IAWA Journal, Vol. 19 (2), 1998: 191-206

WOOD AND BARK ANATOMY OF CARICACEAE; CORRELATIONS WITH SYSTEMATICS AND HABIT by Sherwin Carlquist

Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, CA 93105, U.S.A.

SUMMARY

Wood and bark anatomy are described for four species of three genera of Caricaceae; both root and stem material were available for Jacaratia hassleriana. Wood of all species lacks libriform fibers in secondary xylem, and has axial parenchyma instead. Cylicomorpha parviflora has paratracheal parenchyma cells with thin lignified walls; otherwise, all cell walls of secondary xylem in Caricaceae except those of vessels have only primary walls. Vessels have alternate laterally elongate (pseudo­ scalariform) pits on vessel-vessel interfaces, but wide, minimally bor­ dered scalariform pits on vessel-parenchyma contacts. Laticifers occur commonly in tangential plates in fascicular secondary xylem, and rarely in xylem rays. Proliferation of axial parenchyma by zones of tangential divisions is newly reported for the family. Bark is diverse in the spe­ cies, although some features (e.g., druses) are common to all. Wood of Caricaceae is compared to that of two species of Moringaceae, recently designated the sister family of Caricaceae. Although the wood and bark of Moringa oleifera, a treelike species, differ from those of Caricaceae, wood and bark of the stem succulent M. hildebrandtii, the habit of which resembles those in Caricaceae, simulate wood and bark of Caricaceae closely. Counterparts to laticifers in Moringaceae are uncertain, how­ ever. Phloem fibers of Caricaceae form an expansible peripheral cylin­ der of mechanical tissue that correlates with the stem succulence of most species of Caricaceae. Key words: Anomalous secondary thickening, cambial variants, Cap­ parales, Caricaceae, glucosinolate families, parenchyma proliferation, succulents, systematic wood anatomy.

INTRODUCTION

In recent years, Caricaceae have been transferred from Violales (Cistales or Parietales of earlier authors) to Capparales. Although Dahlgren (1975) pioneered an expanded version of Capparales, the order as newly defined with the aid of DNA evidence con­ tains fifteen families. The history of the expansion of Capparales so as to include Caricaceae and other families previously placed elsewhere has been well rendered by Rodman et al. (1996) and need not be repeated here. Capparales as presently consti-

Downloaded from Brill.com10/07/2021 06:52:03AM via free access 192 IAWA Journal, Vol. 19 (2), 1998 tuted are all plants that contain glucosinolates except for Koeberliniaceae, which lack them; glucosinolates occur outside the new Capparales only in Drypetes of the Euphor­ biaceae (Rodman et al. 1996). Comparison of woods of the expanded Capparales with recent cladistic trees of the order is a goal of the series of papers of which the present paper is a part. Habit ap­ pears to have diversified rapidly and dramatically in the glucosinolate families, and thus this order gives us an opportunity to examine the comparative roles played by phyletic relationship and by similarity or difference in habit where wood anatomy is concerned. Data are available on Akaniaceae (Carlquist 1996), Brassicaceae (Carlquist 1971), Bretschneideraceae (Carlquist 1996), Capparaceae (Metcalfe & Chalk 1950; see also Gregory 1994), Koeberliniaceae (Gibson 1979), Limnanthaceae (Carlquist & Donald 1996), Resedaceae (Carlquist 1998), Tovariaceae (Carlquist 1985), and Tropaeolaceae (Carlquist & Donald 1996). In Caricaceae, Fisher (1980) has contrib­ uted detailed descriptions of the wood and bark of Carica papaya L.; consequently, that species is not included in the present survey. Other data may be found in Metcalfe & Chalk (1950) and some references cited by Gregory (1994). A summary paper is planned when sufficient data become available for wood of Capparales. Recent studies (e.g., Rodman et al. 1996) pair Moringaceae closely with Caricaceae using both macromorphology and evidence from DNA. This treatment differs mark­ edly from previous arrangements, because Moringaceae was considered close to Bras­ sicaceae but Caricaceae were placed in Violales near such families as Passifloraceae and Tumeraceae. Consequently, wood and bark of two species of Moringaceae, Morin­ ga hildebrandtii Engl. and M. oleifera Lam., have been included in the present study. Wood data on Moringa peregrina (Forssk.) Fiori are given by Fahn et al. (1986); the woods of M. oleifera and M. peregrina are very similar. Mark Olson, currently col­ lecting woods and other materials of Moringaceae for a doctoral thesis, will likely make available much additional information on wood anatomy of Moringaceae, which will thereby provide a much more complete picture of the family. The diverse habits of Moringaceae are likely related to different modes of xylem structure, and those species of Moringa that are stem succulents, as are Caricaceae, may show a greater degree of resemblance to Caricaceae in wood anatomy. Because secondary xylem of Caricaceae is so highly parenchymatous, attempts to dry 'wood samples' using ordinary techniques are not successful, and the secondary xylem usually rots. The fiber-rich secondary phloem does not deteriorate so markedly under these circumstances. Thus, most specimens of Caricaceae to be found in xylaria are samples of secondary phloem with little or no secondary xylem. Samples of sec­ ondary xylem of Caricaceae should be preserved in a liquid fixative. The prominence of secondary phloem fibers in stems of Caricaceae as well as the fiber-free nature of the secondary xylem is very likely related to the distinctive habit of the family: a succulent or semisucculent sparsely branched shrub or tree. Stem bases are notably swollen in Carica quercifolia (St. Hil.) Solms-Laubach. Although species of Carica and Cylicomorpha are tall enough to qualify as trees, Jacaratia and Jarilla are smaller. Jarilla, a monotypic genus restricted to Mexico, is herbaceous.

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One genus occurs in Africa: Cylicomorpha, with two species (one included in the present study). The three remaining genera (which together contain about 38 species) range from Bolivia to Chihuahua and Sonora, Mexico (Gentry 1942; Heywood 1978). Carica and Jacaratia are included in the present study, but no material of Jarilla was available.

MATERIALS AND METHODS

Material of Cylicomorpha parviflora Urban was obtained from a xylarium specimen (SJRw 27462, colI. by H. J. Schlieben in Tanzania) provided by the U. S. Forest Prod­ ucts Laboratory, Madison, Wisconsin. Carica pentagona Heilborn (documented by the specimen Carlquist 8179, SBG) was cultivated in my garden; the habit of the plant is similar to that of C. papaya. Stems of C. quercifolia were obtained from the University of California at Santa Barbara. This species is cultivated as an ornamental because of the swollen basal stem, but cylindrical upper stems were employed for this study. Plants of Jacaratia hassleriana Chodat were cultivated in my garden; these plants, which were about 1.5 dm tall, were obtained from Arid Lands Greenhouses, Tucson, Arizona. The stems in this species are not notably succulent, but they grade into markedly succulent roots. Material of Cylicomorpha parviflora was boiled in water and stored in 50% aqueous ethanol. For the remaining species, portions from living plants were selected and fixed in 50% aqueous ethanol. These methods were applied also to the two species of Moringa, both of which were obtained from Arid Lands Greenhouses and cultivated in my garden (plants about five years old, with stem bases 2 cm in diameter). Segments to be sectioned were soaked in ethylene dia­ mene, infiltrated, and embedded in paraffin, a method described in Carlquist (1982). Paraffin sections were stained in a safranin-fast green combination corresponding to Northen's modification of Foster's tannic acid-ferric chloride method (Johansen 1940). Macerations were prepared with Jeffrey's Fluid and stained with safranin. Measurements of vessel diameters are lumen diameters, measured as a mean be­ tween wide and narrow diameters. Vessel grouping is measured as a solitary vessel = 1.0, a pair of vessels in contact = 2.0, etc. When transections were scanned to obtain figures for vessel density, ray areas were included in the areas surveyed.

WOOD ANATOMY

Secondary xylem of all Caricaceae lacks growth rings (Fig. 1,5, 7, 15). Vessels are more commonly solitary, but groups can contain as many as eight vessels (Fig. 1); Fisher (1980) reported a maximum of seven in Carica papaya. The mean number of vessels per group for the collections studied is 1.61 (Table 1, column 1). Grouping is not perceptibly more in a radial direction than in a tangential direction (Fig. 1,5, 7, 15). Mean vessel diameter for species studied ranges from 48 Ilm to 116 11m (Table 1, column 2). Vessels in Jacaratia (Fig. 5, 7) are narrowest among the species studied, those of Cylicomorpha (Fig. 15) widest. Vessel element length (Table 1, column 3) of the various collections deviates little from the mean for collections studied, 157 11m.

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Table 1. Wood characteristics of Caricaceae.

1 2 3 4 5 VG VD VL VM ME Carica pentagona 1.55 91 160 11 1300 Carica quercifolia 1.75 64 182 21 555 Cylicomorpha parviflora 1.87 116 169 34 577 Jacaratia hassleriana stem 1.59 49 161 53 125 Jacaratia hassleriana root 1.30 48 114 12 452 Above collections, averaged 1.61 74 157 26 602 Key to columns: 1 (VG), mean number of vessels per group 2 (VD), mean vessel lumen diameter, J.III1 3 (VL), mean vessel element length, 11m 4 (VM), mean number of vessels per mm2 5 (ME), mesomorphy ratio (vessel diameter times vessel element length divided by number of vessels per mm 2)

What variation is present probably relates to degree of juvenilism: the longest vessel elements were observed in the specimen with the least secondary xylem accumula­ tion, Carica quercifolia. This would accord with the gradual decrease in length of fusiform cambial initials in dicotyledons that show paedomorphosis (Carlquist 1962). Vessel density (Table 1, column 4) varies inversely to vessel diameter except for the root of Jacaratia hassleriana. In the root of this species (Fig. 7, 8), vessels are much sparser than in the stem (Fig. 5, 6). V~SSy~s baveexclusively simple perforation plates. Lateral wall pitting varies con­ siderabiy, but this variation occurs not so much among species as within a sample. The range of patterns is illustrated in Figures 9-12. Figures 9 and 10 are both from stems of C. quercifolia. The wider, minimally bordered pits shown in Figure 9 are characteristic of interfaces between vessels and axial parenchyma. The much smaller pits shown in Figure 10 are on contact areas between adjacent vessels. The pits of Figure 10 are more prominently bordered and associated with greater wall thickness. Metcalfe & Chalk (1950) and Fisher (1980) describe these pits as 'gash-like' because of the tapered ends of the pit apertures as seen in face view. Both the wide vessel­ axial parenchyma pits and the smaller vessel-vessel pits can be traversed by axially­ oriented strands of secondary wall material (Fig. 9, 10); such strands are figured by Fisher (1980) in C. papaya. The basic pattern of pitting is alternate, as shown in Fig­ .ure 12. Where alternate pits are markedly elongate laterally, the pattern may be termeq pseudoscalariform. The pits on vessel to axial parenchyma interfaces (Fig. 9) can be considered truly scalariform, since the length of these pits corresponds to a vessel wall facet. A vessel-to-vessel pattern transitional to a vessel-to-axial parenchyma pat­ tern is shown in Figure 11; the wider pits at the left and right edges of the vessels are in contact with parenchyma, the smaller pits in the central strip of vessel wall are vessel to vessel pits.

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No librifonn fibers or other imperforate tracheary elements are fonned in second­ ary xylem of Caricaceae; this agrees with observations by Metcalfe & Chalk (1950) and Fisher (1980). Axial parenchyma is present in fascicular xylem instead of imper­ forate tracheary elements. A measure of the amount of axial parenchyma that substi­ tutes for the imperforate tracheary elements one might expect in a woodier species can be shown in Cylicomorpha parviflora (Fig. 15). In this species, the paratracheal axial parenchyma, one or two cells in thickness around vessels or vessel groups, has thin lignified walls, whereas axial parenchyma not adjacent to vessels has thin non­ lignified walls and has collapsed in this specimen, which was dried. The collapsed axial parenchyma in C. parviflora is likely equivalent in quantity and location to fibrous tissue in woods of non succulent dicotyledons. The thin nonlignified primary walls of axial parenchyma are shown in Figures 2, 3, 5, and 7. Axial parenchyma cells are not subdivided, or may be in strands of two to three cells (Fig. 4, 16); undivided cells are more common. In areas of secondary xylem that were fonned in years prior to a current year's accumulation, axial parenchyma cells can undergo tangential divisions and also elon­ gate radially. These processes are shown for Carica pentagona in Figure 2. This ra­ dial proliferation of axial parenchyma occurs to the degree that there is stem widen­ ing. The base of C. pentagona is appreciably wider than upper portions of the stem, and the area shown in Figure 2 was taken from a stem base. The proliferation of axial parenchyma, because it occurs in an orderly fashion, can be considered a kind of cambial variant ('anomalous secondary thickening'). The result of this activity in Carica is an increase in stem diameter with attendant increase in cell volume avail­ able for water storage. Rays are composed of thin-walled nonlignified cells (Fig. 1,2,4-8,14). Rays are wide and multi seriate (Fig. 6, 8); few narrow rays are present (a biseriate ray is vis­ ible in Fig. 4, at right). Radial sections show that in all species, procumbent cells predominate, although some upright and square cells are present. The abundance of procumbent cells may be related to some extent to radial expansions of stems and roots, and may represent a slow pace of divisions in cambial ray initials, but in any case is characteristic in the family. Starch is present in axial parenchyma, especially near vessels and in rays. The amount of starch accumulation may vary with season, and needs to be investigated. Druses were observed in secondary xylem of Caricaceae only in the rays of Cyli­ comorpha parviflora. Laticifers in secondary xylem occur in tangential plates (Fig. 1,3,4, 13). The tan­ gential plates of laticifers are characteristic of fascicular areas, but a few may also extend from fascicular areas across rays. Conspicuous gaps fonn in cell walls where resorption of walls between adjacent cells of the articulated laticifers occur (Fig. 13, arrows); a drawing of similar appearance is provided by Fisher (1980). Storied structure is evident in secondary xylem in some axial parenchyma cells (Fig. 4) for all species. Where storied axial parenchyma is adjacent to vessels, one can see that the vessels confonn to the storied pattern of the vessels. Axial parenchyma is not unifonnly clearly storied; some areas show no apparent storying. (text continued on page 202)

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Fig. 1-4. Wood sections of Carica. - 1-3: C. pentagona; wood transections. -1: Outer second­ ary xylem (below) and inner secondary xylem (above); vessels are embedded in an axial paren­ chyma background. - 2: Inner secondary xylem in which vessels above and vessels below have become separated due to tangential divisions and radial elongation in intervening axial parenchyma, center; also some radial divisions and tangential elongation to left of vessels, above. - 3: Vessels embedded in axial parenchyma; a tangential band of laticifers (horizontal line of narrow cells) is present a few cells above the vessels. -4: Carica quercifolia; tangential section; group of laticifers, lower right; storied axial parenchyma to left of the biseriate ray at right. - Magnification scale for Fig. 1,2, above Fig. 1; scale for Fig. 3, 4, above Fig. 3 (divi­ sions = 10 J.lm in scales).

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Fig. 5-8. Wood sections of Jacaratia hassleriana. - 5,6: Stem. - 5: Transection; vessels are relatively small, densely placed. - 6: Tangential section; rays are two to eight cells wide at their widest points. - 7,8: Root. - 7: Transection (inner secondary phloem above); vessels much less common than in stem. - 8: Tangential section, showing portion of a vessel to right of center; rays identifiable by smaller diameter of their cells; axial parenchyma cells are larger but not vertically elongate. - Fig. 5-8, scale above Fig. 1.

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Fig. 9-12. Vessels from tangential sections of Caricaceae, to show types of pitting in vessels and nature of associated axial parenchyma cells. - 9, 10: Carica quercifolia. - 9: Vessel to axial parenchyma pitting. - 10: 'Gash-like' pits on a vessel-to-vessel interface. - 11, 12: Jacaratia hassleriana. - 11: vessel with vessel-to-vessel pitting (center) and, at right and left edges of vessel, portions of vessel to parenchyma pitting. - 12: Vessel to vessel pitting; as in other vessels shown, vertical strands of secondary wall material traverse some of the pits. - Fig. 9-12, scale above Fig. 9 (divisions = 10 filll).

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Fig. 13-16. Sections of wood ofCaricaceae. -13: Carica quercifolia; laticifers from tangential section, gaps in wall shown by arrows indicate extensive resorption of intervening cell wall areas. -14: Jacaratia hassleriana; stem radial section to show predominance of procumbent cells in a multiseriate ray. - 15, 16: Cylicomorpha parviflora. - 15: Transection; vasicentric axial parenchyma cells with thin lignified walls form circles around vessels, but other paren­ chyma cells, with nonlignified walls, have collapsed. - 16: Radial section; thin-walled cells associated with the vessels are vasicentrlc axial parenchyma with lignified walls. - Fig. 13, scale above Fig. 9; Fig. 14, 15, scale above Fig. 1; Fig. 16, scale above Fig. 3.

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Fig. 17-21. Sections of wood of Moringa (Moringaceae) for comparison with wood of Cari­ caceae. - 17-19: M. oleifera. - 17: Transection; thin-walled libriform fibers are present.- 18: Tangential section: vessel at right; rays are uniseriate to triseriate. - 19: Radial section; vessels and ray (near top) with procumbent cells; vasicentric parenchyma, bottom left. - 20, 21: M. hildebrandtii. - 20: Transection of wood and (top) inner secondary phloem; vessels are sparse, in an axial parenchyma background. - 21: Tangential section. Vessel near right; rays and axial parenchyma have nonlignified walls; vague storying in some axial parenchyma. - Fig. 17, 18, 20, 21, scale above Fig. 1; Fig. 19, scale above Fig. 3.

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Fig. 22-26. Transections of bark of Caricaceae. - 22, 23: Carica quercifolia. - 22: Phloem fibers (lower left); druses (dark spots) in outer cortex, periderm at top. - 23: Portion of second­ ary phloem to show alternating tangential bands of phloem parenchyma and phloem fibers. - 24, 25: Carica pentagona. - 24: Cells from dilated portion of phloem ray, to show radially­ oriented divisions (mostly near center). - 25: Portion of outer cortex and periderm; druses present in a scattering of parenchyma cells and a few of the thin-walled sclereids. - 26: Cyli­ comorpha parviflora; secondary phloem to show three strips of phloem fibers (with phloem parenchyma in strands and other patterns within the strips) and, alternating with the fiber zones, three rays. - Fig. 22, 26, scale above Fig. 1; Fig. 23-25, scale above Fig. 3.

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Information on wood anatomy of Moringa (Fig. 17-21) is presented here for pur­ poses of comparison. In M. oleifera (Fig. 17-19), growth rings are absent (faint in M. peregrina: Fahn et al. 1986). Vessels are solitary or in small groupings. Vessels have simple perforation plates and alternate lateral wall pitting composed of oval pits with slit-like pit apertures. Libriform fibers with thin walls are present (Fig. 17, 18); wall thickness varies according to season. Axial parenchyma is scanty vasicentric in M. oleifera; vasicentric to aliform in M. peregrina (Fahn et al. 1986); as seen in longi­ sections, the strands are composed of two to six cells with thin lignified walls and relatively large pits (Fig. 19, to right of vessel). Rays are uniseriate to narrow multi­ seriate (usually three cells at widest point); the majority of ray cells are procumbent (Fig. 19), and therefore the rays correspond to Heterogeneous Type II of Kribs (1935). Ray cells may have either lignified or nonlignified walls. No laticifers or other idioblasts occur in rays except for M. peregrina, in which druses occur in rays but laticifers are absent (Fahn et al. 1986). In Moringa hildebrandtii (Fig. 20, 21), wood of the lower stem of a relatively young plant (about five years old) shows no growth rings. Vessels are solitary or in small groupings (Fig. 20). Vessels have simple perforation plates and alternate lateral wall pitting; the lateral wall pits have an oval pit cavity outline and a relatively wide ('gaping') pit aperture as seen in face view. Imperforate tracheary elements are ab­ sent, replaced by axial parenchyma (in a sample from the upper stem, axial paren­ chyma predominates in secondary xylem, but a few tangential bands of thin-walled libriform fibers are present). Axial parenchyma is scanty paratracheal. The scanty paratracheal axial parenchyma has thin nonlignified walls with relatively large circu­ lar pits, although the axial parenchyma not adjacent to vessels has thin nonlignified walls. A differentiation can be made between paratracheal parenchyma and other axial parenchyma because paratracheal parenchyma is subdivided into strands, whereas the remaining axial parenchyma, which substitutes for libriform fibers, is not subdi­ vided (Fig. 21). Most rays are multi seriate and relatively wide (Fig. 21) compared to rays of M. oleifera. Most ray cells in M. hildebrandtii are procumbent, and rays cor­ respond to Heterogeneous Type II of Kribs (1935). Ray cells have thin nonlignified walls. Wood is vaguely storied (Fig. 21, especially at left, center). A few gum cells were observed in multiseriate rays, but these cells do not differ from neighboring cells in shape, and are idioblastic only in terms of contents. No laticifers were re­ ported in M. peregrina (Fahn et al. 1986). Secretory canals were observed in the pith of M. drouhardii (Olson and Carlquist, unpublished), but these are intercellular struc­ tures, in contrast with laticifers.

BARK ANATOMY

Bark of Caricaceae retains cortical features; periderm is added, and secondary phloem fibers and rays are added proportionately to age (Fig. 22). A full range of anatomical features is shown for Carica quercifolia (Fig. 22, 23). Phellem is approximately 10 to 20 cells thick, and consists of cells with tangential walls that are thicker than those of radial walls. As seen in gross aspect, phellem peels in thin sheets and sloughs as growth proceeds in older stems. About five layers of phelloderm were observed

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(Fig. 22). Chloroplasts are present in phelloderm, as well as in the outer cortex. Occa­ sional druses are present in the outer cortex, as are scattered sclereids with only mod­ erately thick walls. Laticifers are clearly present in the cortex, as seen both in tran­ sections and in radial sections. The secondary phloem (Fig. 22, 23) consists of fibers, variously interrupted by bands or strands of phloem parenchyma (in younger second­ ary phloem, sieve tube elements and companion cells are present, but these are crushed within a year or two). Sieve tube elements and companion cells are storied as seen in tangential section. Carica pentagona (Fig. 24, 25) is distinctive in showing radial lines of cell divi­ sions in the wedges of secondary phloem rays (Fig. 24). These cell divisions are re­ sponsible for dilation of phloem parenchyma; in other species, dilation is present, but is not accomplished by such clearly aligned and localized divisions. Phellem consists of about 10-20 cell layers; tangential walls bear moderate thickenings (Fig. 25). One or two layers of phelloderm are present. In the outer cortex, moderately thick-walled sclereids are common (Fig. 25); druses are present both in some sclereids and some parenchyma cells. Chloroplasts were not observed. Phloem fibers are much as shown for C. quercifolia. A few laticifers are present in phloem (probably the narrowest thin­ walled cells in Figure 24, above, are laticifers; identification of laticifers must be made from longisections, in which interconnection between vertical files of laticifer cells can be seen). In Cylicomorpha parvijlora (Fig. 26), cortex -derived bark portions were not present in the xylarium specimen available. Most of the specimen consisted of fibrous sec­ ondary phloem (Fig. 26). Strands or bands of phloem parenchyma are encased in the abundant fibers. Secondary phloem rays range from thin-walled (Fig. 26, above, right) to sclerenchyma-like (Fig. 26, near left). facaratia hassleriana stems have phellem with thickenings on tangential walls. Only a single layer of phelloderm was observed. Outer cortex contains scattered thin­ walled sclereids in phloem ray parenchyma. Druses are present in cortical parenchy­ ma or in outer phloem of ray areas. Secondary phloem is like that of Carica querci­ folia. No laticifers were observed in cortex or in phloem rays. Roots of f. hassleriana have bark like that of stems except that sclereids are absent, druses are fewer, and areas of secondary phloem fibers are smaller. Moringa oleifera has a phellem like that described above for Caricaceae; two to three layers of phelloderm are present, but neither in phelloderm nor in outer cortex were chloroplasts observed. Scattered druses and rhomboidal crystals (the latter one per cell) occur in cells of the cortex, phloem rays, and axial phloem parenchyma. Moderately thick-walled sclerenchyma cells are scattered in the inner cortex. Phloem fibers form wedges that contain bands and strands of phloem parenchyma. No laticifers are present, but some gum cells, with shapes like those of neighboring cells but mas­ sive red-staining contents (shrunken away from the wall, likely due to fixation and dehydration) are present in cortex and in phloem parenchyma. In M. hildebrandtii, layers of phellem like those in M. oleifera are present. The several phelloderm layers and the adjacent cell layers of outer cortex contain chlo­ roplasts. A few druses occur in the cortex. Phloem fibers (Fig. 20) are present; phloem

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SYSTEMATIC CONCLUSIONS

Because the recent system of Rodman et al. (1996) for Capparales pairs Caricaceae with Moringaceae, whereas previous systems have placed Caricaceae in Violales, se­ lected data on wood and bark of two species of Moringa have been presented. These data show that wood and bark of the lower stem of M. hildebrandtii, a 'tank-tree' succulent, correspond closely to those of Carica. Carica is a succulent or semi succulent tree, so that a comparable habit is available for comparison. Features of M. hildebrandtii not found in Carica include the oval pits of M. hildebrandtii (in contrast to the 'gash­ like' pits of Carica), but the difference is small. The lignified paratracheal parenchy­ ma of M. hildebrandtii is like that of Cylicomorpha parviflora. The bark of M. hilde­ brandtii not also seen in one or more species of Caricaceae. The wood of Moringa oleifera is unlike that of Caricaceae in its narrower rays and libriform fibers, and thus might be considered equivalent to a 'pycnoxylic' version of the 'mannoxylic' pattern of Caricaceae, the difference attributable to divergent habit. Moringa oleifera has a treelike habit and is woodier than any species of Caricaceae, all of which are succu­ lent by comparison. The near-identity between the wood and bark of M. hildebrandtii and those of Caricaceae is unlikely to be wholly the product of convergent habit, however. Such features as presence of druses in cortex, presence of Heterogeneous Type II rays, and presence of storied structure do not seem likely due to similarity in habit or ecology. The chief point of difference between the two families is the pres­ ence of laticifers in Caricaceae. The 'gum cells' of Moringaceae are conceivable homologs, although the gum cells are idioblastic in contents only, not in shape, and do not form networks as do the articulated anastomosing laticifers of Caricaceae. However, we have information on a minority of species in both families, and articu­ lated laticifers have been evolved independently in a number of angiosperm families or orders: the distribution of laticifers in dicotyledons (Metcalfe & Chalk 1950: 1347, 1349) when compared to phylogenetic systems can lead to no other conclusion. In sum, wood evidence supports the concept that Caricaceae and Moringaceae are closely related, provided one takes into account the relationship between types of wood struc­ ture and distinctive habits. Bark characters are certainly worth further investigation, and likely will prove to contain some features of systematic significance.

HABITAL CONCLUSIONS

The most striking feature of wood of Caricaceae is the absence of mechanical cells in secondary xylem (the few paratracheal parenchyma cells with lignified walls in Cyli­ comorpha might playa role in protecting vessels from torsion). The clear correlation in structure of stems in Caricaceae is that of peripheral placement of mechanical

Downloaded from Brill.com10/07/2021 06:52:03AM via free access Carlquist - Wood and bark of Caricaceae 205 tissue, which is abundant in secondary phloem. Because the wedges of secondary phloem fibers are separated from each other by thin-walled rays in most instances, this mechanical cylinder is expansible, allowing the parenchymatous secondary xy­ lem to expand indefinitely and to serve as a water-storage and starch-storage tissue. The placement of mechanical tissue in a cylinder close to the periphery of a stem is a theme seen in numerous plants, including fossil lycopsids (Carlquist 1975). The tangential divisions in axial parenchyma cells of the secondary xylem and the radial enlargement of these cells permits widening of the base in Carica pentagona. This widened base is likely to have a structural significance primarily, although the increased volume of parenchyma produced by this cell proliferation makes water stor­ age a possible function. Delayed expansion of stems by means of enlargement of xylem parenchyma cells was noted by Holbrook and Putz (1992) for Jacaratia. How­ ever, cell divisions combined with cell enlargement are evident in my material of Carica pentagona (Fig. 2). If one considers Caricaceae to be stem succulents of a sort, on can compare them to a sample of stem succulents from dicotyledons at large (Carlquist 1975: 206). The quantitative values are very close for vessel diameter in this sample (72 flm) and in Caricaceae (74 flill); vessel density is lower in Caricaceae (26 flill) than in the dicoty­ ledon sample (64 flill), but the difference, if anything, favors interpretation of wood of Caricaceae as that of a succulent. Also, succulents from habitats more xeric than those typical of Caricaceae tend to have higher vessel densities, in accord with the general trend in dicotyledons. If one computes the so-called Mesomorphy Value for species of Caricaceae individually and collectively (Table 1, column 5), one obtains a figure that shows Caricaceae are clearly far from xeromorphic. Caricaceae, taken as a whole, exemplify a balance between parenchyma related to succulence and sclerenchyma related to mechanical strength that functions in self­ support of the stems. This balance is appropriate for the rosette-tree or rosette-shrub form characteristic of the family. In other groups of succulents, such as sparsely branched cacti or thicker-stemmed species of Crassula, the balance favors turgor pressure in thin-walled cells as a means of achieving both mechanical self-supporting tissues and water storage, with sclerenchymatous tissues present in taller succulents (e.g., tree cacti) but often much less well represented in shorter succulents.

REFERENCES

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