16159 (GH). N. pumila (Timm) DC.— of strands, penetrated by holes that apparently extend through Small pond on Mount Sinjuha, Siberia, Russia. August 1963, Crow et al 93 – to an adjacent tracheid. These features are confi rmed when one 369 (NHA). views end-wall pit membranes from the outer surface of the Hybrid: N. × rubrodisca Morong (= N. microphylla × N. variegata ). Culti- vated on W side of West Road, Adams, Berkshire Co., Massachusetts, USA tracheid (Fig. 8). In this pit membrane, the dense spongiform from material collected in Vermont, USA. Hellquist 17086 (MASS). nature of the pit membrane is evident. Note that the primary Nymphaea: N. cv ‘ Marliac Carnea ’ — Cultivated at Lotusland Foundation, wall between pit membranes ( Fig. 8 , top and bottom), portions Montecito, Santa Barbara Co., California, USA. ( Living collections accession underlying secondary wall strips and therefore between pit number 1992 – 417 ). membranes, are homogeneous and do not have any meshwork. All materials were available in fresh condition from cultivated specimens. The section shown in Fig. 9 illustrates the outer surface of a Portions of roots and stems were preserved in 50% aqueous ethanol. Sections were cut by hand with a single-edged razor blade, washed with three changes of tracheid, shaved away by sectioning more at right than at left. distilled water, placed between clean glass slides with pressure suffi cient to Not surprisingly, the longitudinal strands are exposed where produce fl atness, and dried on a warming table at 50° C. Portions of the dried more has been shaved away, whereas pit membrane portions at sections that appeared to contain xylem in longisection were placed onto elec- left have (at low magnifi cation) a meshwork much like that troconductive pads on aluminum stubs, sputter-coated with gold, and examined seen in the membrane in Fig. 8 . with a Hitachi S2600N SEM. The sections averaged between 1 – 2 mm thick- A view of the inner tracheid surface of a stem of N. advena ness, and thus were suffi ciently thick so that handling was unlikely to result in excessive breakage of xylem cells. Such breakage can occur in thin sections, subsp. ozarkana (Fig. 10) illustrates several pits with longitudi- such as paraffi n sections when they are prepared for study with light micros- nal strands nearer to the lumen, with a dense perforated mesh- copy. Hand sections made with a single-edged razor blade show views of the work (darker gray) farther back toward the outside of the cell. inside surfaces of tracheids as well outside surfaces; various layers of a primary Narrow pits with slanting longitudinal strands probably are re- wall can be seen in portions of outer surfaces that are shaved away. The meth- lated to the sinuous contour of some tracheids in stems, as op- ods employed here are essentially the same as those in recent studies of ferns posed to the very straight ones seen in roots. This same condition (Carlquist and Schneider, 2007) and are similar to those of Sano (2005). To observe whether pit membranes of Nuphar stems were lignifi ed, we examined is illustrated for the stem tracheid pit membranes shown for N. sections cut by hand and stained with safranin-fast green by means of light advena subsp. advena, Hellquist 16722 (Fig. 11). The longitu- microscopy. dinal strands in Fig. 11 are notably coarse and may group to- The term stem here is considered equivalent to rhizome. All observations gether. Other pit portions of tracheids from the same collection refer to metaxylem tracheids unless otherwise specifi ed. The term fi bril is used ( Fig. 12 ) have a meshworklike structure, but with fewer longi- for strands of wall material shown in pit membranes because these structures tudinal strands. are much thicker in diameter than the microfi bils that have been demonstrated in cell walls by means of transmission electron microscopy. The term porosity Diminution of longitudinal strands in a pit membrane is and the adjective porose are used to describe holes in the primary walls (pit shown in stem tracheids of N. variegata ( Fig. 13 ). This pit membranes) of tracheid end walls. membrane is probably a lateral wall pit. Correlatively with less- January 2009] Carlquist et al. — Early angiosperm xylem 209

Figs. 1– 6. SEM micrographs of pits of tracheary elements of roots. 1 . Nuphar advena subsp. ozarkana, porose end wall pits, viewed from inside of tracheid. 2. N. advena subsp. ozarkana, nonporose lateral wall pits, viewed from inside of tracheid. 3. N. polysepala, end wall of tracheid, viewed from outer surface of tracheid. 4 . N. japonica, end wall pits, viewed from outside of tracheid. 5. Nymphaea ‘ Marliac carnea,’ end wall pits, viewed from outside of tracheid. 6 . Nymphaea ‘ Marliac carnea,’ lateral wall tracheid to parenchyma pits; wall of facing cell has been removed by sectioning from pits at left. Bars = 5 µ m. 210 American Journal of Botany [Vol. 96

Figs. 7 – 12. Views of pits from stem tracheids of Nuphar advena. Figs. 7– 9. N. advena subsp. advena, Hellquist 17082. 7. Portions of pits from end wall, seen from inside of tracheid; coarse longitudinal strands traverse the pit aperture. 8. Portion of pit from end wall, seen from outside of tracheid; coarse fi brils disposed in spongiform manner. 9. Pits from end wall, seen from outside of tracheid; spongiform portion of pit membrane mostly sectioned away (fragments, at left), showing some longitudinal strands. 10 . N. advena subsp. ozarkana , narrow pits as seen from inside of tracheid. Figs. 11– 12. N. advena subsp. advena, views of pits from inside tracheids. 11. View showing thick nature and grouping of coarse fi brils. 12. Area lacking longitudinally oriented strands (probably pit facing parenchyma cell), showing random nature of fi bril meshwork with some porosities at left. Bars: Figs. 7– 11, 5 µ m; Fig. 12, 2 µ m. January 2009] Carlquist et al. — Early angiosperm xylem 211 ening of longitudinal strands, there are no porosities evident in dominate, but they do not occur at a level different from that this membrane, which thus appears to be a pit membrane inter- of the randomly oriented fi brils with which they are inter- connecting a tracheid with a parenchyma cell. An end wall tra- mixed. Porosities are clearly evident in the meshwork. A cheid pit of N. variegata ( Fig. 14 ), seen from the inside of the view of the outside of a Nymphaea stem tracheid end wall tracheid, has an abundance of longitudinal strands, which merge (Fig. 25) shows a reticulate network. The reticulum probably with the porose, denser portion (darker gray) farther away from represents only a portion of a pit membrane between two the cell lumen. The portion shown in Fig. 14 illustrates that the adjacent tracheids. A denser aggregation of fi brils, as in Fig. longitudinal strands are not in a layer separate from the back- 24, would be expected if the entirety of a pit membrane were ground meshwork, but merge with it. present. A wall layer has evidently been sectioned away in Similar features are illustrated by the stem tracheids of N. this preparation. polysepala. The narrow pits in Fig. 15 dramatically illustrate the three-dimensional nature of the coarse fi brils in the pit; ren- dering these photographically is diffi cult, in fact, because distal DISCUSSION portions of the meshwork are recessed so far behind the longi- tudinal strands. Longitudinal strands and porosities can even be The root tracheids of Nuphar and Nymphaea are much like seen in protoxylem tracheids of N. polysepala ( Fig. 16 ). The those fi gured in earlier studies ( Schneider et al., 1995 ) and re- presence of the strands in protoxylem might not be expected semble closely the vessel-like tracheids fi gured for Brasenia of because of the elongation one often sees in protoxylem trache- the Cabombaceae ( Schneider and Carlquist, 1996a ). On end ids. Such elongation, which often fractures the primary wall walls of the root tracheids of Nuphar and Nymphaea , nontex- into fragments, may not occur because the stems of Nuphar tured pit membranes containing prominent porosities are present. polysepala are very thick and elongate so little during growth. The relatively large size of the porosities suggests that root xy- A section of the outer surface of a tracheid ( Fig. 17 ) reveals the lem in Cabombaceae and Nymphaeaceae has attained a close result of sectioning. The three-dimensionality of the pit mem- approach to vessel element characteristics. In woody, vessel- branes is evident because the meshwork strands have been cut bearing angiosperms, presence of imperforate tracheary ele- by the sectioning process; the cut ends point upward. Note that ments allows use of criteria other than end wall characteristics: no meshwork is present on the primary wall portions that un- vessel elements are wider and longer than the tracheid they ac- derlie strips of secondary wall (one portion of secondary wall company and often have different lateral wall pitting. Nympha- left, at upper right). A portion of a pit membrane at a higher eaceae, like monocotyledons, do not have division of labor into magnifi cation ( Fig. 18 ) shows a clearly spongiform structure. vessel elements plus imperforate tracheary elements, and thus The sections, made by hand, and stained with safranin and fast only the criterion of end-wall structure is available when cate- green, did not demonstrate to us any lignifi cation in the pit gorizing these elements as tracheids or vessel elements. The membranes of N. polycephala stem tracheids. However, our highly porose pit membranes in these two families indicate that preparations were not ideal, and our observations of this mate- absence of a pit membrane over most of a perforation is a crite- rial are tentative. rion of vesselhood very nearly achieved. However, end-wall pit Stem tracheids in Nuphar section Nuphar have a wide range membranes of metaxylem root tracheids in Cabombaceae and of structure. The end-wall pit membranes of N. japonica ( Fig. 19 ) Nymphaeaceae seem not to be swept away by the conductive show patchiness in distribution of porosities. These pit mem- stream and/or hydrolyzed away. The removal of most or all of branes also show a relative paucity of longitudinal fi bril strands. pit membranes in end walls of tracheary elements by these nat- Pit membranes of N. japonica when seen from the outer surface ural processes would seem to be a criterion by which to desig- of a tracheid (Fig. 20) have some strands of moderate promi- nate vessel elements. However, implementation of that criterion nence on a compact meshwork background in which porosities requires use of SEM and thus may be resisted by those in search can be seen. In N. microphylla , pit membranes on end walls of of easy defi nitions. tracheids as seen from the inside of the cell have inconspicuous The stem tracheary elements of Nuphar have end-wall pit longitudinal strands superimposed on a fl at meshwork, much as membranes that do contain porosities, but the porosities are in N. japonica . The lateral walls of N. microphylla stem trache- not large enough or abundant enough that one would be ids have a reticulate structure ( Fig. 21 ) facing the solid, untex- tempted to call these cells vessel elements. The existence of tured wall of an adjacent parenchyma cell. such porosities in pit membranes of stem tracheid end walls Stem tracheids of N. pumila have prominent longitudinal (but not lateral walls) does suggest a rudimentary step to- strands on the inner surfaces of pit membranes of end walls ward acquisition of vessel elements. However, retention of ( Fig. 22 ). When part of a pit membrane is sliced away by sec- tracheids as a means of restricting spread of embolisms tioning ( Fig. 23 ), the nature of the coarse fi brils is revealed. At within a xylary system is a strategy of some groups (most the top of Fig. 23 , the secondary wall is intact; the secondary ferns, for example), and lack of vessels should not be con- wall at the bottom of the pit has been removed by sectioning. sidered a failure to evolve conductively effi cient conductive Related to this, many of the fi brils are cut in the lower half of cells (Carlquist, 1975). the photograph. Although there may have been some displace- The pit membranes of end walls in stem tracheids of Nuphar ment of the fi brils as a result of the sectioning, the three-dimen- and Nymphaea are remarkable for their three-dimensional net- sional nature of the meshwork is clearly evident throughout the works of coarse fi brils. Similar appearances have been reported photograph. Pores that interconnect the adjacent tracheids are in stem tracheids of Cabomba (Schneider and Carlquist, 1996b). evident in the pit membrane. The fi brils closer to the interior of a tracheid are sparse, thick, The stem tracheids of Nymphaea have essentially the same and longitudinally oriented, whereas those at the outside of the microstructural features as in Nuphar. Examination of an pit membrane are denser and randomly oriented. The longitudi- end wall pit membrane as seen from the inside of a tracheid nally oriented fi brils do not seem to be homologous to the lon- ( Fig. 24 ) shows that longitudinally oriented fi brils are pre- gitudinal strands that occur as pit membrane remnants in vessel 212 American Journal of Botany [Vol. 96

Figs. 13– 18. Pits from stem tracheids of Nuphar . Figs. 13 – 14. N. variegata, pits seen from inside of tracheids. 13. Tracheid to parenchyma pits; fi bril- lar strands are slender and sparse. 14. Tracheid end wall pits; longitudinally oriented fi brillar strands predominate, fading into more random fi brillar mesh- work. Figs. 15 – 18. N. polysepala. 15. View from inside of tracheid, showing marked three-dimensional disposition of longitudinally oriented strands in end wall. 16 . Portion of inner surface of protoxylem tracheid, with long axis oriented horizontally, showing strands and porosities on wall surface. 17. View from outside of tracheid, showing tracheid-to-tracheid pits; secondary wall mostly sectioned away (one fragment, at upper right), revealing three-dimen- sional nature of pit membrane (ends of reticules broken by sectioning pale gray). 18. Pit membranes of tracheid-to-tracheid pits at higher magnifi cation, showing spongiform appearance. Scales: Figs. 13, 14, 5 µ m; Figs. 15 – 18, 2 µ m. January 2009] Carlquist et al. — Early angiosperm xylem 213

Figs. 19– 25. Pits from stem tracheids of Nuphar (19 – 23) and Nymphaea (24– 25). Figs. 19, 20. Nuphar japonica . 19. View from inside of tracheid, tracheid-to-tracheid pits, showing relative sparsity of coarse fi bril strands. 20. View from outside of tracheid; some coarser fi brils appear on surface of otherwise uniform (but porose) pit membranes. 21. N. microphylla: tracheid to parenchyma interface, wall shaved away above, revealing porose nature of membrane on tracheid side. Figs. 22, 23. N. pumila, views of inner surfaces of end wall pits. 22. Appearance typical of end wall pits, with longitudinally oriented strands superimposed on porose meshwork pit membrane. 23. Secondary wall and portion of pit shaved away (lower half of photograph), allowing broken fi bril ends to separate from three-dimensional meshwork of pit membrane. Figs. 24– 25. Nymphaea ‘ Marliac carnea,’ views of tracheid end walls. 24. Coarse fi brils form porose meshwork on a pit membrane seen from inside of tracheid. 25. View from outer surface of tracheid: highly porose reticulate appearance may refl ect removal of wall layer by sectioning process. Scales: Figs. 19 – 24, 2 µm; Fig. 25, 5 µ m. 214 American Journal of Botany [Vol. 96

Table 1. Comparison of tracheary element wall microstructures in Nymphaeaceae and woody angiosperms.

Nymphaeaceae: Stem tracheids inner surface remnants of perforation plates Woody angiosperms: Pit membrane microstructure

In primary xylem tracheids In secondary xylem vessel elements On all tracheid walls In perforation plates of vessels only Composed of coarse fi brils Composed of thin fi brils Two-layered wall in pits: reticulate outer plus axial strands inside One-layered wall Reticulate layer spongiform with pores Laminar with pores or, if fi brils sparser, variously network-like Axially oriented strands superimposed on the reticulate layer Axially oriented strands remnants of the reticulate layer after hydrolysis of the primary wall

elements of Illicium (Carlquist and Schneider, 2002 ), various Carlquist , S. 1983 . Wood anatomy of Bubbia (Winteraceae), with Chloranthaceae ( Carlquist, 1987 , 1990 , 1992a , b ) or other genera comments on origin of vessels in dicotyledons. American Journal of (Carlquist, 1992c). The differences are summarized in Table 1 . Botany 70 : 578 – 590 . The S-, G-, and P-type pits of tracheids in earlier vascular plants Carlquist , S. 1987 . Presence of vessels in Sarcandra (Chloranthaceae): ( Hartman and Banks, 1980 ; Kenrick and Crane, 1997 ; Fried- Comments on vessel origins in angiosperms. American Journal of Botany 74 : 1765 – 1771 . man and Cook, 2000) clearly differ from what one sees in Carlquist , S. 1988 . Comparative wood anatomy, 1st ed. Springer-Verlag, Nuphar stems. Berlin, Germany. The distinction in structure between the stem tracheids of Carlquist , S. 1990 . Wood anatomy of Ascarina (Chloranthaceae) and the Nuphar and Nymphaea and those of the root is puzzling. The tracheid — Vessel element transition. Aliso 12 : 667 – 684 . roots of Nuphar are adventitious, so that the conductive charac- Carlquist , S. 1992a . Wood anatomy and stem of Chloranthus ; summary teristics of the two organs, although coordinated, might show of wood anatomy of Chloranthaceae, with comments on relationships, some differences. However, that consideration would apply to vessellessness, and the origin of monocotyledons. IAWA Bulletin 13 : monocotyledons, in which differences of degree in pit mem- 3 – 16 . brane remnant presence may be found with respect to organog- Carlquist , S. 1992b . Wood anatomy of Hedyosmum (Chloranthaceae) raphy ( Carlquist and Schneider, 2006 ). No difference in and the tracheid — Vessel element transition. Aliso 13 : 447 – 462 . Carlquist , S. 1992c . Pit membrane remnants in perforation plates of microstructure of tracheary elements with respect to organogra- primitive dicotyledons and their signifi cance. American Journal of phy is as yet evident in monocotyledons, however, even in Botany 79 : 660 – 672 . aquatic genera such as Acorus (Carlquist and Schneider, 1997). Carlquist , S. , and E. L. Schneider . 1997 . Origins and nature of ves- The root tracheids of Nymphaeaceae have microstructure typical sels in monocotyledons. 1. Acorus. International Journal of of that found in monocotyledons as well as in dicotyledons. The Sciences 158 : 51 – 56 . stem tracheids of Nuphar and Nymphaea show a change in that Carlquist , S. , and E. L. Schneider . 2001 . Vegetative anatomy of the basic pattern, an elaboration. At present, that change can be con- New Caledonian endemic Amborella : New data; relationships with sidered an autapomorphy developed within Nymphaeaceae. The the Illiciales and implications for vessel origin and defi nition. Pacifi c systematic distribution of the stem tracheid peculiarities within Science 55 : 305 – 312 . Nymphaeaceae (and variations of them) is not yet available, and Carlquist , S. , and E. L. Schneider . 2002 . Vessels of Illicium (Illiciaceae): Range of pit membrane presence in perforations and other details. we hope that further studies in progress will clarify the occur- International Journal of Plant Sciences 163 : 755 – 768 . rence of the features we have described. Also needed are defi ni- Carlquist , S. , and E. L. Schneider . 2004 . Pit membrane remnants in tive studies of whether any lignifi cation is present in the complex perforation plates of Hydrangeales with comments on pit membrane pit membranes of Nuphar and Nymphaea stem tracheids. remnant occurrence, physiological signifi cance, and phylogenetic dis- The physiological signifi cance of the stem tracheids of tribution in dicotyledons. Botanical Journal of the Linnean Society Nuphar and Nymphaea may clarify as our knowledge of the sys- 146 : 41 – 51 . tematic distribution of the Nuphar-type stem tracheids, or vari- Carlquist , S. , and E. L. Schneider . 2006 . Origins and nature of vessels ants of them, develops. At the very least, the occurrence of the in monocotyledons. 8. Orchidaceae. American Journal of Botany 93 : patterns described here for Nuphar shows that diversity of trac- 963 – 971 . heary elements in basal angiosperms is more diverse than previ- Carlquist , S. , and E. L. Schneider . 2007 . Tracheary elements in ferns: ously thought and that studies of microstructure with SEM are New techniques, observations, and concepts. American Fern Journal 97 : 199 – 211. highly desirable to demonstrate the variations present. The mi- Friedman , W. E. , and M. E. Cook . 2000 . 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