Secondary growth and wood histology of Welwitschia

BotanicalJournal of the Linnean Society (1995), 118: 107-121. With 22 figures Secondary growth and wood histology of Welwitschia SHERWIN CARLQUIST F. L. S. 1 AND DAVID A. GO WANS, Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara CA 93105, USA. «Received December 1994, accepted for publication June 1995 New observations regarding secondary growth and vascular tissues in Welwitschia are based on roots (used because of the longitudinal orientation of most vascular tissues). Welwitschia has successive cambia that produce xylem and phloem. Contrary to previous reports, the cambia produce secondary phloem with phloem rays and secondary xylem that has readily definable xylem rays and axial parenchyma. Internal sculpturing (grooves associated with pit apertures) of tracheary elements is figured with SEM; observations confirm die descriptions of Bierhorst (1960). Tori and vesturing are apparently absent on pits of tracheary elements. Abundance of gelatinous secondary walls may be related to water economy to a minor extent. A feature recendy reported for Ephedra wood is newly reported for xylem rays, phloem rays and conjunctive tissue of Welwitschia: presence of minute calcium oxalate crystals lining intercellular spaces. This feature is apparendy limited to the two genera. The ontogenetic origin of vascular strands added by secondary activity can be traced to phelloderm. In this feature, Welwitschia differs from Gnetum, in which the ultimate origin of new cambia can be traced to cortical tissues. T. ISJJ5 The Linnean Society of London ADDITIONAL KEY WORDS: cambial variants - Ephedra - Gnetum - successive cambia - wood histology. i CONTENTS Introduction Material and methods Anatomical results Plan of root and nature of vascular strands. Tracheary element structure Cambial activity and origin of vascular strands. Conclusions Acknowledgement References 107 108 109 109 115 116 118 120 120 INTRODUCTION Much attention has been paid by various workers to the vegetative anatomy of Welwitschia mirabilis Hook., especially concerning vascular anatomy (for a 'Address for correspondence: 4539 Via Huerto, Santa Barbara, CA 93110, U.S.A. 107 0024-4074/95/060107+15 $08.00/0 1995 The Linnean Society of London

108 S. CARLQUIST AND D. A. GOWANS review, see Martens, 1971). However, a number of questions have not been answered, a number of interpretations have not been clarified, and even a few features remain to be discovered. Lack of suitable material does not appear to have delayed studies, because cultivated specimens of Welwitschia are numerous and have offered suitable material for decades. Those interested in wood anatomy may have bypassed Welwitschia because of its apparent lack of wood: students of wood anatomy tend to investigate woodier species of plants. In addition, the texture of Welwitschia stems and roots (hard cells scattered in a background of soft tissues) and the sinuous course of vascular strands make study of axial vasculature of Welwitschia difficult. The method of sectioning employed in this paper minimizes problems posed by the texture of the Welwitschia axis. The problem posed by the sinuosity of vascular strands has been minimized by use of root materials only; the vascular tissue of the root is predominantly longitudinally orientated. The fact that the cambia in the vascular strands of Welwitschia are apparentiy short-lived, with the consequent lack of massive accumulations of secondary xylem, has led workers to assume that Welwitschia lacks features of wood histology. Martens (1971: 255), says: "The separation and the very peculiar course of bundles of Welwitschia (hypocotyl, crown, fertile branches) do not permit a useful comparison with this plan [to the wood of Ephedra and Gnetum]". On the contrary, the products of the cambia in Welwitschia are entirely comparable with those of Ephedra and Gnetum. Although root tissue has been selected in the present study, the principles observed surely apply to products of cambia in vascular strands elsewhere in the plant body. The study of Bierhorst (1960) contributes many details on protoxylem, metaxylem and secondary xylem tracheids of Welwitschia. However, as Martens (1971) noted, Bierhorst's drawings suggest presence of tori on pits of tracheary elements whereas other workers claimed tori to be absent in Welwitschia and Gnetum (Martens, 1971), but present in Ephedra (Carlquist 1989, 1992). In fact, tori are present in pits of tracheary elements of some species of Gnetum (Carlquist & Robinson, 1995). Because the present study uses scanning electron microscopy (SEM), which was not available to Bierhorst (1960), confirmation of Bierhorst's results was one of our goals. The meristematic action that results in lateral addition of vascular strands to the axis of Welwitschia has never been clearly established. Martens (1971) asked whether phelloderm is ultimately the source of this meristematic action or whether a special meristem (in either stem or root) is present. Bertrand (1874: 13) claimed never to have found a meristem that ultimately would lead to vascular tissue. Strasburger (1872: 375), De Bary (1884: 616), and Eichler (1889: 126) thought that such a meristem must exist, but did not cite evidence for one. The relatively non-sinuous course of most vascular tissue in the root offers maximal potential clarity for demonstrating the nature of meristematic activity leading to addition of lateral vascular strands. MATERIAL AND METHODS The study of ontogenetic phenomena, which must be traced to thin walled cells, is complicated in Welwitschia by the abundance of tough fibrous structures scattered through a parenchymatous background tissue. Sectioning

WELWTTSCH1A WOOD ANATOMY 109 on a sliding microtome is not feasible because of this texture. Sectioning on a rotary microtome is only a little easier, even given softening techniques. A softening technique incorporating ethylene diamine (Carlquist, 1982) proved successful. Transverse, tangential, and radial sections were prepared from the main root of plants of four ages. The youngest and next youngest ('older seedling' in figure captions) of these were cultivated in the greenhouse at Pomona College. The largest root (Fig. 1) was from a specimen collected in Namibia by Dr Lyman Benson about 1965. He dried the plant in its form as collected; this specimen had several cm of cylindrical root below the hypocotyl, and that portion of the root was studied. This dried specimen was kept in storage at Pomona College. The next to largest root was collected in the wild in Namibia (Carlquist 8071, RSA). Roots of all except the largest plant were preserved in formalin acetic alcohol. The dried root of the largest plant was boiled in water and stored in 50% aqueous ethanol prior to sectioning. Sections for light microscopy were stained with a safranin-fast green combination. Some paraffin sections were mounted on SEM aluminum stubs using the same techniques as with glass slides. Prior to examination with an SEM, paraffin was removed from the sections with changes of xylene, and the sections were sputter-coated with gold. Macerations were prepared with Jeffrey's Fluid and stained with safranin. Terminology follows the IAWA Committee on Nomenclature (1964). The large fusiform cells with gelatinous walls in which crystals are embedded are here termed fibrosclereids. They are homologous with the astrosclereids in leaves and bracts of Welwitschia, which Rodin termed 'crystalliferous sclereids' (1958, 1963). The fibrosclereids of the root were termed 'spicular cells' by Pearson (1929) and Parameswaran & Liese (1979) but 'sclerites' by Martens (1971). The term 'conjunctive tissue' is used here for parenchyma added to the root by lateral meristematic action, in accordance with the use of the term in dicotyledons with successive cambia (see Carlquist 1988). The term 'successive cambia' is also applicable to Welwitschia, because cambial activity characterizes the vascular strands of the hypocotyl and root. The term 'secondary xylem' is used to denote xylem produced by action of cambia in each of the vascular strands. ANATOMICAL RESULTS Plan of root and nature of vascular strands Early vascularization of the root was described by Sykes (1910), Rodin (1953) and Butler, Bornman & Evert (1973). The accounts of a one-year seedling by these authors do not illustrate addition of bundles by lateral meristematic action, but the account of Bower (1881) does illustrate some of these additional vascular strands in a root. Hooker (1863) and De Bary (1884) examined older roots and reported five to eight concentric circles of vascular strands. All but the innermost cycle must be attributed to action of lateral meristematic action. One must conclude that Hooker and De Bary had access to relatively small plants, no larger than the collection Carlquist 8071, in which the root is 4 cm in diameter. In the root shown in Figure 1, more than eight circles of vascular strands are present. However, one

110 S. CARLQUIST AND D. A. GOWANS Figures 1-4 Fig. 1. Transverse section (T.S.) of root from large mature Welwitschia plant. Fig. 2, 3. T.S. root of older seedling. Fig. 2. Periphery of root, phellem at top. showing several series of bundles; phloem fibres abundant. Fig. 3. Higher magnification, to show a recently initiated vascular strand (below) and fibrosclereids (blackish) formed from phelloderm cells. Fig. 4. T.S. root from Carlquist 8071, showing thick phellem and the outermost vascular strands. Fig. 1, life size; Figs 2, 4, to scale above Fig. 2 (divisions = 10 um); Fig. 3, divisions = 10 um.

WEL WTTSCHIA WOOD ANATOMY 111 root, showing secretory canal (lower right), abundance of phloem fibres; tracheary elements as a group, then radiate. Fig. b. Portion of secondary xylem from one vascular strand, higher magnification; ray, centre, contains a larger and a smaller fibrosclereid; tracheids are about four times more numerous than vessel elements. Fig. 5, lo scale of Fig. 2; Fig. fi, to scale of Fig. 3. must stress that strict synchronicity in origin of vascular strands does not occur, and as the outlines of the root become irregular and deviate from the circular outline of younger roots, placement of strands departs increasingly from the plan of concentric circles. The means of origin of the vascular strands and their cambia from lateral meristematic activity is considered in J a later section. A figure of Sykes (1910) shows that cambial activity has occurred in the first-formed bundles of the hypocotyl. However, as mentioned earlier, no author has analysed the histology of the vascular strands in terms of what tissues are added by cambial action and how those tissues are organized. Consequently, a description is offered here. In each vascular strand, the gelatinous fibres of the secondary phloem (Figs 2, 4, 5, 15) are more abundant than are tracheary elements of the xylem. In fact, xylem portions of vascular strands are not evident in gross aspect in the root shown in Figure 1; the dark grey strands seen in Figure 1, forming the bulk of tissue in the root other than background parenchyma,

112 S. CARLQUIST AND D. A. GO WANS are massive strands of phloem fibres. However, the maturation of tracheary elements derived from cambial action precedes maturation of phloem fibres (Fig. 3, bottom; Fig. 4, bottom middle). Fibrosclereids formed from tissue derived from lateral meristematic action mature prior to vascular strands, and thus some fibrosclereids can be found external to die outermost vascular strands (Figs 3-5). Some fibrosclereids occur in conjunctive tissue. In addition, fibrosclereids can originate inside vascular strands (Fig. 6, centre). Secretory canals originate in conjunctive tissue added by lateral meristematic action (Fig. 5, lower right). Thick accumulations of phellem with dark-staining cellular contents (Figs 3-5) characterize older roots. The dim-walled nature of ray cells and axial parenchyma in secondary xylem produced by the successive cambia of Welwitschia may have led workers to confuse ray and axial parenchyma cells with conjunctive tissue. In Ephedra and Gnetum, as in the majority of dicotyledons, ray cells and axial parenchyma of secondary xylem bear secondary walls. The secondary xylem shown in Figure 6 is typical of xylem produced by the successive cambia of Welwitschia. The first-formed xylem of the strand is not shown in Figure 6; first-formed tracheary elements of a strand typically form a single group from which fascicular areas radiate (Figs 3-5). This 'radiation' results from initiation of rays, which are composed of tliin-walled cells. The fact that the fascicular areas become separated by parenchyma is evident in Figure 6, but the organization of the parenchyma as rays is only clear in tangential section (Fig. 7) or a radial section (Fig. 8) of secondary xylem. A tangential section (Fig. 7) shows that the parenchyma between fascicular secondary xylem is distributed in fusiform areas, just as in any vascular plant in which rays occur as a result of secondary growth. Vascular rays of Welwitschia are composed exclusively of upright cells (Figs 7, 8). At the margins of rays, the cells are taller and narrower than tliey are at the centres (Fig. 7). Upright marginal or sheathing ray cells common in rays of vascular plants, and can easily be seen in Ephedra (Carlquist, 1989, 1992). In Ephedra, as in dicotyledons, procumbent ray cells become more abundant with age. This is true in Gnetum gnemon, although rays in the wood of mature G. gnemon trees has procumbent ray cells exclusively (Carlquist, 1994). One can ask whether the presence of exclusively upright cells in the rays of Welwitschia is related to origin of rays from successive cambia rather than from a single cambium. However, in species of Gnetum with successive cambia, ray cells are rarely upright and almost all cells are procumbent (e.g. Carlquist & Robinson, 1995). Thus, the exclusive presence of upright cells in rays of Welwitschia cannot be attributed to presence of successive cambia in that genus. In die central portions of many rays of Welwitschia, there are areas in which minute calcium oxalate crystals line intercellular spaces (Figs 9, 13, 14). These minute crystals are much smaller than the crystals embedded in the walls of fibrosclereids (Figs 11, 12), and have a different mode of occurrence. The contrast in size is shown in Figure 13, in which the crystals dislodged from a fibrosclereid by sectioning (lower half of photograph) are about four times the size of the minute intercellular crystals (top half of photograph). The minute intercellular crystals in Figure 14 are shown at a higher magnification than those in Figure 13. The minute crystals occur on

WEL WITSCHIA WOOD ANATOMY 113 Figures 7-10. Longitudinal sections of secondary xylem of vascular strand from root of older seedling of Welwitschia. Fig. 7. Tangential section; portions of three rays are visible. Fig. 8. Radial section; files of upright cells are visible. Fig. 9. Tangential section, higher magnification; cells of ray, centre, are associated with minute crystals lining intercellular spaces. Fig. 10. Radial section, showing sinuous course of fascicular xylem; sections of rays appear at upper and lower left; conjunctive tissue in right half of photograph. Fig. 7, to scale of Fig. 2; Figs 8, 9, to scale of Fig. 3; Fig. 10, divisions = 10 um.

114 S. CARLQUIST AND D. A. GOWANS Figures 11-14. SEM photographs of structures from Welwitschia root, Carlquist 8071. Fig. 11 Transverse section of gelatinous wall (bearing crystals) of fibrosclereid. Fig. 12. Tip of fibrosclereid (from maceration), showing crystals embedded in wall surface. Fig. 13. Larger crystals (below) dislodged from wall of fibrosclereid plus smaller crystals (above) dial line intercellular spaces of conjunctive tissue. Fig. 14. Minute crystals lining intercellular spaces among conjunctive tissue parenchyma, higher magnification. Scale bars in all figures = 10 urn.

WELMTSCHIA WOOD ANATOMY 115 parenchyma cell surfaces that face air spaces. This occurrence of minute crystals is a peculiar feature, newly reported for Welwitschia, that occurs in only one other genus of vascular plants Ephedra (Carlquist, 1989, 1992). Minute intercellular crystals have never been observed in Gnetum, although that genus has relatively large crystals inside ray and axial parenchyma cells (Carlquist, 1994; Carlquist & Robinson, 1995, and Carlquist, unpublished data). The minute intercellular crystals of Welwitschia are not restricted to xylem rays, but also characterize the phloem rays, and are also scattered in conjunctive tissue, in a fashion much as they are in the ray in Figure 9. Where present, the minute crystals line intercellular spaces among groups of 5-25 adjacent cells. Axial parenchyma occurs in Welwitschia, and is present in the form of occasional parenchyma cells embedded among tracheary cells (Fig. 6), parenchyma cells that are not in the rays themselves. The fact that axial parenchyma cells are composed of strands of upright cells, as are ray cells, makes the discrimination between axial parenchyma and ray cells difficult in tangential section. In radial sections of Welwitschia vascular strands, as in the radial sections of wood of other vascular plants, ray cells appear in radial rows (Fig. 8), whereas axial parenchyma cells do not correspond to these rows. In addition, ray cells in Welwitschia are wider and not horizontally subdivided, or subdivided at most into a pair of cells (Fig. 8), whereas axial parenchyma is narrower and horizontally subdivided into strands of two to five cells. The peculiar curvature in the vascular strand shown in Figure 10 may be due to contraction of the root during early ontogeny, similar to the curvature of vascular strands in contractile roots in angiosperms. The relative abundance of vessel elements and tracheids in fascicular areas of vascular strands of Welwitschia has not received comment in the literature. Based on relative diameters of the two cell types in macerations, where the two cell types can be differentiated widi certainty, one can identify the two cell types as seen in transection. In the transection shown in Figure 6, there are approximately 25 vessels. There are relatively few narrow vessels (or unusually wide tracheids) that are intermediate between the diameter typical of the two respective cell types. Tracheids are about four times as common as vessel elements in the vascular strands of Welwitschia roots. In a maceration taken from the outer part of the large root shown in Figure 1, the mean tracheid length was 617 urn, and the mean vessel element length was 547 urn. Tracheary element structure Excellent data on details of tracheary elements of Welwitschia have been contributed by Bierhorst (1960) and, at an ultrastructural level, by Eicke (1957, 1962). As noted by Bierhorst (1960), perforation plates consist of a single circular perforation in most vessel elements of Welwitschia; two superposed circular perforations are rare and three are extremely rare. Prominent borders occur on the perforations. The internal sculpture and conformation of pit apertures of vessel elements and tracheids of Welwitschia

116 S. CARLQUIST AND D. A. GOWANS are accurately figured by Bierhorst (1960, 282), who did not have the advantage of SEM. Wider vessel elements have slit-like pits (Fig. 16) that are approximately as long as the diameter of the pit cavity (pit membranes) with which they are associated. The narrower the tracheary element, especially tracheids, the more this pit aperture is elongate, so that the pit aperture becomes intercontinuous with a groove that extends around the cell. These grooves appear as depressions in the wall (Figs 17, 18). When seen with SEM, the grooves and intervening ridges form a corrugated appearance on the inside surface of a tracheid. These are not superimposed 'helical thickenings'. As seen in optical section with a light microscope, the ridges and grooves are quite evident, and are as figured by Bierhorst (1960), who described them as follows: "... all of the apparent irregularities on the inner surface of the wall... are the edges of elongate inner pit apertures". We were not able to confirm Bierhorst's observation that multiple apertures occur on some pits. Martens (1971) noted that Bierhorst's (1960) figures of tracheary elements in sectional view, some of which Martens reproduced, tend to show elliptical pit membranes. Because of our interest as to whether Gnetum and Welwitschia lack tori on pit membranes of tracheary elements, as Eicke (1957, 1962) and Martens (1971) claimed, we have carefully examined material of Welwitschia both with light microscopy and SEM. We are unable to demonstrate presence of tori in Welwitschia, although there are clear instances of their presence in Ephedra (Carlquist, 1992) and Gnetum (Carlquist & Robinson, 1995, plus Carlquist, unpublished data). If one views with light microscopy a pit of a Welwitschia tracheary element in near-sectional view, there is some tendency to see the membrane as slightly thicker in the middle, perhaps because one is often viewing the membrane obliquely. If the pit membrane is rigorously observed when it is perpendicular to the plane of view, the membrane appears flat, not elliptical. Parameswaran and Liese (1974) figured "large wart-like protuberances" (wording from legend to figure) on a tracheary element pit membrane of Welwitschia. The text of their paper does not mention this figure in any way. The 'wart-like protuberances' are much larger than the vestures seen in pits of Gnetum (in which genus the vestures occur in the cavities of pits, not on the membranes). Our studies of pit membranes and pit cavities of Welwitschia revealed no 'wart-like protuberances'. We conclude that the Welwitschia pit membranes figured by Parameswaran & Liese (1974) may represent droplets of a secondary plant product. Such droplets occur on pit membranes in Ascarina (Carlquist 1990). Gale (1982) has cautioned against identifying droplets of secondary compounds as vestures when viewing them with SEM. Careful examination with light microscopy can reveal that such formations are droplets rather than vestures; observing distribution of these formations within a wood is also indicative: vestures do not show patchy distribution within a given wood, as far as is known, but droplets of secondary compounds are rarely uniformly present within a wood. Cambial activity and origin of vascular strands Martens (1971) questioned whether bundles might ultimately originate from the phelloderm. We have shown that this is, in fact, true. The feature that

WELW1TSCHIA WOOD ANATOMY 117 Figure 15-18. SEM photographs from sections of Welwitschia root. Carlquist 8071. Fig. 15. Transverse section of gelatinous fibres from secondary phloem of vascular strand. Fig. 16. Longitudinal section of inside of vessel wall, showing slit like pit apertures that lack vestures. Figs 17, 18. Inner surfaces of tracheids to show wall relief. Fig. 17. Pit apertures are deep, widened into grooves. Fig. 18. Wall appears ridged, elongated pit apertures are between the ridges. Scale bars in all figures = 10 urn.

118 S. CARLQUIST AND D. A. GOWANS permits identification of vascular strand origins is the tendency for cell division plates to remain evident for long periods of time, because a subdivided cell in Welwitschia parenchyma (or similar cells) tends to retain the shape of the cell prior to division indefinitely. One sees many oval cells subdivided by a transverse cell division (Figs 21, 22). This feature aids in tracing cell lineages. Figures 19-22 are all orientated with phellem at top, and show extensive files of phelloderm cells. Files continuous with phelloderm extend inward past vascular strands (Fig. 19), indicating that no tissue other than the phelloderm can be a source for these strands. The files are just as clear in radial longitudinal sections as in transverse; such extensive phelloderm is unusual in vascular plants, and does not occur in Gnetum species with successive cambia (Carlquist & Robinson, 1995). Because vascular strands are not added with great frequency over short periods of time, numerous stages in their origin cannot be found. However, several early stages of vascular strand formation within phelloderm like the one shown in Figure 19 were observed. Conjunctive tissue origin can be traced to phelloderm: all the tissue that does not mature into vascular strands becomes, by definition, conjunctive tissue. Fibrosclereids, as well as vascular strands and conjunctive tissue, originate from some of the phelloderm cells (Figs 20, 21).» CONCLUSIONS The recognition that Welwitschia vascular strands have cambia that produce fascicular areas, axial parenchyma, and ray parenchyma like those of other woody plants brings Welwitschia into line with Ephedra and Gnetum and permits comparisons with those genera. The previous view, that Welwitschia lacked wood features such as these was probably due to lack of good preparations. The predominance of upright cells in the rays of Welwitschia could potentially mark Welwitschia as similar to Ephedra rather than Gnetum, because ray cells are procumbent in mature rays of Gnetum, even in those species with successive cambia. Exclusive presence of upright cells in rays is an indicator of juvenilism in dicotyledons (Carlquist 1962, 1988), and that interpretation is available for Welwitschia also. The fact that Welwitschia produces only one pair of functional leaves in addition to the cotyledons has been seen as an indicator of permanent juvenilism ('neoteny') by many authors (see Martens, 1977, who demurs), so the ray cells may add evidence for the interpretation of Welwitschia as juvenilistic. Tracheids are slighdy longer than vessel elements in Welwitschia, as in Ephedra (Carlquist 1989, 1992). The low ratio in length between tracheids and vessel elements underlines the primitive nature of Welwitschia and Ephedra wood, because progressive division of labour with phyletic advance leads to more strongly divergent lengths between vessel elements and imperforate tracheary elements (Carlquist, 1975). The lengths of vessel elements in Welwitschia are very similar to those of Ephedra. If shortness of vessel elements is an indicator of wood xeromorphy (Carlquist, 1975), wood of Ephedra and Welwitschia qualifies as xeromorphic in comparison with that of Gnetum (Carlquist, 1994). A feature newly reported here for Welwitschia is the presence of minute

WELW1TSCHIA WOOD ANATOMY W 119 Figures 19-22. Sections of periphery of root of larger seedling of Welwitschia, innermost pheliem at top of each photograph. Figs 19-21. Transverse sections. Fig. 1!). Radial files of phelloderm extend inward past young vascular strand. Fig. 20. Long radial files of phelloderm cells; portion of a fibrosclereid formed from a phelloderm cell near centre. Fig. 21. Phelloderm cells and (left margin) a fibrosclereid formed from a phelloderm cell; cell division sequences are visible because mouier cells retain original form well after a division has occurred. Fig. 22. Radial section of root periphery; Long radial files of phelloderm cells are evident. Figs 19-22, to scale of Fig. 3.

\20 S. CARLQUIST AND D. A. GOWANS crystals lining intercellular spaces in phloem rays, xylem rays, and conjunctive tissue. This feature also occurs in Ephedra, but not in Gnetum; it has apparently not otherwise been reported in vascular plants. SEM studies of the vessel elements and tracheids of Welwitschia confirm most of the features reported by Bierhorst, whose studies were done with light microscopy. Walls of tracheary elements are somewhat gelatinous, as shown by shrinkage patterns, and the walls of phloem fibres and fibrosclereids are quite markedly gelatinous, with many layers. In view of the abundance of cells with gelatinous secondary walls in Welwitschia, there is a possibility that short-term storage of small amounts of water might occur in these walls. The nighdy absorption of condensed moisture by leaves of Welwitschia is a well-known phenomenon (see Bornman, 1978) that might accord with water storage not only in parenchyma, as seems likely, but to a minor extent in gelatinous cell walls as well. The origin of vascular strands and the cambia they contain can be traced to phelloderm in Welwitschia. This origin is highly unusual, and places Welwitschia in contrast to Gnetum. In the lianoid species of Gnetum, successive cambia do not originate from phelloderm, but can be traced to cortical parenchyma (La Riviere, 1916; Carlquist & Robinson, 1995). Thus, lateral meristem activity and origin of vascular strands are non-homologous in the two genera. The occurrence of minute crystals in intercellular spaces, presence of upright ray cells, and the development of vascular strands from phelloderm are three newly reported features in which Welwitschia resembles Ephedra instead of Gnetum. The interrelationships of the three genera continue to be studied with great interest. As features of Ephedra, Gnetum, and Welwitschia are clarified, we can develop a more accurate picture of phylogeny within the gnetalean genera. In addition, features of the three genera may be of value in clarifying how Gnetales are related to other groups of vascular plants. A summary of characters of vascular tissue of Ephedra, Gnetum, and Welwitschia is planned as a final paper in this series of studies. ACKNOWLEDGEMENT D. A. G. is a member of the Plant Scholars Program, Santa Barbara Botanic Garden. REFERENCES Bertrand CE. 1874. Anatomie comparee des tiges et des feuilles chez les Gnetacees et les coniferes. Annates des Sciences Naturelles, Bolanique, ser. 5, 20:.5-1.53. Bierhorst DW. 1960. Observations on tracheary elements. Phytomorphology 10: 249-305. Bornman CH. 1978. Welwitschia. Cape Town & Johannesburg: C. Struik. Bower FO. 1881. On die further development of Welwitschia mirabilis. Quarterly Journal of Microscopical Research 21:.571 594. Butler V, Bornman CH, Evert RF. 1973. Welwitschia mirabilis: vascularization of a one-year old seedling. Botanical Gazette (Crawforasville) 134: 63-73. Carlquist S. 1962. A theory of paedomorphosis in dicotyledonous woods. Phytomorphology 12: 30-45. Carlquist S. 1975. Ecological strategies of xylem evolution. Berkeley and Los Angeles: University of California Press. Carlquist S. 1982. The use of ediylene diamine in softening hard plant materials for paraffin sectioning. Stain Technology 57: 311-317.

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