Aliso: a Journal of Systematic and Evolutionary Botany Wood Anatomy of Cercidium (fabaceae), with Emphasis on Vessel Wall Sculpture Wood Anatomy of Cercidium (fabaceae), with Emphasis on Vessel Wall Sculpture

ABSTRAcr Quantitative and qualitative data are presented for wood anatomy of the seven species of Cercidium (including two subspecies of C. floridum as well as the hybrid C. x sonorae) currently recognized. Data on wood of Parkinsonia are presented for purposes of comparison. Vessel walls of Cercidium show unusual sculpture: coarse excrescences termed verrucae here, crateriform pits, and grooves interconnecting pit apertures. These plus crystal distribution, presence of septa in fibers, pit diameter, presence of vasicentric tracheids, and presence of diagonal vessel aggregations are probably species distinctions to various degrees. The crystal-bearing fibers of Cercidium, some of which have very thin walls, exemplify fiber dimorphism. Wood of Parkinsonia aculeata is amply distinct from that of Cercidium. Vessel element dimensions and density in Cercidium do not indicate exceptional xero-morphy for woods, perhaps because some species grow in desert washes, perhaps because other features (leaflessness, thick cuticle on stems) reduce transpiration. Vessel wall sculpture, vasicentric tracheid presence, and degree of vessel grouping are considered likely indicators of wood xeromorphy within the genus, however.


INTRODUCTION
The report by Cozzo (1953) of crateriform pits in vessels of certain species of Cercidium led me to make preparations of the wood in order to illustrate these structures by means of SEM in a book (Carlquist 1988).While examining sections of Cercidium wood under a light microscope, I discovered that protuberances could be found on vessel walls of some species.These structures provided an unusual opportunity for study by means of SEM, because adequate resolution of them could be achieved only by that means.
The diversity of wall sculpture in Cercidium was sufficient to make examination of all species desirable.Therefore, wood samples of all currently recognized taxa were obtained a!ld sectioned.According to the treatments ofJohnston (1924) and Carter (1974), Cercidium consists of seven species, one with two subspecies (see Table 1 ); a natural hybrid (C.x sonorae Rose & Johnston) is also known.The wood preparations of Cercidium revealed other anatomical features of interest, so a survey of wood anatomy of the genus has been undertaken here.
The highly disjunct nature of the genus prevented collection of mature wood samples for all taxa, so some woodier portions of herbarium specimens were utilized.The latter samples very likely have vessel sculpture representative for these species, but such features as vessel element length and ray dimensions represent less mature conditions than in the species for which larger wood samples were available.The presence of different degrees of maturity in wood patterns in various samples is regarded as of interest in its own right, and for this reason, a figure for stem diameter is given for each sample studied (Table 1, column 1).Material of C. x sonorae was sufficient for study of vessel wall characteristics, but reliable data on other wood features could not be obtained.
The occurrence of certain types of vessel sculpture in dicotyledons may represent adaptation to dry ecological conditions (Carlquist 1982).In addition, certain other features of wood anatomy represented in Cercidium have been cited as indicative ofxeromorphy: narrow vessel diameter (Carlquist 197 5), increased degree of vessel grouping (Carlquist 1984), presence of vasicentric tracheids (Carlquist 1985), and presence of diagonal vessel aggregations (Carlquist 1987).These and other features of wood anatomy possibly related to xeromorphy are analyzed here.The species of Cercidium can all be said to characterize desert sites or areas of equivalent dryness.The geographical ranges ofthe species are as follows (data from Johnston 1924 and Carter 197 4) As a generalization, one could say that Cercidium favors low-elevation deserts that tend to receive summer rainfall.There is, however, a considerable range of climates or at least microclimatic situations (e.g., underground water availability) within the areas occupied by Cercidium.Other than the pervasive theme oflimited rainfall, generalizations are not useful; one notes that equatorial as well as temperate deserts are occupied by the genus.Although wood features can be analyzed with respect to ecology, one must remember that other vegetative features can play significant if not overriding roles in the water economy of desert plants.In this regard, the leafless habit during the dry season and the presence of very thick cuticle on the photosynthetic bark surfaces are noteworthy.
The diversity of vessel wall sculpture as well as the range of other wood anatomical features in Cercidium invites comparison to the taxonomic system at the species level.Cercidium has been included within Parkinsonia by some authors, segregated by others (see Carter 1974 for a history).A species of Parkinsonia, P. aculeata L., has been included in the present study in order to determine if differences between Parkinsonia and Cercidium wood support recognition of the two genera.
The presence of chambered crystals in some libriform fibers of Cercidium wood leads to examination of an interesting phenomenon, fiber dimorphism (Carlquist 1958(Carlquist , 1961)), which has not hitherto been reported in Fabaceae.Wood of Cercidium contains interesting forms of fiber dimorphism that are analyzed here and discussed in relation to probable instances elsewhere in caesalpinoid Fabaceae.

MATERIALS AND METHODS
Wood samples derived from herbarium specimens are mostly or more than twig thickness (Table All wood samples were available in dried form.Samples were boiled in water and stored in aqueous 50% ethyl alcohol.Despite this treatment, starch remained in some samples.Woods were sectioned on a sliding microtome.Sections were stained with a safranin-fast green combination in order to differentiate septa in libriform fibers and pits on vessel walls.Unstained sections dried between glass slides to assure flatness were used for SEM studies.SEM photomicrographs were obtained with the lSI WB-6 at the Rancho Santa Ana Botanic Garden.Macerations were prepared with Jeffrey's Fluid and stained in safranin.The quantitative data in Table 1 were based on 25 measurements each except for vessel wall thickness, vessel-vessel pit diameter, libriform fiber wall thickness, and ray cell wall thickness, in which typical conditions were measured.Vessel diameter represents lumen diameter and was measured at the widest point of a vessel as seen in transection.

ANATOMICAL RESULTS-CERCIDIUM Growth Rings
All species could be said to be semi-ring porous because growth rings are present but are not well marked (Fig. 1, 9).By noting the date on which stems were collected, and noting the diameter of the most recently formed vessels on stems, one can demonstrate that earlywood of growth rings is initiated in response to water availability rather than increase in temperatures at the end of a cold season.
Vessel diameter (Table 1, column 3) might be expected to be inversely related to vessel density (column 2), and in Cercidium, a rough inverse correlation (with several exceptions) can be seen by comparing the two columns.One should note that the range in mean vessel diameter (24 ,.,m in C. andicola, Fig. 1, to 43 ,.,m in C. floridum subsp.jloridum) is not great.Age of stem may be related to mean vessel diameter: the two collections with the greatest vessel diameter (Table 1, column 3) have the greatest stem diameter.Had wood from larger stems been available, very likely wider vessels would have been reported.
With respect to vessel grouping, the collections of Cercidium mostly show moderate figures for number of vessels per group (Table 1, column 4), as in C. andicola (Fig. 1) and C. australe (Fig. 9), but C. microphyllum and C. texanum (Fig. 27) have much higher figures.The latter two species are also the only ones in which diagonal vessel aggregations (Fig. 27) and vasicentric tracheids occur.In all other species, one can say that vessel grouping tends to take the form of short radial multiples (Fig. 1, 9); solitary vessels are common in the species with a smaller degree of vessel grouping.
Vessel element length (Table 1, column 5) is notably short in Cercidium.The collections with the longest mean vessel element lengths are those in which sample diameter is greatest.
Vessel wall thickness (Table 1, column 6) is also greatest in the collections with the largest sample diameter.

Vessel Wall Sculpture
Characteristically some Cercidium species form conspicuous excrescences on the inside of vessel walls, whereas others do not.In the species in which they occur, the excrescences range from inconspicuous (Fig. 4) to densely present (Fig. 5,6,11,12,13,24).The term "vesturing," proposed by Ohtani et al. (1984) for structures sometimes known as warts, is not regarded as applicable here because the excrescences on Cercidium vessel walls are so much larger; they also tend to be angular or otherwise irregular in shape (Fig. 5, 6).A number of adjectives (e.g., "budlike, coralloid") are used by other authors (Parameswaran andGomes 1981, Castro 1988).The term "verrucae" is consequently proposed for the structures in Cercidium.All species of Cercidium have vestured pits (Fig. 11,16,21,32), but verrucae do not appear to be merely vestures extending onto the inner vessel surface.Vesturing can be present without any verrucae on the vessel surface (Fig. 16).
Grooves on vessel walls are also discussed here.By "grooves" I connote the presence of shallow depressions that interconnect pit apertures laterally.Grooves vary in length and depth, as shown in Figure 8, 13, 15, and 26.The concept of helical thickenings is not applicable here, because thickenings do not account for the presence of grooves (see also illustrations of Parkinsonia grooves, Fig. 29, 30).Grooves such as those found in Cercidium and Parkinsonia account for adoption of the term "helical sculpture" as an inclusive term in dicotyledon woods, with grooves and thickenings representing distinctive respective types (Carlquist 1988).The crateriform pits reported by Cozzo (1953) in Cercidium are also considered a form of vessel wall sculpture.
In C. andicola (Fig. 4-8), a wide range of helical sculpture may be found.
Cercidium australe is the most distinctive species of the genus with respect to vessel wall sculpture.The crateriform pits (Fig. 11, 12) have collarlike or cylindrical form and are much higher forms of relief than the relatively low crateriform pits of C. praecox (Fig. 22) or C. x sonorae (Fig. 25).The nature of these pit orifices is perhaps most clearly shown in Fig. 12, lower portion.In addition, the verrucae of C. australe are always united to various degrees into polygonal networks (Fig. 11, 12) or similar aggregations coarse enough to be seen under the light microscope.Grooves were not observed on vessel walls of C. australe.
In C.jloridum subsp.jloridumthere is a wide range in sculpture ofvessel walls.
Verrucae are present in some vessel walls, where they range from minimal (Fig. 13, 14) to absent in others (Fig. 15).Various degrees of groove presence may be found, ranging from very deep and furrowlike (Fig. 13) to shallow (Fig. 15) to minimal (Fig. 14).
In the collection studied of C. floridum subsp.peninsulare (Fig. 16), neither verrucae nor grooves are present on vessel walls; pit apertures range from circular to oval in outline, and vestures are clearly visible within pits.Vessel walls of C. macrum (Fig. 1 7) may be smooth or shallowly grooved; walls bear sparsely or densely what must be called vestures (or warts), less than half the diameter of the structures termed verrucae here.These vestures do not appear to be artifacts, because they seem more uniform in size and presence than droplets of resinlike materials would be.
In C. microphyllum (Fig. 18), vessel walls were observed to be uniformly smooth, with no appreciable degree of groove presence.For C. praecox, four SEM photographs have been selected to show the range of vessel wall sculpturing (Fig. 21-24 ).Verrucae may be sparse, with inconspicuous striate extensions of bases (Fig. 21 ).Verrucae may be moderately conspicuous but separated from each other (Fig. 22).On portions of walls free from pits, verrucae may be fused into striate structures spaced closely (Fig. 23) or coralloid aggregations (Fig. 24).The laterally striate or laterally aggregated pattern is very common in C. praecox.Pits have orifices very slightly (Fig. 21) to more moderately (Fig. 22) raised into crateriform shape; these apertural extensions are not so conspicuous as in C. australe, however.Grooves interconnecting pit apertures were not observed in C. praecox.Cercidium x sonorae has vessel walls in which verrucae cannot be said to be present, but the striate patterns like those of C. praecox are present inconspicuously to markedly (Fig. 25 exemplifies the latter condition).Grooves were not observed.Crateriform pits may be seen in C. x •sonorae vessels (Fig. 25), but do not have markedly upraised orifices.
In C. texanum (Fig. 26), vessel walls do not bear any verrucae.Grooves are marked (Fig. 26) or nearly absent, depending on which portion of a vessel wall one views.

Libriform Fibers and Dimorphic Fibers
Libriform fiber lengths (Table 1, column 8) parallel vessel element lengths, and the comments above on vessel element length apply here.Libriform fiber wall thickness is relatively uniform in the genus (Table 1, column 9); note should be taken that tension wood fibers have been excluded from these observations.
Examination of crystal-bearing fibers of Cercidium yields a diversity of appearances.As illustrated for C. andicola (Fig. 3), crystal-bearing fibers commonly are subdivided into chambered crystals, single rhomboidal crystals separated by thin primary walls.However, libriform fibers may bear crystals along only part of their length (Fig. 3, fiber one-third the distance from the left edge of photograph).Portions of such libriform fibers that are devoid of crystals are separated from the crystal-bearing portions by lignified walls similar to those in axial parenchyma strands.Sections and macerations show that a large proportion of crystal-bearing fibers have walls that are very thin, much thinner than one would expect in ordinary libriform fibers; walls of intermediate thickness may also be found on crystal-bearing fibers.The fibers that contain chambered crystals are wider and shorter than ordinary libriform fibers of Cercidium.If devoid of crystals, these shorter fibers may be subdivided by one (rarely two) walls like those of axial parenchyma strands or they may not be subdivided.
The shorter (and in Cercidium, often crystal-bearing) libriform fibers tend to occur in groups, as suggested by their abundance in the tangential section of Fig. 3, and these groupings or patches occur at various points within a growth ring.
Dimorphic fibers occur in all collections of Cercidium examined here, but only a fraction of the shorter fibers are crystal bearing.Crystal-bearing fibers were observed to be more abundant in C. andicola (Fig. 3), C. australe, C. microphyllum, and C. praecox.Crystal-bearing fibers occur mostly at the juncture between latewood and earlywood.Crystal-bearing fibers were not observed in C. jloridum subsp.floridum or C. macrum, and they were observed to be very scarce in C. j/oridum subsp.peninsulare and C. texanum.Several crystals per cell (chamber) were seen in some crystal-bearing fibers of C. texanum.
Starch was observed to be abundant in alllibriform fibers of C. jloridum subsp.jloridum, C. microphyllum (Fig. 20), and C. praecox.Absence of starch in the other samples may be due to season of collection or method of drying or preparation rather than to any inability to form starch.The material of C. floridum subsp.jloridumshowed starch somewhat less common in the short libriform fibers than in the long ones.
Reaction wood (tension wood) libriform fibers occur commonly in Cercidium (Fig. 1, 10, dark patches).Stems in the present study devoid of tension wood probably are main stems or upright stems rather than diagonally leaning branches.

Axial Parenchyma
Axial parenchyma occurs in the form of sheaths one to three (or even four) cells in thickness in all taxa; most commonly the sheaths are two cells in thickness.Strands of axial parenchyma are composed of two to six cells, most commonly four cells.Starch was observed in axial parenchyma of C. floridum subsp.floridum and C. microphyllum (Fig. 19).Crystals were not observed in axial parenchyma in any of the collections.

Vasicentric Tracheids
Small numbers of vasicentric tracheids were seen in sections (presence confirmed in macerations) in C. microphyllum and C. texanum.They occur intermixed with vessels in diagonal vessel aggregations (Fig. 27).
Uniseriate ray height (Table 1, column 10) and multiseriate ray height (column 11) are rather small compared to values found in dicotyledons at large.Mean uniseriate ray height is rather uniform throughout the genus (Table 1, column 1 0); the lower height in C. jloridum subsp.floridum and C. microphyllum may relate to the fact that stems studied for these species are larger, and most taller rays that may have begun as uniseriate rays near the pith would have been converted to multiseriate rays in outer portions of the stems.Multiseriate rays are also relatively uniform in height (Table 1, column 11) except for those of C. floridum (both subspecies).
Ray cell walls (Table 1, column 13) range in mean thickness from 0.7 to 2.3 ~m.Some borders were observed on ray cell pits, especially on tangentially oriented walls, in both species with thick (C.andicola) and thin (C.floridum subsp.floridum) walls.
Crystals were observed in ray cells in all species except C. microphyllum.They were observed only in a primary ray in C. macrum.Crystals typically are not found at random in ray cells where present, but at the margins of growth rings, often in the radially shorter cells produced in latewood portions of rays.Crystals are rhomboidal and formed singly per cell where present.

Storying
No evidence of storying was observed in Cercidium wood.Only qualitative data are given here because only one species of Parkinsonia has been studied.In the comparison between Cercidium and Parkinsonia, qualitative data appear to be more important than quantitative data.In P. aculeata, vessels are solitary or in short radial multiples.Vessels have simple perforation plates.Vessel walls have narrow grooves interconnecting pit apertures (Fig. 29,30).Vessel-vessel pits are alternate, circular to oval in shape but often with polygonal outline (Fig. 30).The pits are vestured (Fig. 31) in pit cavities.Vessel walls bear no vestures or verrucae (Fig. 29).Libriform fibers are monomorphic, nonseptate, and devoid of crystals.Axial parenchyma is both initial (narrow bands of various width present to various degrees in earlywood) and para tracheal (sheaths 2-4 cells wide around vessels or vessel groups).Rays are Homogeneous Type I, with multiseriate and uniseriate rays about equally abundant.Crystals are present in axial parenchyma cells, but not in ray cells.Wood is nonstoried.

MORPHOLOGICAL CONCLUSIONS
Vessel wall sculpture of Cercidium shows a remarkable range.The crateriform pits of C. australe have much taller orifices than do the crateriform pits of C. praecox or C. x sonorae.These crateriform pits were figured by Cozza (1953), and the SEM studies in the present paper yield shapes similar to his drawings.
The coarse vessel wall excrescences of Cercidium are termed verrucae here because they are much larger than the structures termed vesturing by Ohtani et al. (1981) or the structures often termed warts in conifer tracheids.A comparable type of excrescence has been figured in Ligustrum lucidum Ait.(Oleaceae) by Parameswaran and Gomes ( 19 81 ), who term the structures "budlike protrusions" from the vessel wall.More pertinent is the report of structures comparable to the verrucae of Cercidium in another genus ofFabaceae, Prosopis (Castro 1988).Warts on vessel walls of certain Fabaceae mentioned by Schmid and Machado (1964) are smaller in size.The diversity of verrucae in Cercidium is surprising, and is summarized under Taxonomic Conclusions below.
In addition, attention is called to helical sculpture in vessel walls of Cercidium.Helical sculpture has all too often been equated with helical thickenings in the literature on wood anatomy.Cercidium does have, intermittently, striate vessel wall thickenings, but these are considered here merely a form oflow fused verrucae.In addition, grooves interconnecting pit apertures occur widely in Cercidium as well as in Parkinsonia; these do not involve any form of thickening in the vessel wall.Grooves in vessel walls occur in a number of dicotyledons, notably many Asteraceae.Occurence of grooves in vessel walls has been reviewed elsewhere (Carlquist 1988).The occurrences are doubtless more common than present literature, which has neglected them and stressed helical thickenings instead, would tend to indicate.
Fiber dimorphism is a phenomenon described earlier (Carlquist 1958(Carlquist , 1961)); it has been discovered in a number of taxa (for a review, see Carlquist 1988).Fiber dimorphism can, in some dicotyledons, be regarded as a way of achieving a parenchymalike function for a portion of the libriform fibers in a wood.In Cercidium, libriform fibers are apparently living fibers in all species, with a prominent starch storage function in some if not all species.Therefore the dimorphism of Cercidium libriform fibers adds a different function: crystal accumulation.The chambered crystals of Cercidium libriform fibers are noteworthy not merely because they occur in fibers that are shorter and wider, but because they occur in fibers with a range of wall thickness from thick, as is normal for libriform fibers, to thin, like that of a primary wall.This range prevents one from considering the crystal-bearing fibers of Cercidium, even when thin walled, as a form of axial parenchyma.Axial parenchyma cells in Cercidium are paratracheal, as is very common in Fabaceae, whereas the crystal-bearing fibers are diffuse in distribution.Diffuse axial parenchyma is not characteristic ofwoods ofFabaceae.Chambered crystals like those of Cercidium have been reported by Metcalfe and Chalk (1950) in other caesalpinoid genera: Bauhinia p.p., Bussea, Copaifera, Cryptosepalum, Englerodendron, Hardwickia, Humboldtia, Peltophorum, Saraca, Schotia, Sindora, and Tamarindus.In addition, I have observed them in Cassia armata Wats.and Castanospermum austra/e Cunn.(new records).As noted above, they do not occur in Parkinsonia aculeata.The thin-walled septate fibers mentioned by Metcalfe and Chalk (1950) for Poeppigia may be a kind of fiber dimorphism, as may be the fusiform parenchyma cells they mention for Dinizia, Kingiodendron, Macrolobium, and Pterogyne.With more exploration of wood of caesalpinoid legumes and perhaps other legumes as well, we may expect further reports of fiber dimorphism.
Vasicentric tracheids occur in two species of Cercidium, and these cells are associated with the formation of diagonal aggregations of vessels as seen in transection.The association between vasicentric tracheids and diagonal vessel aggregations has been reviewed elsewhere (Carlquist 1987), and Fabaceae are among the instances in which this phenomenon occurs.
In all of the Cercidium species for which larger wood samples were available, multiseriate rays average more than three cells wide at their widest points.We may assume that rays of this width would be found had larger-sized wood samples ofthe remaining species been available.Vessel element length and libriform fiber length also tend to be appreciably longer in the species for which larger wood samples were available.This is not surprising, because in typically woody dicotyledons, fusiform cambial initial length rises with age (Bailey and Tupper 1918).In the samples studied, vessel wall thickness also appears to increase with age.

TAXONOMIC CONCLUSIONS
Although the number of collections available was limited, one may cite characteristics of the taxa of Cercidium tentatively.
Cercidium andicola: vessels moderately grouped; vessel-vessel pits medium sized; vessel walls often with shallow grooves; verrucae on vessel walls dense, often aggregated into striate patterns; crystals in both fibers and ray cells; septate fibers present.
Cercidium australe: vessels moderately grouped; vessel-vessel pits medium sized, crateriform, with elongate cylindrical extensions; vessel walls not grooved; verrucae in vessel walls aggregated into polygonal groupings; crystals in both fibers and rays; septa not seen in fibers.
Cercidium floridum subsp.floridum: vessels little grouped; vessel-vessel pits large; vessel walls with grooves or without grooves; verrucae if present not ag-gregated, either present in grooves or walls smooth; crystals present in rays but not seen in fibers; fibers prominently septate.
Cercidium macrum: vessels moderately grouped; vessel-vessel pits medium sized; vessel walls not grooved, smooth or with small wartlike verrucae; crystals observed only in a primary ray; septa not seen in fibers.
Cercidium microphyllum: vessels in moderately large groups, diagonal vessel aggregations that include vasicentric tracheids present; vessel-vessel pits medium sized; vessel walls not grooved, smooth; crystals seen in fibers but not in rays; septa not seen in fibers.
Cercidium praecox: vessels in moderately small groups; vessel-vessel pits medium sized, crateriform; vessel walls not grooved but with verrucae dense to very dense, forming striate patterns when sparser, coralloid aggregations when denser; crystals present in some ray cells and fibers; septa not seen in fibers.
Cercidium texanum: vessels in large groups, forming diagonal aggregations in which vasicentric tracheids are present; vessel-vessel pits large; vessel walls grooved or lacking grooves, smooth; crystals present in fibers (sometimes more than one crystal per chamber) and ray cells; septa not observed in fibers.
The above tabulation illustrates a number of probable specific distinctions.Vessel wall sculpture distinguishes some species, although note must be taken that within these species, the sculpture shows a range of expressions.Clear groupings of species in the sense of subgenera are not evident on the basis of wood features alone unless one wishes to stress particular features (e.g., presence of crateriform pits).The data from wood anatomy support the decision by Carter  (1974) to treat as C. peninsulare subsp.peninsulare a taxon formerly considered a species.
Although wood of C. x sonorae was not analyzed with respect to all features, vessel walls of this hybrid do show intermediacy between those of its putative parents.The crateriform pits characteristic of C. praecox are present in C. x so no rae, but verrucae are present in reduced form, like the striate patterns beneath lines of verrucae in C. praecox.
Parkinsonia has traditionally been considered to be close to Cercidium, so close that union of the two genera has been advocated by some.Hybrids between Cercidium and Parkinsonia are known to exist (Carter 1974;Carter and Rem 197 4).Wood anatomy speaks clearly for separation of Cercidium and Parkinsonia.In Parkinsonia, the following features are present, with contrasting character states in parentheses: vessel walls with long narrow grooves interconnecting pit apertures (grooves where present wide, shallow, relatively short); vessel walls smooth (vessel wall sculpture where present consisting of verrucae; crateriform pits in some species); libriform fibers monomorphic (libriform fibers dimorphic, some of the wider and shorter with chambered crystals); axial parenchyma paratracheal and initial (axial parenchyma paratracheal only).
With respect to wood anatomy, resemblances between Cercidium and Parkinsonia are no closer than resemblances between Cercidium and some other caesal-pinoid genera (e.g., Cassia has dimorphic crystal-bearing libriform fibers like those of Cercidium ).
ECOLOGICAL CONCLUSIONS If one computes the value "Mesomorphy" (Table 1, column 14), one finds that the values are higher than are typical for desert shrubs in southern California (Carlquist and Hoekman 1985).However Mesomorphy values that are only moderately low also characterize other desert legumes, such as Prosopis (Carlquist and Hoekman 1985).The tendency ofthe larger shrubby legumes of desert regions to grow in streambed or wash habitats and the tendency of desert legumes to have extensive roots systems were cited as reasons for their moderate Mesomorphy values (Carlquist and Hoekman 1985).In the genus Cercidium, C.jloridum subsp.j/oridum occurs in washes, canyons, and similar habitats with probable protracted water availability (Carter 1974); Carter says of C. j/oridum subsp.j/oridum that "its water requirements appear to be greater than those of C. microphyllum."This idea is supported by the Mesomorphy values of Table 1 (column 14).
Ifhigh degree of vessel grouping is indicative ofxeromorphy, as claimed earlier (Carlquist 1984), and ifvasicentric tracheid presence is indicative ofxeromorphy (Carlquist 1985), especially when coupled with diagonal aggregations of vessels (Carlquist 1987), the woods of C. microphyllum and C. texanum are notably xeromorphic, and this appears correlated with relatively dry habitats as cited by Carter (1974).
The habitats of some Cercidium populations seem to be just as dry as those of desert shrubs that have lower Mesomorphy values according to the Carlquist and Hoekman (1985) computations.A possible explanation is the modifying effect of vegetative features of Cercidium: short duration ofleaves, with green stems forming minimally vulnerable photosynthetic surfaces.The epidermis of stems of Cercidium have extraordinarily thick cuticles, usually much thicker than those illustrated by Gibson (1983) for C. f/oridum.Characteristics of the photosynthetic stems may therefore be a prime tool in water management for Cercidium, with other features subsidiary.
One should entertain the possibility that vessel wall sculpture may be correlated with xeromorphy (Carlquist 1982).If this correlation is valid, the various forms of vessel wall sculpture in Cercidium (vestured pits, crateriform pits, verrucae, grooves) represent forms of adaptation to dry conditions by increasing the surface area of the vessel wall.That all species do not have degrees of vessel sculpture in exact proportion to dryness of habitat does not necessarily invalidate the possible function hypothesized: vessel grouping and presence of vascular tracheids, for example, may be highly effective features in conferring water column redundancy and safety in C. microphyllum and C. texanum, whereas the dense verrucae of C. andicola, C. australe, and C. praecox may be mechanisms for assuring watercolumn integrity for those respective species.These latter three species may occupy habitats dry even for Cercidium, judging from the data given by Johnston (1924), Shreve andWiggins (1964), andCarter (1974).
: C. andicola Grise b., S. Bolivia and adjacent N. Argentina (Jujuy); C. australe Johnston, central Argentina and adjacent Paraguay; C. jloridum Benth.ex Gray subsp.jloridum, desert areas of California and Arizona plus Sonora, Mexico; C. floridum subsp.peninsulare (Rose) Carter, southern half of Baja California, Mexico; C. macrum Johnston, Tamaulipas and Nuevo Leon, Mexico; C. microphyllum (Torrey) Rose & Johnston, Colorado desert areas of eastern California and southern Arizona plus areas adjacent to the Gulf of California in Baja California and Mexico, Sonora; C. praecox (R. & P.) Harms, southern half of Baja California, eastern and central Sonora plus southern Mexico, Dominican Republic, Leeward Islands and adjacent Venezuela, and extreme middle Western Peru; C. x sonorae, areas of overlap between C. microphyllum and C. praecox in Baja California and Sonora; C. texanum A. Gray, Rio Grande Plain northwest to Val Verde Co., southward to Starr Co., Texas, and into Mexico as far as Coahuila and Nuevo Leon.

Table 1 .
Wood characteristics of

1, column 1 )
. I am particularly grateful to Dr. Juan Hunziker for the material of C. andicola, without which representation of all taxa could not have been achieved.Xylarium samples are indicated in Table1by the abbrevi-