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Stem and leaf anatomy of the arborescent Cucurbitaceae

Dendrosicyos socotrana with comments on the evolution

of pachycauls from lianas

M. E. Olson

Instituto de Biologıa, U.N.A.M., Departamento de Botanica, Ciudad Universitaria, Mexico

Received March 11, 2002; accepted January 23, 2003Published online: July 31, 2003� Springer-Verlag 2003

Abstract. Stem and leaf anatomy of Dendrosicyossocotrana, the only arborescent Cucurbitaceae, areexamined for correlations with life form andecology and are used to test hypotheses regardingfeatures adaptive in scandent plants. The stemconsists mainly of ray and conjunctive parenchymawith small strands of xylem forming an anastomo-sing net throughout the trunk. Xylem strands bearvascular cambia that produce secondary phloem,representing the first report of successive cambia inCucurbitales. Some features characteristic of lianas,such as very wide vessel elements with thick walls,are absent from Dendrosicyos. Other features, suchas very wide rays and abundant axial parenchyma,are present in both Dendrosicyos and lianas butappear to serve differing roles in these different lifeforms. It is suggested that lianas have numerousfeatures that are readily co-opted in the evolutionof pachycaul trees and that the evolution ofpachycauls from lianas has happened repeatedlyin the core eudicots.

Key words: Adaptation, Cucurbitaceae, Dendro-sicyos socotrana, evolution, anatomy, lianas, suc-cessive cambia, Socotra.

The arborescent Dendrosicyos socotrana Balf.f. is a conspicuous exception to the almostexclusive scandent habit within Cucurbitaceae.

With its bloated trunk several meters tall, andfew, thick main branches (Fig. 1), D. socotranais an example of the pachycaul ‘‘bottle tree’’habit. Dendrosicyos is found in shrublandcommunities on Socotra, the dry tropicalisland off the Horn of Africa, and differs frommany bottle trees in having twigs and smallbranches that are conspicuously pendent.Although the pachycaul habit is unusual inCucurbitaceae, it is common in many familiesthroughout the seasonally dry tropics (partic-ularly conspicuous examples are members ofMalvaceae such as Adansonia in Africa, Mad-agascar, and Australia, and Chorisia in SouthAmerica).

The anatomy of Dendrosicyos has elicitedinterest at least as early as 1835, with a reportthat ‘‘The whole diameter of their trunksconsists of a soft, whitish cellular substance,so easily cut through that we could divide thelargest of them with a common knife’’ (Well-sted 1835, quoted in Balfour 1888, p. 101). Incomments accompanying the naming of theplant, Balfour (1888, p. 100) noted that ‘‘Theexamination of the stem would be of greatmorphological interest, and I hope to havesome specimens from Schweinfurth’s plant [in

Plant Syst. Evol. 239: 199–214 (2003)DOI 10.1007/s00606-003-0006-1

cultivation in Cairo], of which an account willbe given in the Appendix’’. Balfour apparentlynever received the specimens, but an anatom-ical study of Dendrosicyos is still of interest onvarious fronts. Because they are of little timbervalue and grow in understudied tropical dryhabitats, the anatomical features of bottle treeshave received little attention. One aim of thispaper is therefore to examine the leaf and stemfeatures associated with the bottle tree habitand the ecological conditions that Den-drosicyos experiences, especially wind andseasonal drought. A second goal is to examinethe anatomy of Dendrosicyos in the context ofanatomical features known from other Cucur-bitaceae. A further goal is to examine howanatomical features of scandent ancestors aretransformed in the evolution of a pachycaultree. It is proposed that scandent plants havefeatures conducive to the evolution of pachy-caul trees and shrubs and that this pheno-menon has occurred repeatedly throughout thecore eudicots.

A final objective is to use Dendrosicyos totest hypotheses regarding anatomical featuresthat are adaptive within the liana habit ingeneral and Cucurbitaceae in particular. Be-cause Dendrosicyos is not scandent, it is likelythat features that are adaptive in a scandentmorphology would be of differing selectivevalue in a pachycaul tree. These features andhypotheses include:

1. The presence of wide, thick-walled ves-sels with band-like reinforcements. The verywide vessels of vines are interpreted as adap-tations to meet the osmotic needs of a largeleaf biomass with limited stem transectionalarea (Carlquist 1975, 1985, 2001a). Ewers andFisher (1991) noted that vining species of thelegume genus Bauhinia had wider vessel ele-ments than species of the same genus withfreestanding life forms. A similar trend hasbeen noted between species in Gnetum (Fisherand Ewers 1995), and even between vining andself-supporting clones of the same geneticindividual of Toxicodendron diversilobum(Gartner 1991). Features that have beenobserved to be associated with wide vessel

diameter in Cucurbitaceae are increased wallthickness and bandlike wall thickenings, whichmay help maintain vessel integrity in the faceof torsion of the stem and low xylem pressure(Carlquist 1992). Thus it might be expectedthat the vessels of the rigid mature stem ofDendrosicyos would lack band-like reinforce-ments and be narrower than those of scandentcucurbits.

2. Sclerified parenchyma adjacent to ves-sels. Carlquist (1992) postulated that vessels inCucurbitaceae are protected by adjacent thick-walled paratracheal parenchyma cells (andlibriform fibers) and cushioned by thin-walledparenchyma and ray cells in other parts of thestem. Protected within a massive trunk, thevessels of Dendrosicyosmight be expected to besurrounded by axial parenchyma cells that arenonlignified.

3. Vasicentric tracheids. As the conductiveefficiency of vessels increases with increasingdiameter, the danger of air embolisms alsobecomes greater. Carlquist (1992) noted thepresence of vasicentric tracheids in contactwith the vessels of vining Cucurbitaceae as afeature of likely adaptive significance in miti-gating the effect of embolized vessels. Narrowtracheids are much less likely to becomeembolized than vessels, so in the event of theconductive column of a vessel being broken,the vasicentric tracheids should maintain someflow to the organs supplied by their associatedvessel. If, as is hypothesized above, vessels inDendrosicyos are narrower than those oflianas, the selective value of vasicentric tra-cheids seems low.

4. Abundant axial parenchyma that buffersstem torsion. The very long stems of vines andtheir dependence on other plants or objects forsupport means that events along the length ofthe stem, such as a falling support branch,leave them vulnerable to pinching or breakingof the conductive column due to stem torsion.Axial parenchyma may serve a cushioningfunction, in which the energy of a damagingevent to the stem is absorbed by thin-walledparenchyma cells, sparing the vessels fromdamage (Schenck 1893 fide Carlquist 2001a,

200 M. E. Olson: Anatomy of Dendrosicyos

Putz and Holbrook 1990, Carlquist 2001a).Carlquist (1992) hypothesized that the abun-dant axial parenchyma in the stems of scan-dent Cucurbitaceae serves such a bufferingfunction. It is of interest to examine the stemof Dendrosicyos for this cell type and to ask ifit is serving the cushioning function that itserved in the scandent ancestor, or whether it isserving a new function.

5. Very wide, tall rays. In addition to axialparenchyma, the very wide, exclusively mul-tiseriate rays observed in scandent Cucurbita-ceae have been hypothesized to provideresilience to torsion (Carlquist 1992). Fisherand Ewers (1989) noted that the very wideunlignified rays in Coccinia provided radiallyoriented zones of weakness between plates ofxylem. When subjected to torsion, the stemwas found to split along these rays, preservingthe water column in the vessels surrounded bylignified fibers and parenchyma cells. Themature stem of Dendrosicyos is unlikely toexperience torsion to a great degree and mighttherefore be expected to lack massive raysforming radial planes of weakness.

6. Starch storage in parenchyma of lianastems. Carlquist (1985) hypothesized thatparenchyma in long vining stems could serveas a repository of starch that is in proximity tovessels along the entire length of the stem.Such starch reserves could be drawn upon tofuel massive reproductive events, leaf flushes,or raise vessel osmotic pressure to regulateconduction. Therefore the distribution andpossible role of starch in the stem of Den-drosicyos is compared with that of viningCucurbitaceae.

Materials and methods

In 1967, J. Lavranos collected live material ofD. socotrana at Ras Hebak on the northern coast ofSocotra. Live second-generation seedlings werekindly provided by T. Metcalf of the Universityof California at Davis. The plants were cultivatedin a heated greenhouse in Santa Barbara, Califor-nia, for an additional two years before harvesting.Phenological and growth pattern observations were

made during this period. At the time of sampling,the plants had reached a height of 1.5 meters and abasal stem diameter of 12 cm. One of these plants isdepicted in Fig. 1; the locations of the majority ofstem samples are indicated by arrows. The samplefrom the area indicated by the upper arrow isreferred to as the ‘‘young stem’’ where the stem was2 cm in a diameter. That from the area of the lowerarrow was taken from just above the widest part ofthe trunk and is referred to as the ‘‘mature stem’’.Samples taken from the distalmost branches thatstill had traces of epidermis are referred to as‘‘twigs’’. Two sets of preparations (trans- andlongitudinal sections of the three stem ages andleaves) were made, one from each of two plants,with the exception of the mature stem, which wasonly made from one specimen. A voucher isdeposited at the Missouri Botanical Garden her-barium (MO) as Olson, s.n. Because of the highlysucculent nature of the stem, it was necessary topreserve samples immediately in 70% aqueousethanol to avoid rot and collapse of the water-filled cells. After fixation, stem chips were softenedin 10% ethylene diamine prior to embedding inparaffin and sectioning on a rotary microtome at13 lm (method of Carlquist 1982). Leaves werealso fixed in 70% ethanol, dehydrated, and embed-ded in paraffin. Sections were stained with North-en’s modification of Foster’s ferric chloride-tannicacid staining series (Johansen 1940). Macerationswere prepared using Jeffrey’s solution (Johansen1940) and stained in safranin. Terminology followsCarlquist (2001a). For scanning electron micro-scope (SEM) observation, sections were mountedon aluminum stubs, cleared in xylene, sputtercoated with an Emtech K550 and observed with aHitachi S-2460N in the SEM facility of theInstituto de Biologıa of the Universidad NacionalAutonoma de Mexico. Measurements of cells areoutside dimensions except for vessel diameter, forwhich the lumen diameter is provided. Means givenare based on n¼ 25 (± standard deviation).

Results

Habit

The pendent leaders of Dendrosicyos areparticularly easily observed in young seedlings(Fig. 1). The erect stem of the young seedlinggradually bends until its apical meristem is no

M. E. Olson: Anatomy of Dendrosicyos 201

longer the highest point of the plant (blackstem at upper left in Fig. 2). A branch thatbecomes the leader emerges from the highestpoint (light gray branch at upper left inFig. 2). This process repeats iteratively until amassive sympodial trunk is formed, presum-ably the product of numerous apical meris-tems (as depicted in Fig. 2 at lower right, withthe adjacent products of different meristemsshaded differently). The leaves of the culti-vated Dendrosicyos were never completelyshed or only tardily so. This is in greatcontrast to other dryland pachycauls in thesame greenhouse (Moringa spp.; Jacaratiaspp.), which were completely leafless duringthe dry season.

Stem anatomy

Stem structural plan. Adult stem structuralplan is characterized by vascular bands alter-nating with bands of conjunctive tissue, withno growth rings distinguishable (Fig. 3). Vas-cular bands consist of rays and plates of xylem;plates are made up of vessels, libriform fibers,vasicentric axial parenchyma, and sometimesapotracheal axial parenchyma. Phloem is pro-duced by vascular cambia on the abaxial facesof the xylem plates. In transections, xylemplates appear to be separated from one anoth-er as islands in parenchyma. However, thesestrands frequently contact each other byweaving and occasionally bifurcating, forminga vascular net within the stem (Fig. 8). Mostweaving of vascular strands is tangential, withthe result that contact occurs mostly withinvascular bands. The extension of a vascularstrand radially into another band is lesscommon.

Phellem, phelloderm, and cortex. The out-ermost stem of Dendrosicyos bears 15–25layers of phellem and 2–5 layers of chloro-plast-bearing phelloderm. The cortex of ayoung twig (4 mm in a diameter) was about30 cells thick and consisted of rounded paren-chyma cells generally of uniform size andshape with most of the divisions observedbeing transverse. The epidermis has numerous

cystolith-bearing trichomes similar to thosefigured on the undersides of the leaves.

Cortical features persist in larger stems. Inthe young stem (2 cm in diameter), the phello-derm was underlaid by a cortical area with anumber of cell layers similar to that in the twigbut with much more divisional activity andresultant variety in cell size and shape. In themature stem, the cortex was also about 30 cellsthick, with more abundant cell division ob-served than in the twig or the young stem.Because of the great increase in diameter thatthe base of the stem undergoes, it is likely thatvirtually all of the cells observed in the cortexof the mature stem are the products of divisionof the primary cortical cells and division of thelateral meristem. Thus it is reasonable to referto this region in the mature stem as secondarycortex. Excluding divisions of the lateral mer-istem, in the 2 cm stem radial divisions out-number tangential divisions by 3:1 and obliquedivisions by 2:1. In the mature stem, similarproportions were observed between radial andoblique divisions, but tangential divisions wereeven rarer than in the 2 cm stem.

Small fields of septate fibers were observedin the young stem every 60–300 lm around thestem, 5–10 cells beneath the phelloderm. Thesefibers averaged 22 (±5.4) lm in diameter withsegments 274 (±73.8) lm long, whereas thefibers themselves exceeded the 2.5 mm heightof the longisection available. Similar fiberswere not observed in either the twig or themature stem.

The lateral meristem, cambia, and their

products. The outer stem is notable for its lackof a single clearly defined vascular cambium(Fig. 3). Instead, cells in a general area of theinner cortex give rise internally to ray cambia,conjunctive tissue, or vascular cambia thatproduce the xylem plates and associated phlo-em. Tangential divisions of the lateral meri-stem also produce small amounts of corticalparenchyma externally.

As a xylem plate increases radially, raycambia vaguely aligned with the vascularcambia produce ray cells internally as thephloem impinges into the secondary cortex.


Alternately, the lateral meristem producesconjunctive parenchyma. In this way, a vascu-lar cambium and its products become sur-rounded by parenchyma on all sides (Fig. 4).

In the young stem, the plates of xylemformed in the first season of growth arecounter to the orientation of mature xylemplates, being associated with cambia thatproduce intraxylary phloem toward the pith.These plates reach their largest tangentialdimensions close to the pith and are narrowestat their outermost extent (Fig. 6). Just externalto these xylem plates are small patches ofxylem each with 1–3 vessels. Each of thesepatches has a small cambium that is usually onthe abaxial face of the patch, producingphloem toward the outside of the stem. Occa-sionally, these small cambia are located on theadaxial face of the patch and produce phloemtoward the pith, as do the innermost xylemplates.

Occasional strands of interxylary phloemranging in size from 3–20 cells wide wereobserved in the ray cells between xylem plates(Fig. 7). Phloem strands produced by vascularcambia include sieve tube elements and com-panion cells, which are often exceeded innumber and size by phloem parenchyma.Small strands of phloem similar to thosefigured as occurring in rays occur sporadicallyin the outer cortical parenchyma. Thesestrands may represent incipient formation ofvascular cambia.

Vessels. In the mature stem, mean vesselelement dimensions are 180 (±79.6) lm longand 48 (±12) lm in diameter. Vessels weresolitary in 84% of the cases examined. Theremainder were in mostly radial or tangentialpairs or, much more rarely, radial multiples orglobular clusters of up to five vessels. Vesseldensity is 2 (±2) vessels per mm2, occupying0.7% per unit area of transection. Perforationplates are simple with a distinct border andusually have an angle of 90�–45� with respectto the longitudinal axis of the vessel element.Lateral wall pitting is alternate. Vessel-vesselpit cavities are narrowly oval, sometimesapproaching rhomboidal, with narrowly oval

apertures 4 (±0.8) lm wide and 1 (±0.4) lmtall. Pits at vessel-axial parenchyma interfacesare 10 (±4.1) lm wide and 4 (±0.8) lm talland have usually very broadly oblong cavitieswith wide, oval, or oblong apertures (Figs. 10,11), forming distinct borders 1–2 lm wide.Sometimes 2–4 pit apertures are interconnect-ed within the same groovelike pit cavity.Vessel-axial parenchyma pits are sufficientlybroad as to approach scalariform arrangementin some areas (Fig. 10). Although vessels weresometimes observed to contact libriform fibers,no vessel-fiber pitting was observed. Vesselsare usually slightly radially oblong and angularin cross section, with each side of the polygoncorresponding to the face of a contacting cell.Mean vessel wall thickness is 2 (±0.8) lm.Vessels are sometimes obstructed by numeroussmall (ca. 50 lm in diameter) tyloses. Meanvessel dimensions in the young stem were 75(±21.4) lm diameter with a wall thickness of3 (±0.8) lm.

Libriform fibers. Dendrosicyos exhibits akind of fiber dimorphism. Large-pitted paren-chyma-like fibers that are usually non-septateform the bulk of this cell type (Fig. 12).Occasionally intermingled with parenchyma-like fibers are very long fibers that are alwaysseptate and usually bear only small pits(Fig. 13; to distinguish them from the paren-chyma-like fibers, which are only occasionallyseptate, these cells are referred to here as‘‘septate fibers’’). Mean dimensions of paren-chyma-like fibers are 482 (±139.3) lm longand 25 (±6.2) lm in diameter. None of theseptate fibers persisted through the macerationprocess intact; mean dimensions of the frag-ments were 1329 (±378.3) lm long and 22(±5.4) lm in diameter, figures sufficing todemonstrate that the septate fibers are muchlonger and somewhat narrower than theparenchyma-like ones. The mean number ofsegments observed in these fragments was 6,and the mean length of each segment was 274(±73.8) lm. Parenchyma-like fibers havemore abundant and larger pits: parenchyma-like fiber pits are 4 (±1.8) lm wide in theirlargest dimension, whereas those of septate


fibers are 3 (±0.8) lm wide (Fig. 13). Pits inboth fiber types are exclusively non-bordered(Fig. 13 shows pits of parenchyma-like fibers).Mean libriform fiber wall thickness is 2(±0.6) lm (septate vs. parenchyma-like fibers

were not distinguished in transection, but therarity of septate fibers means that this figurelikely represents the wall thickness of the non-septate fibers). Parenchyma-like fibers arebroadly fusiform or with cylindrical main


bodies with long intrusive tails that togetherequal or exceed half of the total cell length.Tails often bifurcate or have other irregularramifications at their tips (Fig. 12).

Axial parenchyma. Three types of axialparenchyma cells are found in Dendrosicyos.Vasicentric scanty parenchyma contacts theradial walls of the vessels, with libriform fibersbarely contacting the tangential walls. In othercases, especially when vessels are abundant,vasicentric parenchyma cells form a completesheath around vessels (Fig. 6) or even extendsbetween vessels. These cells are of highlyirregular shape (Fig. 11 shows such a cell froma maceration) with constrictions between thelarge vessel-axial parenchyma pits and walls 1(±0.6) lm mean thickness. Vasicentric paren-chyma-libriform fiber pits are small and oval(Figs. 9, 11). Clear cases of vasicentric paren-chyma cells divided into strands were notobserved. The second type of axial parenchy-ma in Dendrosicyos is apotracheal, in bandsthat interrupt the plates of xylem. Bands ofapotracheal axial parenchyma were by farmost common in the young stem (two bandsare indicated by arrows in Fig. 6) and wererare in the mature stem (note the lack of suchbands in the xylem plate in Fig. 5). The thirdtype of axial parenchyma is conjunctive tissue(labeled ‘‘CT’’ in Fig. 3; also at bottom ofFig. 5) that fills the area between the vascularbands. These cells were rarely observed to be instrands of two, but are usually not so divided.In the material available, bands of conjunctive

tissue equaled the vascular bands in radialextent or were narrower.

Rays. Very wide multiseriate rays areinitiated between xylem plates. Even the pri-mary rays are multiseriate, with no uniseriateprecursors (Fig. 6 shows part of one such rayand its point of contact with the pith). Theserays may be 20–50 cells wide or more and areoften 5 mm or more in height. Cells are for themost part procumbent, though square andupright cells may be found, especially on theray periphery. No uniseriate wings were ob-served on multiseriate rays, and uniseriate raysare lacking. Ray cell pit density is not evenlydistributed on all faces and appears to be leaston the tangential faces of the cells. Ray andconjunctive parenchyma cells, which form thebulk of the stem of Dendrosicyos, are notdistinguishable based on shape and are notexpected to be functionally different.

Crystals, starch, and storying. Crystalswere not observed in the stem, but starchgranules are common in cortex, ray, conjunc-tive parenchyma, and pith cells (Figs. 5–8), lessso in libriform fibers (Fig. 9 at top). Clearcases of storying were not detected in any ofthe preparations made.

Leaf anatomy

Leaves average 10 cm in length and diameter,and bear prickly margins. The mean thick-ness of the leaf from the upper cuticle to thelower cuticle and away from veins or surface

b

Figs. 1–4. Dendrosicyos socotrana, habit and growth patterns. 1 Habit. Mature plant in middle, one of theseedlings from which samples were drawn is shown at right. Low arrow indicates location of ‘‘mature’’ sample.Upper arrow indicates location of ‘‘young stem’’ sample. Note rigid lower trunk and flexible, pendent upperstems. Scale bar ¼ 1m. 2 Contributions of successive leaders in producing the trunk of Dendrosicyos. Tissuederived from different apical meristems is given different shading. 3 Stem transection to show structural plan ofbands of conjunctive tissue (CT) alternating with vascular bands that consist of rays (R) and xylem plates (darkstrips of cells between rays). Outer surface of stem is shown at top; most of the light cells in this area are part ofthe secondary cortex (SC). Scale bar¼ 3 mm. 4 Diagram of xylem plate ontogeny. Dark gray¼ 2� cortex; lightgray¼ ray and conjunctive parenchyma; black¼ phloem; white¼ xylem; black-white interface¼ vascularcambium. Following a band of conjunctive tissue, a new vascular cambium is produced by the lateral meristem(top left). This cambium produces xylem to the inside as the stem expands (upper right and lower left). Thexylem plate becomes distanced from the cortex when the lateral meristem produces another band of conjunctivetissue (lower right)


projections is c. 300 lm. The epidermis of theupper leaf surface is composed of a single layerof cells of ca. 30 lm mean cell height (Figs. 14,15). These cells have slightly lignified cell walls,with the faces corresponding to the upper leaf

surface covered with a thick cuticle (Fig. 16).Cuboidal or slightly rhomboidal crystals areoccasionally found singly in the epidermal cells(two are shown beneath pustule in Fig. 14).Stomata occur sparsely on the upper leaf


surface (one of these is shown in Fig. 15). Theupper surface of the leaf does not beartrichomes, but multicellular pustules are com-mon (Fig. 14). These pustules are 2–5 cells talland overlie the main epidermal cell layer,which may become discontinuous beneath thepustules as in Fig. 14. The cells of the pustuleshave walls that are comparable in thickness tothe other epidermal cells, but each is usuallyfilled with a mass of striated material, presum-ably calcareous cystoliths (cf. Metcalfe andChalk 1950). Beneath the epidermis are twopalisade layers totaling ca. 120 lm thick,characterized by numerous open spaces(Fig. 15). Small cuboidal crystals are commonin these cells (upper arrow in Fig. 14). Thespongy mesophyll is ca. 330 lm think and iscomposed of 5–6 layers of highly irregular cellswith large open spaces between them. Chlo-roplasts are common in the cells of bothmesophyll layers but are slightly more so inthe palisade layers. The epidermis of the lowersurface is similar to that on the upper surface,with the cells in the single layer being slightlysmaller, averaging 23 (±5) lm in height (Figs.14, 17). Stomata, which are anomocytic, aremuch more common on the lower surface thanon the upper. Figure 17 shows three stomatain transection, and Fig. 18 shows three in aparadermal section. Stomata on the lowersurface average 23 (±6) lm in their largestdimension and 17 (±4) lm in their smallest,with a mean stomatal aperture of 11 (±4) lmin its largest dimension. Stomata are flush ornearly so with the leaf surface. Pustules are notfound on the lower leaf surface, but two- toseven-celled trichomes are conspicuous in this

area. Most such trichomes have a basal cellthat is constricted with respect to the diameterof the rest of the trichome (Fig. 17). The tips ofthese trichomes are often bulbous or pointed(Fig. 17), with the apical 1–2 cells beingoccupied by cystoliths.

Discussion

Stem anatomical correlations with habit and

ecology. The most conspicuous feature of thestem of Dendrosicyos in connection with thebottle tree habit is the large quantity of ray andconjunctive parenchyma clearly associatedwith water and photosynthate storage as ameans of surviving prolonged drought. Oneconsideration of such storage is expansion andcontraction of the stem associated with uptakeand use of stem water reserves. Although thestem of Dendrosicyos is not ribbed like that ofa columnar cactus, the undulating walls of ray,axial, and cortical parenchyma suggest amechanism to cope with drastic seasonalchanges in stem diameter. Presumably theundulations in the cell walls are reduced asthe cell water content and stem diameterincreases in the rainy season, with an oppositetrend in the dry season.

In plants with a single cambium, rays aretypically involved in radial conduction fromthe single phloem area to or from more centralparts of the stem. But the necessity forextensive radial translocation in plants withsuccessive cambia seems small due to thecomprehensive enervation of the stem byphloem strands and the lack of a singlesubcortical phloem system (Carlquist 2001a).

b

Figs. 5–8. Dendrosicyos socotrana, stem anatomy. 5 Stem transection, image oriented with outermost extent attop, showing one entire xylem plate with part of another at bottom left. Abaxial to xylem is a vascular cambium(arrow) producing secondary phloem toward the outside of the stem. This cambium coincides roughly inalignment with the ray cambia on either side. Scale bar¼ 1 mm. 6 Transection showing pith at bottom andintraxylary phloem being produced from vascular cambia adaxial to xylem plates (torn cells). Bands ofapotracheal axial parenchyma interrupt xylem plates (arrows). Starch granules are common in pith and rays.Scale bar¼ 500 lm. 7 Strand of interxylary phloem with no associated xylem from a transection of maturestem. Scale bar¼ 100 lm. 8 Tangential section of mature stem to show very wide, tall rays, bifurcation of xylemplates (as at right) and reconnection (as at left). Scale bar¼ 1 mm


In Dendrosicyos, the somewhat lighter pittingon tangential than on other faces of ray cellssuggests that, rather than radial translocation,these cells may be chiefly involved in storage ofphotosynthates supplied by local phloem

strands and the possibly rapid translocationof storage products to nearby vessels associat-ed with reproductive events or leaf flushes.

The presence of the gelatinous fibers thatcharacterize reaction wood might be expected


in a heavy, water-laden stem with little lignifiedtissue, especially one with the pendent branch-es of Dendrosicyos (cf. Euphorbia, Carlquist1970; Fouquieria, Carlquist 2001b). The ab-sence of this wall type may be a phylogeneticcontingency. Reaction wood has not beenreported in Cucurbitaceae, and appears to benonexistent or rare in the flexible stems oflianas (cf. Metcalfe and Chalk 1950, Hosterand Liese 1966).

The abundance of radial divisions in thecortical cells is reflective of the large increase incircumference that the trunk of Dendrosicyosundergoes. Likewise, radial divisions in thecells of rays and conjunctive tissue are alsoinvolved in the increase in circumference of themassive stem. The comparatively low numberof tangential divisions observed within thecortical parenchyma, rays, and conjunctivetissue is compensated for by divisions in thelateral meristem, which contributes to theincrease in diameter of the stem. Obliquedivisions are likewise more common thantangential divisions in the cortex and appearto fill the increase in volume created by stemexpansion.

Integration of leaf and stem features. Theleaves of Dendrosicyos have lignified epidermalcell walls and a thick cuticle. These significantinvestments in leaf tissue suggest that Den-drosicyos does not share the strategy of manydeciduous dryland plants, which have highlymesomorphic leaves that are produced imme-diately following rains and are shed just as

suddenly (cf. Fouquieria Carlquist 2001b).Likewise, very low conductive area and narrowvessels do not suggest that Dendrosicyos trans-pires at rates comparable to a mesophyteduring the rainy season. The mean vesseldiameter of a broad sampling of woodymesophytes (108 lm; Carlquist 1975: 206)greatly exceeds that of Dendrosicyos. However,neither are the leaves of Dendrosicyos highlyxeromorphic, lacking such features as stomatalcrypts or dense indumentum. Instead, deadleaves clothe the branch tips, likely providingshade and still air around living leaves, buf-fering them from water loss. Thus a combina-tion of anatomical and phenological traitscombine to form leaves that probably persistlonger than those of many other drylandpachycauls in similar environments. Leaf lon-gevity has been linked to reduced photosyn-thetic rate (Reich et al. 1992). The narrowvessels of Dendrosicyos relative to more rapid-growing tropical dryland pachycauls (e.g.Moringa, Olson and Carlquist 2001) may berelated to a relatively lower rate of photosyn-thesis.

Features of scandent Cucurbitaceae that are

absent in Dendrosicyos. Of the features hy-pothesized to be adaptive in a scandentmorphology, the first three categories dis-cussed in the introduction are absent fromthe arboreal stem of Dendrosicyos:

1. Wide, thick-walled vessels with band-likereinforcements. The vining Cucurbitaceae ex-amined by Carlquist (1992) had a mean vessel

b

Figs. 9–13. Dendrosicyos socotrana, stem cell shape and pitting. 9 SEM image from a tangential sectionshowing vessel-vasicentric parenchyma pitting as seen from the inside of the two vessels labeled ‘‘V’’. On the farwall of the vasicentric parenchyma cell labeled ‘‘VP’’ can be seen vasicentric parenchyma-libriform fiber pitting.The three thick-walled cells at upper middle and right are libriform fibers (labeled ‘‘LF’’), and show a few fiber–fiber pits. Part of a starch grain can be seen in left fiber at top. Scale bar¼ 50 lm. 10 Radial section showingvessel-vasicentric parenchyma pits on the vessel elements at center. Vasicentric parenchyma cells on either sideof the vessel at top are labeled ‘‘VP’’. Fiber–fiber pits can be seen at lower left. Scale bar¼ 100 lm. 11Vasicentric parenchyma cell from maceration to show irregular shape and constriction of cell between vessel-parenchyma pits (broad light patches). Smaller, lighter patches on far wall of cell are parenchyma-fiber pits.Scale bar¼ 50 lm. 12 Portion of parenchyma-like fiber from maceration to show broadly fusiform shape,abundant pits, and bifurcate tip. Scale bar¼ 100 lm. 13 Tangential section showing parts of three libriformfibers. The two cells at right are parenchyma-like fibers with large, simple pits. On the left is a septate fiber witha septum at lower left and no pits visible in this image. Scale bar¼ 50 lm


diameter of 164 lm among the four species. Ingreat contrast, the nonscandent Dendrosicyoshas a much smaller mean vessel diameter of48 lm in the mature stem. It may be assumedthat in the seasonally dry habitat of Den-drosicyos, large vessels and their attendant

vulnerability to embolism are a liability whilethe arborescent habit of Dendrosicyos obviatesthe requirement for large vessels by permittinga higher number of vessels. The combinedmean vessel wall thickness of the four viningspecies that Carlquist (1992) examined was

Figs. 14–18. Dendrosicyos socotrana, leaf anatomy. 14 Leaf transection, upper surface at top. Dark cellsforming pustule at top bear cystoliths. The upper arrow indicates two crystals. Lower arrow highlights stoma inlower surface. Scale bar¼ 100 lm. 15 Leaf transection showing stomata in upper leaf surface and palisade cellswith abundant air spaces. Scale bar¼ 100 lm. 16 Leaf transection showing outer surface of epidermal cells andthick cuticle (slightly detached at left). Scale bar¼ 10 lm. 17 Leaf transection showing three stomata (thelowermost one indicated by an arrow), spongy mesophyll at right, and trichome with cystolith-bearing cells atits apex. Scale bar¼ 50 lm. 18 Leaf paradermal section showing three anomocytic stomata. Scale bar¼ 25 lm


4.9 lm, greatly exceeding the wall thicknessfound in the mature stem of Dendrosicyos of2.36 lm. Consistent with the hypothesis ofCarlquist (1992) regarding the adaptive roleof bandlike thickenings on vessel walls of vinesis the absence of similar features in the maturestem of Dendrosicyos. The young stem ofDendrosicyos was found to have vessels thatare 64% wider than those of the mature stem,with slightly thicker walls, suggesting that thelong, slender branches are subject to similarconsiderations of limited stem area per unitleaf mass as are scandent plants. The very lowvessel density, likely one of the lowest everrecorded for a tree, suggests that vessel ele-ments are not of significance in support of thestem. Therefore, selection for increased me-chanical support is probably not the factorresponsible for narrowing of vessels withrespect to scandent ancestors, but rather it isselection for conductive safety.

2. Sclerified parenchyma adjacent to ves-sels. Significantly, no dimorphism in paren-chyma wall thickness was observed inDendrosicyos, with the vasicentric parenchymabeing similar in wall thickness to the conjunc-tive and ray parenchyma cells. With vesselsbeing less vulnerable to crushing or twistinginside of the thick trunk of Dendrosicyos,thick-walled parenchyma cells around vesselsappear to be of little adaptive value.

3. Vasicentric tracheids. Consistent withthis hypothesis is the observation that vasicen-tric tracheids are absent in Dendrosicyos.Relative to a vine, the large number of narrowvessels in Dendrosicyos seem much less likelyto embolize. Also, the large quantity of water-filled parenchyma and parenchyma-like fibersin Dendrosicyos could supply vessels withwater, preventing excessively low xylem pres-sures.

Features shared by scandent Cucurbitaceae

and Dendrosicyos. The remainder of the fea-tures discussed in the introduction appear tohave an adaptive role both in scandent plantsand in the pachycaul Dendrosicyos. The exam-ination of Dendrosicyos therefore does notprovide information that is consistent with or

that will falsify these hypotheses (but thatthese features are adaptive in pachycaul trees isnevertheless testable using comparative meth-ods):

4. Abundant parenchyma that buffers stemtorsion. Dendrosicyos stems do support verylarge quantities of conjunctive parenchyma,but the lower stems are certainly not subject totorsion to the degree that would be experi-enced by a vine. Parenchyma in Dendrosicyosseems to serve mainly in the adaptive role ofstorage of water and starch and mechanicalsupport via turgor pressure.

Therefore, examination of this trait in themature stem of Dendrosicyos reveals littleabout the role of parenchyma in scandentcucurbits. However, the terminal branches ofDendrosicyos, while not scandent, are flexibleand pendent and might be expected to besubject to some of the selective pressureaffecting vines, especially considering the ex-tremely high winds reported during the south-west monsoon (Miller and Guarino 1994,Gwynne 1968, reports that there are an aver-age of 23 gale days in July around Socotra).Indeed, it is much more common to find theplates of xylem in the young stem to beinterrupted by bands of apotracheal axialparenchyma. Such a configuration in theeasily-bent terminal branches seems likely toserve the same buffering function that suchcells are hypothesized to serve in scandentspecies. The distribution of axial parenchymain young stems thus appears consistent withthe hypothesis of Carlquist (1992).

5. Wide, tall rays. Extremely wide, tall raysare found in the stem of Dendrosicyos, but itseems unlikely that they are serving as zones ofweakness along which the stem fractures undertorsion for at least two reasons. First, thehighly interconnected vascular system forms adense net through the parenchymatous stem,probably strengthening the stem rather thancreating planes of weakness. Second, it seemsunlikely that the lower stem of Dendrosicyoswould experience sufficient torsion as to makefracture planes of adaptive value. The verywide and tall rays of Dendrosicyos must


therefore be serving a different purpose, mostlikely filling an adaptive role identical to thatof conjunctive parenchyma.

6. Starch storage in stem parenchyma. Twolines of reasoning suggest that storage ofphotosynthates in the stem of Dendrosicyos isadaptive in ways that may be similar to that inlianas. First, as in many lianas, leaf and floralproduction is highly seasonal and a massing ofnutrients would no doubt help fuel suchepisodic events. Also, as in many lianas, theroot system of Dendrosicyos appears to pro-vide limited volume for storage relative to stemvolume.

Co-opting of features of scandent plants to

pachycauls. The last three numbered catego-ries of features described above have appar-ently adaptive roles in both scandent plantsand in the pachycaul Dendrosicyos. Whereasabundant parenchyma and rays serve both aprotective and a storage function in scandentplants, it seems most likely that these cell typesare involved in storage only in Dendrosicyos.This storage probably fuels episodic floweringor vegetative events in the members of both lifeform classes. These cell types were present inscandent ancestors for reasons adaptive in thatbiomechanical context, but are seen in newroles in Dendrosicyos. Such features can besaid to be evolutionarily co-opted or exapted.

The abundance of ray and axial parenchy-ma in the stems of many vines seem to befeatures readily co-opted for life as a drylandpachycaul. If this is the case, then repeatedevolution of species with pachycaul habitsfrom scandent ancestors would be expected.This is probably the case in such genera asCyphostemma (Vitaceae), Adenia (Passiflora-ceae), Pyrenacantha (Icacinaceae), and othertaxa mostly from Africa and Madagascar.

The evolution of increased stature in islandplants has been associated with the invasion ofmoist habitats from drier ones (Givnish 1995).Carlquist (1974) noted that the main exceptionto this trend are arborescent stem succulents.Indeed, the selective forces driving the evolu-tion of arborescence in Dendrosicyos do notseem to be entirely addressed by any of the

prominent models proposed to explain evolu-tionary increase in stature. For example, theopen, rocky habitat of Dendrosicyos does notseem a probable context for evolutionaryheight increase driven by competition withneighbors for light, as is likely in denser,moister habitats. Alternatively, massive stor-age may be allocated to stems when rockysubstrates prevent extensive formation of un-derground tubers. Dendrosicyos and numerousother pachycauls occur largely on rock (e.g.Moringa, some Adansonia, etc.).

Systematics. Dendrosicyos is the first tax-on reported with successive cambia in Cucur-bitaceae and apparently in the Cucurbitales(cf. Carlquist and Miller 2001). Given the greatrange of morphological diversity in Cucurbit-aceae, that successive cambia have not beenpreviously reported may indicate the need formore extensive sampling rather than the rarityof the phenomenon.

The assumption that Dendrosicyos hasevolved from scandent ancestors has neverbeen challenged, but the relationships of thisspecies to the rest of the family are not clear.Dendrosicyos is currently included within thetribe Melothrieae (Jeffrey 1962), but there isevidence that this group is para- (or poly-)phyletic (S. Renner, pers. comm). It seemspossible that any one of the several cucurbitspossessing conical aboveground tubers andannual vines in the drylands of the westernIndian Ocean (e.g. Corallocarpus, Xerosicyos,Zygosicyos, etc.) could be the sister taxon toDendrosicyos.

Until such phylogenetic issues are resolved,the evolutionary polarities of the anatomicalfeatures in Dendrosicyos will remain hypothet-ical. For example, the ray and conjunctiveparenchyma of Dendrosicyos have convergedto a striking degree on similar cell form andprobably function presumably because ofselective pressure related to storage. The raysof Dendrosicyos seem clearly homologous tothe very broad rays of cucurbits described byprevious authors (see Carlquist 1992). In allthese species, rays are exclusively multiseriateand occur between plates of xylem. Whether


conjunctive tissue in Dendrosicyos is homolo-gous to banded axial parenchyma or othertissue will be more readily addressed once thesister taxon of Dendrosicyos is identified andthe diversity of anatomical modes in Cucur-bitaceae more extensively sampled.

Summary. The absence in Dendrosicyos ofvery broad, thick-walled vessels with band-likereinforcements, lignified parenchyma in prox-imity to vessels and vasicentric tracheids offersa comparative confirmation of the hypothesisthat these features are adaptive in lianas.Abundant parenchyma, very large rays, andstarch storage in the stem appear to be featuresthat were adaptive in the scandent ancestors ofDendrosicyos and have been evolutionarily co-opted to serve adaptive roles in a new biome-chanical context. It is hypothesized that thesefeatures facilitate the evolution of the pachy-caul habit from vines and that this is the casenot only in Dendrosicyos but also in a diversityof families. These evolutionary hypothesesdepend on their phylogenetic assumptions;the assumptions made here should be testedwith broad-scale phylogenetic reconstructionwithin Cucurbitaceae.

For live material and its care, I am grateful toTim Metcalf and Joe Olson. I am indebted toSherwin Carlquist for help with everything fromlaboratory equipment to patient discussions andinspiration. Thank you to Susanne Renner andAmanda Neill for discussions regarding cucurbitphylogenetics. Josefina Barajas Morales and Cali-xto Leon Gomez kindly helped with many labora-tory issues. Berenit Mendoza Garfias is gratefullyacknowledged for skillful assistance with the SEM.Frank Ewers, Ken Sytsma, and an anonymousreviewer provided many helpful suggestions.

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Address of the author: Mark E. Olson (e-mail:[email protected]), Instituto de Biolog-ıa, U.N.A.M., Departamento de Botanica, Circuitoexterior s/n, Ciudad Universitaria, Copilco,Coyoacan, A.P. 70-367 Mexico, Distrito Federal,C.P. 04510 Mexico.


stem and leaf anatomy of the arborescent cucurbitaceae ...ww.explorelifeonearth.org/people/olson...

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