Wood and Bark Anatomy of Salvadoraceae: Ecology, Relationships, Histology of Interxylary Phloem’ Sherwin Carlquist2 Santa Barbara Botanic Garden

Journal of the Torrey Botanical Society 129(1), 2002. pp. 10—20

Wood and bark anatomy of Salvadoraceae: ecology, relationships, histology of interxylary phloem’ Sherwin Carlquist2 Santa Barbara Botanic Garden. 1212 Mission Canyon Road. Santa Barbara, CA 93105

CARLQuIsr, S. (Santa Barbara Botanic Garden. 1212 Mission Canyon Road, Santa Barbara, CA 93105). Wood and bark anatomy of Salvadoraceae: relationships, ecology, histology of interxylary phloem. J. Torrey Bot. Soc. 129:10—20. 2002.—Quantitative and qualitative data are presented for Stem wood of one species each of Azinza. Dobera. and Salvadora and for root wood of Salvadora. The liquid-preserved material of Salvadora permitted analysis of interxylary phloem: abaxial to most strands a “residual meristem” adds sieve-tube elements and companion cells to each strand, crushing earlier-formed phloem. Current opinion that Salvadoraceae are a sister family to Bataceae is supported by wood anatomy: they share such features as bands of mostly nonsubdivided axial parenchyma cells, wide multiseriate rays, and and storied structure. Nonbordered perforation plates. found also in other Capparales, are newly reported for Salvadoraceae. Koeberliniaceae have tracheids, Bataceae fiber tracheids, thought more primitive than the libriform fibers of Salvadoraceae. Wood of -Azima is moderately mesomorphic, perhaps because it grows near beaches where saline or brackish water is available to roots: Dohera and Salvadora have highly xeromorphic wood. The terms “foraminate included phloeni’ and ‘concentric in cluded phloem” are misnomers and must be rejected. Key words: Azima. Bataceae, cambial variants, Capparales, Dohera. ecological wood anatomy, glucosinolate bearing plants, “included phloem.” residual meristem, Salvadoro, successive cambia

In phylognetic treatments prior to Dahlgren’s vors this placement is striking. Wood data are (1975) inclusion of all glucosinolate-bearing not needed to confirm the placement of Salva families (excepting Drypetes of the Putranjiva doraceae in Capparales: that treatment is already ceae) in Capparales, Salavadoraceae were placed well supported. Rather, the ways in which wood elsewhere. Cronquist (1981) and Takhtajan data reflect this systematic placement and are (1987) situated Salvadoraceae in Celastrales consistent with it are explored here. The present near Stackhousiaceae. Also current was the idea paper is one in a series surveyinging wood anat that Salvadoraceae belong to Oleales (Metcalfe omy of Capparales; a review of wood of Cap and Chalk 1950, Melchior 1964, Heywood paraceae will conclude the series. 1978, Goldberg 1986). The order Capparales Wood data are useful for demonstrating ge was defined as including little more than Bras neric distinctions within the family, however. sicaceae, Capparaceae, and Resedaceae. DahI Melchior (1964) groups Aziina with Dohera on gren (1975) exapnded the order, by including all the basis of choripetaly; Salvadoru is sympet families (with the exception of Dryperes of Pu alous. Azima (four species) ranges from South tranjivaceae) known to have glucosinolates Africa and Madagasgar to Indomalesia and (mustard oils). The studies by Rodman (199 Ia, southeast Asia. Dohera (three to five species) 1991b) clarified familial relationships. Molecu occurs in eastern Africa, whereas Salvadora lar studies offered a greater degree of precision (five species) occurs in North Africa, the Middle (Karol et al. 1999; Rodman et al. 1993, 1996, East, and northwest India (Melchior 1964, Hey- 1998). In these recent schemes, whether on the wood 1978). basis of DNA evidence or morphological data The majority of localities for the family are (see comparison by Karol et al. 1999), Salva rather dry, even saline, sometimes with brackish doraceae are paired with Bataceae, and Koeber or saline subsurface water availability (Dale and liniaceae are basal to those two families in this Greenway 1961; Fahn et al. 1986). The wood dade. The uniformity with which evidence fa anatomy of Salvadoraceae offers insights into xeromorphy. The main sources for wood data on Salvador Received for publication April 27, 2001, and in re vised form September 10, 2001. aceae include Metcalfe and Chalk (1950). den Dr. Regis B. Miller of the Forest Products Labo Outer and van Veenendal (1981) and Fahn et al. ratory, Madison, WI, kindly furnished one of the sam (1986). Data on wood of the family can also he ples used in this study. found in papers listed by Gregory (1994). 2 Address for editorial correspondence: Sherwin Ca rlquist. 4539 Via Huerto. Santa Barbara, CA 931 10- The occunence of interxylary phloem in Sal 2323 vadoraceae is of especial interest. Tnterxylary

I0 2002j CARLQU)ST: SALVADORACI3AE WOOD AND BARK phloem (sensu Cariquist 198$, 2001) is not to slides, sputter coated, and examined with a be confused With the phiOCiTi produCCd by suc Bausch and Lomb Nanolab scanning electron cessive cambia (e.g., IVlenisperinaceae, Cary— microscope (SEM). Macerations were prepared ophyllales, Onetales, etc.). In the latter, phloem with Jeffrey’s Fluid and stained in safranin. Ves is produced outwardly from each of the succes sel lumen diameter is computed here instead of sive eambia and therefore lies between conjunc the traditional outside diameter of vessels he-- tive tissue (which is not secondary xylem) and cause lumen diameter has hydrological signifi the secondary xylem produced by each of the cance (see l-largrave et al. 1992), whereas out camhia. Thus, the phloem is neither ‘‘intcrxy— side vessel diameter has no known physiological lary’’ nor “included’’ in dicotyledons with suc significance. The mean number of vessels per cessive cambia. Interxylary phloem, on the con group is based on the formula that a solitary trary, is produced centripetally by a single cam- vessel = 1, a pair of vessels in contact = 2. etc. hium and thus occurs as strands intercalated in and a series of such figutres is averaged. All a secondary xylem background. Details of his means are derived from averaging 25 measure tology of interxylary phloem and hark anatomy ments. The terms vasicentric tracheid and ray- are illustrated exceptionally well here by SaL adjacent parenchyma conform to the definitions cadora persica because the specimen was pre of Carlquist (1988, 2001). The term “residual served in liquid and sectioned on a rotary mi— meristem” is newly introduced here. Other crotome. terms conform to the IAWA CoiTmittee on No menclature (1964) usages. Materials and Methods. The stem and root portions of S’ali’adora persica L. were obtained Wood Data. A:inia teiracantha (Fig. l—-5). from a living plant cultivated in the University Growth rings inconspicuous. earlywood possibly of California. Santa Barbara, greenhouses (cul definable by larger vessels in wider hands of axial tivation number $3160). Stem and root material parenchyma (Fig. I). Vessels solitary or grouped of this specimen were fixed in 50% aqueous eth (Fig. 1), mean number of vessel per group, 4.2, anol. Wood of this species consists of delicate Mean vessel lumen diametei 25 p.m. Mean num cells, notably the interxy lary phloem, intermixed ber of vessels per mm2. 45. Mean vessel element with zones of thick-walled fibers and vessels, a length, 229 p.m. Perforation plates simple. non- difficult texture for sliding microtome section bordered. Lateral wall pitting of vessels oval to ing. The method of Carlquist (1982) proved suc circular, alternate, nonvestured, about 4 p.m in di cessful for this material. The specimen of Azirna ameter. Grooves interconnecting pit apertures etracaniha Lam. was collected on consolidated (“coalescent pit apertures” of some authors) pre sand dunes, 3 km E of Wilderness, Cape Prov sent on vessel walls. Mean vessel wall thickness, ince, South Africa, October 3, 1973 (Carlquist 5 p.m. A few vasicentric tracheids observed in 4733, RSA). The stems of Azima tetracantha macerations. the i mperforate tracheary elements were preserved by drying. The dried sample of otherwise all lihriform fibers (Fig. 1, 2) on which Dobera glabra (Forsk.) .luss. ex Poiret studied only simple pits were observed. Mean libriform here (SJRw 26260. Guiseppe Paoli in 1913, So fiber length, 946 p.m. Mean libriform fiber wall maliland) was provided by Forest Products Lab thickness, 6 p.m. Axial parenchyma variously dis oratory, Madison. Wisconsin. Dobera niacalusot tributed (Fig. I). most conspicuously as tangential Mattei has been referred to D. glabra (as D. c/a hands, most of which contain vessels, but some bra var. ,naca/uroi (Mattei) Fiori, and 1). tar— of which do not. Some diffuse, ray adjacent, and anihok/es (Warb.) Warb. cx Harms has been vasicentrie parenchyma also present. Axial paren considered conspecific with D. glabra (Dale and chyma commonly in strands of two cells, but also Greenway 1961). Only one of these three taxa frequently not subdivided (Fig. 3, 4; only a few has been studied here, because observation of nonsubdivided parenchyma cells at lower right in samples of these taxa supplied by Forest Prod Fig. 4). Rays mostly multiseriate (Fig. 2, 3). Rays ucts Laboratory revealed no appreciable differ composed of square to procumbent cells (Fig. 5). ences among them. Portions of wood ot A. te Mean height of multiseriate rays. I ,200 p.m. Rays tlv(clfllha and D. glabra were boiled in water, notably wide (Fig. 2. 3); mean width of multis stored in 50% aqueous ethanol. and sectioned on eriate rays at widest point, 8.1 cells. Mean height a sliding microome Some sections were stained of uniseriate rays, 117 p.m. Crystals present in with a safranin—fast green combination, others axial parenchyma (Fig. 4) and rays (Fig. 5). were left unstained, dried between clean glass Starch present in axial parenchyma and rays. En— 12 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 129

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FIGs. 1—5. Wood sections of Azimctj teti-acantha. Fig. 1. Transection, showing wide vessels, below, and various axial parenchyma distributions. Fig. 2. Tangential section through zone of libriform fibers: rays are mostly multiseriate. Fig. 3. Tangential section through an axial parenchynsa band, showing storying. Fig. 4. Portion of tangential section to show crystals (arrows) in axial parenchyrna. Fig. 5. Radial section of ray to show rhomboidal crystals (arrows) and cell shape. Magnification scale for Figs. 1—3 above Fig. 1 (divisions = 10 p.ns). Scale for Figs. 4. 5, above Fig. 4 (divisions = 10 p.m). 20021 CARLQUIST: SALVADORACEAE WOOD AND BARK 13 terxylary phloem present in bands of axial paren sel lumen diameter, 12 p.m. Mean vessel element chyma only near the periphery of the stem (2 cm length, 131 p.m. Perforation plates simple, non- in diameter); earlier-formed secondary xylem de bordered. Lateral walls of vessels with alternate void of interxylary phloem (Fig. 1); Axial paren oval to circular nonbordered pits about 4 p.m in chyma and vessels storied (Fig. 2—4). diameter. Grooves interconnecting pit apertures, Dobera glabra (Fig. 6—9). Growth rings not especially conspicuous in more mature wood. readily perceptible, earlywood possibly distin Mean vessel wall thickness, 5 p.m. A few vasi guishable by wider vessel diameter. Vessels sol centric tracheids seen in macerations (mean itary or in small groups (Fig. 6), mean number length, 196 p.m); all other imperforate tracheary of vessels per group, 2.0. Mean vessel lumen elements are libriform fibers with simple pits. diameter, 15 p.m. Mean number of vessels per Mean libriform fiber length, 4.2 p.m. Mean li mm2, 179. Mean vessel element length, 134 p.m. briform fiber wall thickness, 4.2 p.m (Fig. 12, Perforation plates simple, mostly nonbordered. 13, top). Axial parenchyma in tangential bands Lateral wall pitting consisting of alternate oval that traverse rays (Fig. 10), often aliform in to circular nonvestured pits, about 2 p.m in di shape as seen in transection. Axial parenchyma ameter; grooves interconnect pit apertures on the bands may include some vessels or no vessels, inner face of the vessel wall. No vasicentric tra or strands of interxylary phloem (Fig. 10—13). cheids observed, but very narrow vessels com Axial parenchyma mostly with nonlignified mon. All imperforate tracheary elements are li walls, but some areas of axial parenchyma with briform fibers with simple pits. Mean libriform lignified walls present distal to the phloem fiber length, 665 p.m. Mean libriform fiber wall strands. Axial parenchyma mostly not subdivid thickness, 4 p.m. Axial parenchyma chiefly ed, but strands of two cells occasional (Fig. 15, banded (Fig. 6), with nonlignified walls adjacent 16). Multiseriate rays (including biseriate rays) to phloem strands but with lignified walls else about as common as uniseriate rays (Fig. 15, where; bands often spanning the tangential dis 16). Rays composed of procumbent, square, and tance between rays (Fig. 6). Diffuse, ray-adja upright cells in about equal numbers. Ray cell cent, and vasicentric axial parenchyma also pre walls lignified except where rays traverse phlo sent (Fig. 6). Axial parenchyma in strands of one em-bearing axial parenchyma, ray cell wall to two cells; some axial parenchyma cells sub thickness about 0.6 p.m. Mean height of multis divided by a thin septum. Multiseriate rays much eriate rays, 244 p.m. Mean width of multiseriate more common than uniseriate rays (Fig. 7). Ray rays at widest point, 2.4 cells. Mean height of cells predominantly procumbent, some square as uniseriate rays, 91 p.m. Rhomboidal crystals oc seen in radial section. Mean height of multiser casional in ray cells; smaller rhomboidal crystals iate rays, 183 p.m. Mean width of multiseriate occasional in axial parenchyma cells. Starch rays at widest point, 3.8 cells. Mean height of dense in ray cells (Fig. 17), some starch grains uniseriate rays, 74 p.m. Ray cell walls lignified, paired but most single; starch grains smaller but except for a few ray cells adjacent to the bands common in axial parenchyma. Interxylary phlo of nonlignified axial parenchyma that contain em strands one to several per band of axial pa phloem. Crystals present in ray cells, either cu renchyma (Fig. 11, 12), strands various in size boidal to rhomboidal (Fig. 8) or polyhedral (Fig. (Fig. 12, 13). Residual meristems add moderate 9). Virtually all ray cells contain a solitary crys amounts of new phloem, resulting in crushing of tal in mature wood. Crystals smaller and fewer older phloem (e.g., Fig. 12, bottom of transec in axial parenchyma. Starch grains present in tion of phloem strand). Vessels, axial parenchy rays and in axial parenchyma cells. Interxylary ma, sieve-tube elements, and some uniseriate phloem present in the tangential bands of axial and biseriate rays storied (Fig. 15, 16). parenchyma (Fig. 6). Axial parenchyma and ves Salvadora persica root (Fig. 14). Features as sels storied. in stem except for data below. Vessels in exten Salvadora persica stem (Fig. 10—13, 15—17). sive groups (Fig. 14), mean number of vessels Growth rings not readily perceptible, but with a per group, 26.8. Mean vessel lumen diameter, 9 tendency for wider vessels be in concentric p.m. Mean number of vessels per mm2, 384. rings. Vessels usually in groups, some groups Mean vessel element length, 160 p.m. Mean fi extensive (Fig. 11). Mean number of vessels per briform vessel element length, 208 p.m. Lateral group, 5.19. Mean number of vessels per mm2, wall pits about 3 p.m in diameter. Mean vessel 251. Narrow vessels are often of about the same wall thickness, 4.8 p.m. Vasicentric tracheids diameter as libriform fibers (Fig. 11). Mean yes- moderately frequent, mean length, 205 p.m. 14 JOURNAl. OF THE TORREY BOTANICAL SOCIETY [VoL. 129

FIGS. 6—9. Wood sections of Dobera glabra. Fig. 6. Transection of earlier-lormed wood: interxylary pliloem has collapsed due to drying of sample. Fig. 7. Tangential section; rays are all multiseriate. short and wide. Fig. 8—9. SEM photographs of ray cells containing crystals from tangential section. Fig. 8. Cuboidal crystals. Fig. 9. Polygonal crystals. Figs. 6, 7, scale above Fig. I. Figs. 8, 9. scales at lower left = 10 in, 20021 CARLQUIST: SALVADORACEAE WOOD AND BARK 15

Fios. I 0— 13. Transections of stem secondary xylem of SaIiadora persica. Fig. 10. Axial parenchyma present as vide aliform ellipses. Fig. II. Axial parenchyma contains interxylary phloem strands of various sizes (iden lifted by means of enlarged versions in Figs 12 and 13). Fig. 12. 13. Strands of interxylary phloem, libriform lihers at tops of photographs: arrows indicate residual meristem ahaxial to strand. Fig. 12. Larger strand; crushed sieve tube elements and companion cells below in strand. Fig. 13. Intermediate-sized strand (lower left) and two smaller strands (near center, lefti(l right) of sieve tubes and companion cells. Figs. 10, scale above Fig. 1; Fig. II, scale above Fig. 4: Figs. 12. 13. scale above Fig. 12 (divisions = 10 pm). ______

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Fins. 14—17. Wood sections of Salvadora persica. Fig. 14. Transection from root; axial parenchyma area is large, groups of vessels (some very small, about the same diameter as fibers) are quite extensive. Figs. 15—17. Tangential sections from stem. Fig. 15. Section through axial parenchyma zone (libriform fibers at left and right edges); rays are not wide. Fig. 16. Section to show storying of axial parenchyma and some rays; axial paren chyma mostly not slLbdivided. Fig. 17. SEM of ray cells (vertical axis oriented horizontally) to show starch grains. some of which are paired; most are single. Figs. 11, 16, scale above Fig. 4: Fig. 15. scale above Fig. 1:

Fig. 17, scale at lower left = 10 m. 2002] CARLQUIST: SALVADORACEAE WOOD AND BARK 17

Mean libriform fiber length, 494 p.m. Axial pa that includes all three families (Karol et a]. 1999; renchyma abundant (Fig. 14), transitional be Rodman et al. 1993, 1996, 1998). tween banded and pervasive. Mean multiseriate Wood and bark characters probably differen ray height, 594 p.m. Mean multiseriate ray width tiate the three genera of Salvadoraceae, although at widest point, 2.9 cells. Mean uniseriate ray the small sampling presented here can only offer height, 151 p.m. Ray cell walls nonlignified. A observations to be applied to future studies with few rhomboidal crystals observed in rays. more thorough sampling. On the basis of the species studied, vessel grouping is greatest in Bark Anatomy. In S. persica, the bark has Salvadora, intermediate in Azima, and least in thick periderm, but is devoid of sclerenchyma Dobera. Axial parenchyma is diffuse, paratra (Fig. 18). The bark interior to the periderm is cheal, ray-adjacent, and banded in Azima; band composed of parenchyma derived from expand ed, ray-adjacent, and paratracheal in Dobera; ed cortex and from secondary phloem parenchy and banded (with a tendency toward pervasive ma. In the phloem formed externally from the in roots) in Sali’adora. Solitary crystals are vascular cambium, phloem parenchyma is abun abundant in rays (but absent in axial parenchy dant, sieve-tube elements and companion cells ma) in Dobera, but present in both rays and ax somewhat less so (Fig. 19). In D. glabra, sec ial parenchyrna of Azirna and Salvadora. Met calfe ondary phloem cells are storied (Fig. 20). Scat and Chalk (1950) reported absence of in tered fibrosclereids grow instrusively though the terxylary phloem in Azinia, but Outer and Vee nendaal (1981) secondary phloem (Fig. 20). A few layers of demonstrate interxylary phloem presence tetracantha. phellem are present (Fig. 21). A band of bark in A. The present study was brachysciereids, derived from cortical cells, based on a stem of A. tetracantha about 2 cm in diameter; interxylary forms a ring around the stem (Fig. 21). There is phloem was present only in the outer some lignification of walls of parenchyma cells 1 mm of the woody cylinder. Outer of outer cortex (Fig. 21, above). and Veenendaal (1981) studied a stem 10 cm in diameter. Thus, A. tetracantha apparently has late-onset interxylary phloem. This may be Conclusions. SYsTEMATIcs. Phylogenies of a primitive character-state expression for the Capparales (Karol et al. 1999; Rodman et al. family (choripetaly in Azima is presumably less 1993, 1996, 1998) propose close a relationship specialized than incipient sympetaly in Salva between Bataceae Salvadoraceae. and The two dora). Interxylary phloem is formed very early families agree in having axial parenchyma bands in Dobera and Salvadora. The other families of composed of storied nonsubdivided cells and the dade to which Salvadoraceae belong lack cells subdivided once; rays predominantly wide interxylary phloem (Metcalfe and Chalk 1950; multiseriate, with few uniseriate rays; nonbor Gibson 1979; Carlquist 1978). dered perforation plates on vessels (Carlquist Azirna retracantha bark is distinctive in hav 1978 and unpublished data). There are only mi ing bundle cap (“pericyclic”) fibers and sclereid nor differences between wood of Bataceae and nests in phloem rays (Outer and Veenedaal Salvadoraceae, such as presence of crystals in 1981). Dobera glabra has a continuous cycle of Salvadoraceae and presence of vestigial borders cortical brachysclereids plus scattered fibroscler on imperforate tracheary elements of Bataceae eids in secondary phloem. Salvadora persica (some vestigial borders reported on imperforate lacks sclerenchyma in bark. tracheary elements of Aziina by Outer and Vee Ecology. The Mesomorphy Ratio (vessel di nedaal 1981, but not by Metcalfe and Chalk ameter times vessel element length divided by 1950). number of vessels per mm2) has proved a sen Wood features of Koeberliniaceae (Gibson sitive guide to ecology, particularly soil moisture 1979) place that family at some distance from availability, in dicotyledonous woods (Carlquist Bataceae and Salvadoraceae. For example, Koe and Hoekman 1985). The values for this ratio in berliniaceae have nonstoried woods, tracheids Salvadoraceae are: A. tetracantha, 127 p.m; D. with fully bordered pits, bordered perforation glabra, 13.5; S. persica stem, 6.3; and S. persica plates (Cariquist, unpublished data), and diffuse root, 0.5. The values for S. persica are notably or diffuse-in-aggregates axial parenchyma. low, especially considering that the plant sam These character states are unspecialized and cor pled for this species was a well-watered culti respond to the placement of Koeberliniaceae vated specimen. The values for D. glabra and S. basal to Bataceae and Salvadoraceae in the dade persica are lower than the mean for desert 18 JOURNAL OF THE TORREY BOTANiCAl SOCIETY Vol. 129

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‘A$’ I, FiGs. 18—21 Sections of bark of Salvadoraceac. Figs. 18, 19. Transections from .Salvadora Jwrxu’a stem. Fig. 18. Phellem above, secondary xylem below; hark ConSists of nonligllilied parenchyma cells. Fig. l9. Juncture between secondary xylem and secondary phloem (pointers indicate vascular cambium): most of secondary phloem is composed of parenchyma; most sieve-tube elements and companion cells are collapsed (arrow indi cates one such strand). Figs. 20, 21. Sections from Dobera glabra stem. Fig. 20. Tangential section through secondary phloem; phloem parenchyma is storied; alTows indicate libroscleieids. Fig. 21. Transection of hark: from top to bottom: phellem. outer cortex with some lignitied cells, brachysciercid hand; Collapsed strands of axial phloem; and secondary xylem. Fig. IS. scale above Fig. 1: Figs. 19—21, scale above Fig. 4. 0O2f CARLQUTST: SALVADORACEAE WOOD AND BARK 19

shrubs of southern California, 20.9 (Carlquist facts. In Salvadoraceae, a single (“normal’’) and Hoekman 1985). The value for A. tetracan cambiurn produces secondary phloem to the out tim is higher than that reported for the other two side and, to the inside, secondary xylem in genera and may relate to the habitat of this spe which there are strands of (interxylary) phloem cies: coastal dunes with underground brackish or embedded within bands of axial parenchyma. saline water steadily available (Dale and Green- Associated with the strands of phloem are some way 1961). This habitat is probably more mesic parenchyma cells, abaxial to the strands, in (if a species can utilize salty water) than the dry which tangential divisions often occur; these are and subdesert flats where Dohera and Salvadora termed “residual meristern” here (Fig. 12, 13). grow (Dale and Greenway 1961). An extensive Over time, these divisions yield additional sieve- survey of ecological wood anatomy in dicoty tube elements and companion cells toward the ledons cannot be undertaken here, but in addi inside of the stem (adaxially or centripetally), tion to comparative and floristic studies of thereby crushing older sieve-tube elements and woods that indicate mopisture availability as ba companion cells in each strand associated with sic to vessel diameter, vessel density. and vessel a residual meristem. Singh (1944) has referred element length, one can cite studies in wood to this process in Saicadora and Strychnos, but physiology. For example, the study of Hargrave his ontogenetic descriptions need critical reas et al. (1994) shows that narrow vessels (which sessmne mit. have greater density per unit transection) than The net effect of addition of sieve-tube dc wider vessels) embolize less readily than wide inents and companion cells by the residual mer— vessels under conditions of drought. isteins is that functioning phloem can be pro The data on vessels per group are revealing duced over a period of years, and thus photo in that they correspond to inverse values for the synthae conduction can occur throughout a Mesomorphy Ratio: A. tetracantha, 4.2 vessels stem. This phenomenon probably corresponds to per group; D. glabra, 2.0; S. pervica stein, 5.2; continued conductive activity in older portions S. pelsica root, 26.8. In species with libriform of the xylem where functioning phloem is main fibers or fiber tracheids (as opposed to tru tained by the residual meristems. Perhaps cor cheids), greater degrees of vessel grouping in related with the increased amount of phloem per dicate greater xeromorphy (Carlquist 1984). stem provided by interxylary phloem, the nurn Leaves are succulent in Dobera giabra (Dale her of sieve-tube elements and companion cells and Greenway 1961) and that may account for observed in secondary phloem of stems and the moderate vessel grouping in that species; tis roots of S. per.sica seemed fewer than is typical sues that confer succulence are associated with of secondary phloeni of dicotyledons at large. less xcromorphic woods (Carlquist and Hock- Phloem parenchyma in axial secondary phloem man 1985). of S. persica is. correspondingly. more abundant lniervyiarv /thloeni. Salvadoraceae are unLisu— compared to sieve-tube elements and companion al among Capparales in having interxylary phlo— cells (Fig. 19). em. This term does not equate to “foraminate lnterxylary phloem is best interpreted as an included phloem,’’ which has been used for in— autapornorphy in Salvadoraceae, since no neigh terxylary phloeni and some products of succes boring family in Capparales produces interxy sive camhia—-ontogenetically quite different phe— lary phloem. The late-onset interxylary phloem noincna. The term “included’’ is a misnomer in in A.i,,ia may represent a more primitive form the case of dicotyleclons with successive cambia of interxylary phloem formation than the earlier because conjunctive tissue in those species is ei onset interxylary phloem of Dobera and Salvo- ther formed as a background tissue (e.g., Neea dora if one interprets absence of interxylary of Nyctaginaceac) or as hands between one vas phloem as a symplesiomorphy in the Bataceae cular hand and anothei; and by definition the Koeberliniaceae-Salvadoraceae dade. More spe phloem is thus not “included” within secondary cialized xylary character states seem to occur in xylem in species with successive cambia. The later-formed xylem rather than the earliest division of some families (e.g., Chenopodiaceae, formed secondary xylem (Bailey 1944). Nyctaginaceae between “forarninate included phlocm” and “concentric included phloem” Literature Cited (e.g., Melcalfe and Chalk 1950) shows that the BAiler, lW. 1944. The development of vessels in an terminology is based on superficial topographic giosperms and its significance in morphological re similarity, but misrepresents the ontogenetic search. Amer. J. Bot. 31: 421—428. 20 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VoL. 129

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