Aliso: a Journal of Systematic and Evolutionary Botany Wood and Stem Anatomy of Phytolaccoid and Rivinoid Phytolaccaceae (caryophyllales): Ecology, Systematics, Nature of Successive Cambia Wood and Stem Anatomy of Phytolaccoid and Rivinoid Phytolaccaceae (caryophyllales): Ecology, Systematics, Natur

Quantitative and qualitative wood features are presented and analyzed for seven species of subfamily Rivinoideae and four of subfamily Phytolaccoideae. All species have nonbordered perforations plates, as elsewhere in suborder Phylocaccineae. Libriform fibers characterize both subfamilies, but vasicentric tracheids occur in two rivinoid species. Axial parenchyma is vasicentric scanty (apotracheal bands and patches in one species). Rays are mostly multiseriate, with procumbent cells infrequent in most species. Rivinoids and phytolaccoids differ from each other in ray height and width and in crystal types. The xeromorphic wood of Petiveria and Rivina is related to their short duration (woody herbs) in disturbed soil that dries readily. Woods of other genera are moderately mesomorphic, correlating with seasonally tropic habitats. Genera of Phytolaccaceae studied here have the same ontogenetic features leading to successive cambia as Stegnosperma. Phytolacca dioica has amphivasal pith bundles in which secondary growth occurs. Vessel restriction patterns are newly reported for the family.


INTRODUCTION
Phytolaccaceae are a curious assemblage because the component groups are diverse with respect to ovary and fruit characters that in other angiosperm alliances are often accepted as familial criteria.Differences in carpel number, stigma number, and fruit texture define subfamilies and genera of Phytolaccaceae (Heimerl 1934).The genera of Rivinoideae (segregated by some as Rivinaceae) have a single carpel but may have samaras (Gallesia, Seguieria); diverse hooked and bristly dry fruits (Monococcus, Petiveria), dry fruits without appendages (Ledenbergia, Schindleria); fleshy spherical fruits (Rivina, Trichostigma); and reticulate semifleshy fruits (Hilleria) (Heimerl 1934).Such diverse fruits often are prime features used to separate otherwise close angiosperm families (e.g., Myrsinaceae from Primulaceae).
Various authors have endorsed removal from Phytolaccaceae of Achatocarpaceae, Agdestidaceae, Barbeuiaceae, Gisekiaceae, Rivinaceae (= Petiveriaceae), and Stegnospermataceae (Cronquist andThorne 1994, Behnke 1997).In addition, Behnke (1997) has re-moved Sarcobatus from Chenopodiaceae as Sarcobataceae and placed it close to Phytolaccaceae (= within suborder Phytolaccineae).The families mentioned above, whether or not one treats them as segregate families of Phytolaccaceae (e.g., Behnke 1997) or subfamilies within a more inclusive Phytolaccaceae (Thorne in Cronquist and Thorne 1994), seem satellites of Phytolaccoideae, as shown by Rodman et al. (1984) and Rodman (1994).However, there is not a consensus at present on the contents of Phytolaccaceae, and therefore I am citing genera (the definitions of which are not so controversial) and subfamilies (by implication, subfamilies of a more inclusive Phytolaccaceae) rather than attempting to use either a Phytolaccaceae s.l, or a Phytolaccaceae s.s.Our concepts of the contents of Caryophyllales and the clades (and therefore family and suborder definitions) within the order are currently in flux, as the study by Williams et al. (1994) shows.Phytolaccaceae as traditionally conceived may not be a monophyletic group, based on cladistic analysis of DNA sequence data (James Rodman, pers. comm.).
Thorne (in Cronquist and Thorne 1994) and Behnke (1997) placed the families (or genera) listed above within a suborder, Phytolaccineae, that also includes Aizoaceae, Nyctaginaceae, and possibly Halophytaceae.Gyrostemonaceae can be found included in Phytolaccaceae by earlier authors (e. g., Walter 1909), but Gyrostemonaceae are one of the glucosinolate families that group with other Capparales; and the exclusion of Gyrostemonaceae from CaryophyllaJes as a whole and therefore also Phytolaccaceae in particular is entirely justified (Goldblatt et al. 1976;Behnke 1977).Bataceae have been similarly repositioned from Caryophyllales to Capparales (Behnke and Turner 1971).
Information from wood anatomy is potentially useful either for segregation of the families listed above from Phytolacceae or for recognition of a more inclusive Phytolaccaceae.Likewise, some wood features, such as the occurrence of nonbordered perforation plates, may prove to unite the families of the suborder Phytolacaccineae or even the families of Caryophyllales.The present paper constitutes one in a series of studies presenting new information about wood anatomy of Caryophyllales.Earlier papers in the series include studies of Caryophyllaceae (Carlquist 1995), Portulacaceae and Hectorellaceae (Carlquist 1998a), Petiveria and Rivina (Carlquist 1998b), Basellaceae (Carlquist 1999a), Agdestis (Carlquist 1999b ), Stegnosperma (Carlquist I 999c), and Barbeuia (Carlquist 1999d).In pursuing this series, some families will not be included because other authors have covered them thoroughly, e.g., Cactaceae (Gibson 1973 and other papers by him) and Didiereaceae (Rauh and Dittmar 1970).The compilation of Gibson (1994) on wood anatomy of Caryophyllales is useful, but as he indicates, the citation of wood features for Phytolaccaceae by Metcalfe and Chalk (1950) unfortunately includes Gyrostemonaceae without citing them separately and thus must be read with care.For citations prior to 1994 on wood anatomy of Phytolaccaceae, see Gibson (1994) and Gregory (1994).Two families consistently appear as outgroups to Caryophyllales: Plumbaginaceae and Polygonaceae.Wood of the former was surveyed recently (Carlquist and Boggs 1996), and a study of wood anatomy of Polygonaceae is in progress.A detailed comparison of the component families of Caryophyllales, including tabular comparisons, will conclude this series of papers, and that paper will be based on newly expanded definitions of Caryophyllales (e.g., Williams et al. 1994).
Wood anatomy of Phytolaccaceae is especially interesting with respect to ecology and habit.Some genera are lianas (Gallesia, Seguieria), whereas others are shrubs (Anisomeria), trees (Phytolacca dioica, Trichostigma octandrum) or woody herbs that are annual or of short duration (Petiveria alliacea, Phytolacca spp., Rivina humilis).Wood features in relation to growth forms and habitats are analyzed in the DIS-CUSSION AND CONCLUSIONS.
The following phytolaccoid genera have been reported to have successive cambia: Agdestis, Anisomeria, Barbeuia, Ercilla, Gallesia, Petiveria, Phytolacca, Rivina, Seguieria, and Stegnosperma (Pfeiffer 1926;Heimerl 1934).Successive cambia have not been reported in Hilleria, Ledenbergia, Lophiocarpus, Microtea , Schindleria, Trichostigma, or in the family now commonly removed from Phytolaccaceae, Achatocarpaceae (Pfeiffer 1926;Heimerl 1934).The phylogenetic question posed by this distribution is whether presence of successive cambia is a plesiomorphy or an apomorphy in phytolaccoids.In a few cases, the appearance of a second cambium is delayed (Horak 1981); successive cambia were not found by Heimerl (1934) in a stem of Stegnosperma 5.5 nun in diameter, for example.Perhaps the genera that lack successive cambia have a genetic basis for formation of successive cambia but also a modifier or repressor gene that delays their appearance for the length of life of the plant.In any case, the generic distribution of successive cambia in Phytolaccaceae s.l. is now known with reasonable accuracy.The three families now included along with phytolaccacoids in Phytolaccineae (Aizoaceae, Nyctaginaceae, and Sarcobataceae) all have successive cambia and are relevant to the phylogenetic status of successive cambia in the order.Very likely, more molecular data may clarify the phylogenetic issue.However, the anatomical nature of successive cambia needs clarification, and each pertinent genus that is studied in detail yields information on this question.Each of the successive cambia in a stem or root is a vascular cambium that produces xylem internally and phloem externally (except for cambia formed in the pith region); that concept is not in question.Rather, the nature and origin of the meristem that leads to production of these cambia and to conjunctive tissue (between the successive vascular bands) form a series of questions that have proved controversial.The papers on this meristematic activity in Phytolacca (Wheat 1997;Mikesell 1979) are not entirely in agreement, and the disagreements in terminology and interpretation widen if we include studies on lateral meristem origins and action in Nyctaginaceae (Studholme and Philipson 1966, Esau and Cheadle 1969, Stevenson and Popham 1973).I have attempted to clarify the nature of lateral meristem activity in several genera of Phytolaccaceae s.l.: Agdestis (Carlquist 1999b ), Barbeuia (Carlquist I 999d), Petiveria (Carlquist 1998b), and Stegnosperma (Carlquist 1999c).In a single phylogenetic unit such as Caryophyllales, the mechanism of lateral meristem origin and action leading to production of successive cambia and conjunctive tissue seems to represent a basic pattern, although variations on that pattern are entirely conceivable on the basis of present information.If there proves to be a single basic type of meristem origin and action in Caryophyllales, then comparisons can be made with various orders of angiosperms as well as with Gnetales.
The specimens of Petiveria alliacea, PhytoLacca americana, P. dioica, and R ivina humiLis were preserved in 50% aqueous ethanol.The material of Petiveria aLliacea and of the three species of PhytoLacca were sectioned in paraffin after softening according to the technique of Carlquist (1982) because mixture of hard and very soft tissues in a stem makes this technique advantageous.Satisfactory sections of the remaining species, which have greater tissue homogeneity and only moderate cell wall hardness, were obtained with a sliding microtome.Sections were stained with a safranin-fast green combination.Some sections were left unstained, dried between clean slides, mounted on aluminum stubs, sputter coated, and observed with a Bausch and Lomb Nanolab scanning electron microscope (SEM).Macerations were prepared with Jeffrey's Fluid and stained with safranin.
Data on Petiveria and Rivina have been published previously (Carlquist 1998b), but quantitative data on these two genera are included here to present a more coherent picture of Phytolaccaceae subfamily Rivinoideae.Vessel diameter in Table I is presented as lumen diameter.No vessel density or vessel grouping data could be presented for Seguieria americana because narrow vessels cannot be differentiated from vasicentric tracheids as seen in transection.Vessel diameter for this species was obtained from macerations.I are based on views of secondary xylem only, and areas of secondary phloem and conjunctive tissue were not included for purpo ses of computation.The transectional area of conjunctive tissue can range from relatively little to about 50% of stem transectional area in Phytolaccaceae with successive cambia.In PhytoLacca dioica, amount of expansion of parenchymatous conjunctive tissue by radial enlargement of cells (possibly a form of succulence) varies from one vascular band to another and from one stem to another.The conductive area per rnrn? of stem transection in that species thus varies so greatly that a vessel density figure based on views that include conjunctive tissue would not be meaningful.Terminology follows the IAWA Committee on Nomenclature (1964) and, for ray types and vasicentric tracheids, Carlquist (1988).The sequence of genera in the plates of the present study follows the generic sequence of Heimerl (1934).

Growth Rings
Growth rings were obsergved in GaLLesia integrifolia (Fig. I, 3) and in Seguieria americana.Although one might expect a growth ring to begin with the initiation of a vascular cambium, the latewood:earlywood boundary in species with successive cambia occurs within a band of vascular tissue, not at the beginning (or end) of it.In GaLLesia and Seguieria, vessel elements do not narrow progressively within a growth ring; instead, narrow late wood vessels appear very shortly before the end of the growth ring (Fig. 3, lower left).Wide, thin-walled libriform fibers (which are not subdivided and therefore are not axial parenchyma) are located at growth ring margins in GaLLesia, but may be found elsewhere within a growth ring also (Fig. I).

Vessel Restriction Patterns
The tendency for vessels to be confined to central portions of fas cicular areas in secondary xylem, and thus for vessels not to be in contact with rays, has been referred to the concept "vessel restriction patterns" (Carlquist 1988).Although a tendency toward this condition is shown in the rivinoids studied here, vessel restriction patterns are most clearly shown here in Anisomeria chiLensis (Fig. 22) and Trichostigma octandrum (Fig. 13).

Vessel ELements
All perforation plates in Phytolaccaceae are simple.They are also nonbordered, a feature that appears to be characteristic for the suborder Phytolaccineae.Nonbordered perforation plates have been figured for Agdestis (Carlquist 1999b), Barbeuia (Carlquist I999d), and Stegnosperma (Carlquist I 999c).A vestigially bordered perforation plates is shown for Trichostigma octandrum in Fig. 19, far right.
Mean vessel lumen diameter (Table I, column 2) is notably small in the two woody herbs, Petiveria and Rivina.In no species, however, is mean lumen diameter wide, considering that the widest mean figure is 56 urn for Phytolacca dodecandra and the figure for the remainder of the species studied is below 50 urn.This figure can be appreciated when one reads that the mean vessel diameter (outside diameter) for dicotyledons as a whole is 94 urn (Metcalfe and Chalk 1950).That figure is probably biased by the tendency for studies to be based on mesic trees rather than on herbs, shrubs, or small trees; the latter range of habits characterizes the species of the present study.Vessel density in Phytolaccaceae (Table I, column 3) is almost perfectly inverse to vessel diameter, with deviation from that relationship in Trichostigma peruvianum, which has comparatively low vessel density.Because of the exclusion of secondary phloem and conjunctive tissue from the computations on vessel density, the vessel density reported in Phytolacca dioica and P. dodecandra is appreciably higher than the figure that would have been obtained had conjunctive tissue been included.The stems of P. americana had only two vascular bands, the first of which was very wide, so conjunctive tissue does not form a large proportion of the stem in that species.
Vessel element length in Phytolaccaceae (Table 1, column 4) ranges from 170 urn to 350 urn.The mean for the species studied as a whole (228 urn) is TOughly half of the vessel element length reported by Metcalfe and Chalk (1950) for dicotyledons as a whole (649 urn).The shortest vessel elements are in the woody herbs Petiveria alliacea (170 urn) and Rivina humilis (176 urn).
The figure for vessel wall thickness (Table I, col-umn 5) ranges widely.Wider vessels tend to be thicker walled, most conspicuously in Gallesia integrifolia and Trichostigma octandrum (Fig. 3).Diameter of lateral wall pits ranges between 5 urn and 10 urn in the species studied.Shape of pit cavities on lateral walls of vessels is uniformly circular to slightly oval.Pit apertures are elongate and slitlike in most species, but circular or nearly so in Phytolacca dioica and Trichostigma peruvianum.Pit apertures of P. dodecandra are widely elliptical in contrast to the narrowly elliptical shape of pit apertures on lateral vessel walls of the remaining species.Grooves of various length interconnecting pit apertures may be seen on the inner surfaces of vessels in Trichostigma octandrum (Fig.

17-19).
There are no helical thickenings on vessel surfaces of any Phytolaccaceae studied.
Vessels are rarely in contact with rays, but rather appear confined to centers of fascicular areas in Seguieria americana (Fig. 9), Trichostigma octandrum (Fig. 13), and Anisomeria chilensis (Fig. 22).This condition is termed a vessel restriction pattern and has been reported in woods of a scattering of dicotyledon families (Carlquist 1988).
Macerations show that vasicentric tracheids are almost as abundant as narrow vessels in Seguieria americana.Mean length of vasicentric tracheids in that spe- cies is 315 urn, Vasicentric tracheids are present but not abundant in Trichostigma octandrum (Fig. 15, right).

Libriform Fibers
Other than the vasicentric tracheids mentioned above, all imperforate tracheary elements in the spe- ALISO cies studied are libriform fibers that bear very small (ca.1-3 urn) slitlike pits.Careful examination of the pits did not reveal any unequivocal borders.Starch was observed in libriform fibers of Petiveria alliacea, Rivina humilis, and Seguieria americana.Cytoplasm remnants were observed in libriform fibers of Trichostigma peruvianum.These four species have living fibers, therefore.No septate fibers were observed.
Mean length of libriform fibers (Table I, column 7) varies greatly within both subfamilies.An interesting consequence of this diversity is that the FN ratio (libriform fiber length divided by vessel element length) ranges from more than 3.0 (Anisomeria chilensis, Gallesia integrifolia, Phytolacca dodecandra) to less than 2.0 (Hilleria latifolia, Seguieria americana, Trichostigma octandrum, and T. peruvianum).

Axial Parenchyma
Scanty vasicentric axial parenchyma was recorded for all but two of the species studied, but in Seguieria americana, axial parenchyma is very scarce.In Gallesia integrifolia and Trichostigma octandrum, scanty vasicentric axial parenchyma is present, but there is also apotracheal parenchyma in the form of patches and radially wide but tangentially short bands.In Seguieria americana, ray-adjacent parenchyma was recorded in addition.Axial parenchyma is in strands of two, less commonly three, cells.The exception to this is Trichostigma octandrum, in which axial parenchyma is dimorphic: either in strands of two cells and lacking in crystals; or not subdivided and containing a large styloid or occasionally several smaller styloidlike crystals (Fig. 16, 21).

Rays
Rays in Phytolaccaceae are mostly multiseriate; appreciable numbers of uniseriate rays occur only in four species of Rivinoideae (Table I, column 10).The mean height of uniseriate rays is much less than that of multiseriate ray s for any given species (Table I, column 8), Rivinoideae have shorter mean multiseriate ray height than the Phytolaccoideae, with no overlap in ranges of mean ray heights between the two subfamilies.The phytolaccoids also have multiseriate rays that are appreciably wider than those of the rivinoids (Table I, column 9).Multiseriate rays of the Rivinoideae are illustrated here by Gallesia integrifolia (Fig. 2; some rays associated with axial parenchyma cells and, left, conjunctive tissue cells) and Seguieria americana (Fig. 10).Multiseriate rays of Phytolaccoideae are illustrated by Anisomeria chilensis (Fig. 23), Hilleria latifolia (Fig. 25), Phytolacca americana (Fig. 27), and P. dioica (Fig. 29).
Ray cell walls are all lignified, but relatively thin: 1.0-1.3urn predominantly.Pits among ray cells are simple or have inconspicuous borders.

Storied Structure
The axial parenchyma of Trichostigma octandrum is indistinctly storied.The storying pattern of the axial parenchyma in this species conforms to similar storying in the narrow vessel elements.

Crystals
In Gallesia integrifolia, styloids or similarly shaped smaller rhomboidal crystals are common in conjunctive tissue (Fig. 5) and in secondary xylem (Fig. 6-8).Crystals are very common in the axial secondary xylem in fibriform cells with thin lignified walls in this species (Fig. 6, right; Fig. 7, left).These fibriform cells are nonseptate and could conceivably be considered either a type of axial parenchyma or a type of libriform fiber.The tissue mentioned above as bands or patches of axial parenchyma are rich in styloids ; these cells in Gallesia integrifolia and Trichostigma octandrum are the same length as libriform fibers in length and nonseptate, although thinner walled than libriform fibers, so they are considered crystal-containing libriform fibers here.Styloids are common in Gallesia integrifolia in the upright sheathing cells of multiseriate rays (Fig. 7, right; Fig. 8); they are less common in procumbent ray cells.Small rhomboidal crystals like styloids in shape occur in phloem parenchyma of Gallesia and the other genera.
In Seguieria americana, rhomboidal crystals occur in wider rays ( Fig. II,12).Some of the rhomboidal crystals are elongate parallel to the stem axis, hence the square crystal transections shown in Fig. II.In Trichostigma octandrum, styloids are conunon in fibriform cells.As in Gallesia, these fibriform cells are not subdivided.One large styloid plus smaller elongate crystals may occur in a single cell (Fig. 16, right), as in Gallesia integrifolia.When tangential sections are viewed with SEM, the large styloids are clearly evident (Fig. 21), although they usually appear broken into segments.A few parenchyma cells in Trichostigma octandrum contain very small rhomboidal crystals (Fig. 20).Rhomboidal crystals were also observed in some tyloses.Styloids were observed in fibriform secondary xylem cells of Hilleria latifolia.
In Anisomeria chilensis, raphides occur idioblastically in the upright and square ray cells (Fig. 24).Raphides also occur in conjunctive tissue of Phytolacca dioica and P. dodecandra.

Starch
Starch remnants (degraded starch grains) were observed in the libriform fibers of Seguieria americana and Trichostigma peruvianum.These fibers are therefore living fibers.Starch was observed in ray cells and conjunctive tissue of Phytolacca dodecandra.

Successive Cambia
Of the species studied here, successive cambia were observed in Gallesia integrifolia (Fig. I, 5), Seguieria americana (Fig. 9), and in all of the species of .Within Phytolaccaceae, most attention has been paid to the successive cambia of Phytolacca (Wheat 1977; Mikesell 1979).Phytolacca dioica offers particularly favorable material because it is a tree that produces an indefinite number of bands of vascular tissue, whereas the other species are mostly annuals that produce one to three bands of vascular tissue.
Three examples of the meristematic region of the stem of P. dioica are illustrated here (Fig. 30-32).At left in each is the secondary xylem of the most recent vascular increment.The vascular cambium is to the right of this, and the secondary phloem to the right of the vascular cambium.Fracturing of cell walls occurs easily in the cambial region, but the sections illustrated show reasonably intact cambial regions.To the right of the secondary phloem and to the left of the sclereid band in Fig. 30-32 are radial rows of secondary cortex (secondary parenchyma).These rows are derived from parenchyma of the inner cortex.The sclereid band demarcates the inner cortex (converted into secondary parenchyma by periclinal divisions in cells of the cortex at an earlier stage) from the parenchyma of the outer cortex (far right in Fig. 30-32), which does not subdivide into radial rows.
Radial rows of secondary cortical parenchyma cells were illustrated for Petiveria and Rivina (Carlquist 1998b).The periclinal divisions in the rows are roughly synchronized in time and in number.In these two genera, the first cambium of the stem, and the second, derived from divisions of cortex, have occurred.Because only one or two bands of vascular tissue have been observed in these two genera, divisions within the radial rows of secondary cortex leading to formation of a third or fourth vascular cambium have not been reported.In Phytolacca dioica and the other genera with numerous bands of vascular tissue in stems (and roots), new vascular cambia develop by an abrupt cylinder of periclinal divisions in the secondary cortex.These new vascular cambia do not develop adjacent to the secondary parenchyma of the preceding vascular band.Instead, several layers of parenchyma cells intervene between secondary phloem and the site of origin of a new vascular cambium.These layers of parenchyma are therefore internal to the secondary xylem that will be produced by the new vascular cambium, and will mature into conjunctive tissue.The conjunctive tissue is a derivative of secondary cortex, not the vascular cambia, and therefore the conjunctive tissue should not be included in the concept of secondary xylem.
Vascular cambia in Phytolacca dioica appe ar abruptly at intervals, whereas periclinal divisions producing secondary cortex occur continuously during the growing season.This ontogenetic pattern is much the same as the pattern evident in Stegnosperma (Carlquist 1999c).The three views (Fig. 30-32) of the meristematic region of the stem of P. dioica are presented to show the nature of periclinal divisions (narrow arrows) in secondar y cortex in different portions of a stem.The vascular cambium is illustrated with wide arrows.The numerous periclinal divisions in the secondary cortex of Fig. 31 may indicate early stages in origin of a vascular cambium.Divisions are less abundant in the secondary cortex of Fig. 32, moderately abundant in the secondary cortex of Fig. 30.

Pith Bundles and Secondary Growth in Them
In the pith of P. dioica, vascular bundles characteristically form.These bundles are amphivasal in construction (Fig. 33) .Secondary growth occurs in the pith bundles.In accordance with the amphivasal organization of the pith bundles, the cambium of the pith bundles produces secondary xylem externally and secondary phloem internally.As secondary growth proceeds, early-formed phloem is crushed (dark gray circle , Fig. 33, center).

EcoLogicaL ConcLusions
Table I, column 12 gives values for a ratio termed Mesomorphy (for definition of this ratio, see Table I).This ratio is not an index of conductive efficiency, but takes into account both conductive safety and conductive efficiency: narrower vessels have been shown to embolize less readily (Carlquist 1975, Hargrave et al. 1994).The lowest figures for the Mesomorphy Ratio in Phytolaccaceae are in Petiveria aLLiacea and Rivina humiLis.These species are short-lived rather weedy woody herbs; as such, their roots are in relatively shallow soil likely to dry readily, so that a xeromorphic wood formulation would be of selective value.The remainder of the species of Phytolaccaceae have relatively moderate Mesomorphy values, but not as as high as those in tropical rain forest trees, in which the Mesomorphy value often lies in the range of 2000-5000.The woods of Phytolaccaceae are typical in vessel features of species from seasonally dry tropical areas .The Mesomorphy values for PhytoLacca are deceptively low because conjunctive tissue, often rather succulent in this genus, has not been included in computations of vessel density (number of vessels per mm-).The Mesomorphy Ratio for the wood of Anisomeria chilensis indicates a wood more xeromorphic than that of PhytoLacca.The higher number of vessels per group in Anisomeria, an indication of xeromorphy (Carlquist 1975(Carlquist , 1984)), is also higher.
Among the genera of Rivinoideae, there are several structural features suggesting xeromorphy other than those that involve vessel dimensions.In Trichostigma octandrum, the number of vessels per group is elevat- ed, and more numerous vessels per group is likely a mechanism that safeguards conductive pathways despite embolisms in some of these vessels (Carlquist 1984).Appreciable numbers of vasicentric tracheids are present in Seguieria americana; Trichostigma octandrum has some very narrow vessels and a few vas- icentric tracheids.Narrow vessels have much the same effect as vasicentric tracheids where conductive safety is concerned (Carlquist 1985).

Systematic Conclusions
All of the genera in the present study have libriforrn fibers and vasicentric parenchyma (GaLLesia integrifolia and Trichostigma octandrum have bands or patches of apotracheal parenchyma, in addition).In these respects, the phytolaccoids and rivinoids are close, and support the suggestion by Brown and Varadarajan (1985) that they constirute the two subfamilies, Phytolaccoideae and Rivinoideae, of Phytolaccaceae.In contrast, tracheids plus diffuse axial parenchyma characterize Stegnosperma (Carlquist 1999c) and Barbeuia (Carlquist ' 1999d) ; Agdestis Carlquist 1999b) has vasicentric tracheids plus libriforrn fibers and vasicentric parenchyma only.Brown and Varadarajan (1985) group these three genera closely; the anatomical data tend to support that these three genera, whether recognized as monogeneric families or not, perhaps as basal elements within the suborder Phytolaccineae, and perhaps a near-basal position in Caryophyllales as a whole, as suggested by Brown and Varadarajan (1985).This arrangement is shared to various extents by the analyses of Downie and Palmer (1994), Manhart andRettig (1994), andRodman (1994), although the disparity among the results of these studies is more impressive than the similarities.Very likely, a sampling of more genera and analyses of more DNA sites will clarify the classification.Thus far, all of the genera of Phytolaccaceae s.l., have been shown to possess nonbordered perforation plates, an unusual feature in dicotyledons as a whole.This feature may prove to be common to all Phytolaccineae, although studies of Aizoaceae and Nyctaginaceae, currently in progress, are needed.Because of their rarity in dicotyledons, nonbordered perforation plates have likely been overlooked by some workers.
Traditionally, Anisomeria has been placed close to PhytoLacca (e.g., Heimerl 1934).Evidence for this treatment is to be found in anatomy: the occurrence of idioblasts containing raphides in ray cells and conjunctive tissues of both Anisomeria and PhytoLacca supports placement of the two genera close to each other.
Rivinoideae differ from Phytolaccoideae on the basis of crystal types.All of the genera of Rivinoideae studied have elongate crystals, ranging from large (styloids) to small rhomboids.Raphides characterize stems of Phytolaccoideae.Species of Rivinoideae differ in size, abundance, and distribution of the elongate crystals (see CrystaLs above), and this suggests that generic and specific criteria may well be evident when all of the species of Rivinoideae are srudied.
One question concerning phylogeny of Caryophyllales that has not been addressed frequently is the likely phylogeny of woodiness within the order.Within the Phytolaccaceae studied here, upright cells compose at least half of the cells in multiseriate rays (and nearly all of cells in the uniseriate rays; upright ray cells predominate in Phytolacca).The rays of the family thus qualify as paedomorphic (Carlquist 1988) and thus may indicate an herbaceous ancestry either for the entire subfamily Phytolaccoideae.

Successive Cambial Activity
An analysis of all aspects relative to origin and action of successive cambia will be attempted when the current series of papers on Caryophyllales is completed .However, my observations at present indicate that radial rows of secondary cortex, formed in the inner cortex, result from periclinal divisions that may be designated a lateral meristem.In Phytolaccaceae S .S ., new vascular cambia originate in the secondary cortex, several cell layers away from the phloem of the preceding vascular band.Those several cell layers mature into conjunctive tissue.Periclinal divisions may increase the radial width of the conjunctive tissue to a minor extent.Each vascular cambium produces secondary phloem to the outside and secondary xylem to the inside, like the vascular cambium of a dicotyledon with only a single cambium.
The above description is very much like those offered for Stegnosperma (Carlquist 1999c) and Simmondsia (Bailey 1980).For the present, an attempt has been made to use a simple descriptive terminology with which to record observations.A number of different terms for the phenomena related to successive cambial activity have been offered by authors who have dealt with successive cambia.Comparisons of the terminology and of the interpretive schemes of these authors must await a more thorough review, in which successive cambia in diverse dicotyledons and in Gnetales can be considered.
Whether occurrence of successive cambia is a plesiomorphic or an apomorphic feature in Caryophyllales is an interesting question that is better addressed when more extensive data are available.For the present, one can say that the families placed basally in cladograms of Phytolaccaceae s.l., including those based on molecular data (e.g., Manhart and Rettig 1994), Caryophyllaceae and Stegnospermataceae, have successive cambia (in at least some genera in the case of Caryophyllaceae).One can find tracheids in at least some genera in these two families (Carlquist 1995(Carlquist , 1999c)); tracheids are considered a primitive type of tracheary element according to traditional criteria (e.g., Metcalfe and Chalk 1950, p. xlv).Families thought to be outgroups of Caryophyllales also possess successive cambia in a few genera: Polygonaceae (e.g., Rumex : Pfeiffer 1926) and Plumbaginaceae (e.g., Aegilitis : Pfeiffer 1926; Carlquist and Boggs 1996); these families, as well as Simmondsiaceae, can be included within a more inclusive Caryophyllales (Williams et al. 1994).The presence of successive cambia in so many families and genera of Caryophyllales, despite the absence of the phenomenon in others (e.g., Cactaceae and Didieriaceae lack successive cambia), leads to another possibility.The basal groups, as well as derived clades and the outgroups of Caryophyllales might have the genetic basis for formation of successive cambia, but one or more genes that delay or suppress formation of successive cambia may have been developed within several clades of Caryophyllales.
Figures for vessel density in Table

Fig . 17
Fig .17-21 .S EM photographs of a tan ge nt ial sec tion of Tric hos tigma oc tandrum wood.-17-19.Inner surfac es o f vessel s.-17.Grooves intercon nect man y pit ap ertures.-18. Pit apertures mo stl y not interconn ected or interconnected in pa irs by groov es; s lender perforat ion plate abo ve center is nonbordered or nearly so .-19.Grooves interconnect ing pit apertures (left.center) a nd a perforat ion plat e that is ves tigially bordered below and nonbordered near top of photograph .-20.Num erous sm all rhomboidal crysta ls within an ax ia l paren chyma cc ll.-2 1. Portion of large sty lo id. to right of center.broken into pie ces by the sectioning process.(Scales in eac h = 5 urn. )

Fig. 30
Fig. 30-33.Phyt ola cca dioica stem.-30-32.Tran sections of the rneri stematic region of the out er stem , outer stem at right ; the pair s of wide arrows ind icate vascular cambia, narrow arrow s denote peri clin al divi sions in secondary cortex .-30.Vascular cambium that has produced seconda ry xy lem and secondary phloem (wid e arrows at left ) and probable origin of a new vascul ar ca mbium (w ide arrows at right.-31 .Vascular cambium that has produced a relatively small amount of secondary xylem and secondary phloem; some of the periclinal divisions in the secondar y cortex indicated.-32.Vascular cambium that has produced only a little secondary xylem and phloem; only a few periclinal divisions are evident in the secondary cortex.-33.Transection of a pith bundle from a stem about two years old. to show secondary growth in an amphivasaJ pith bundle; protoxylern at periphery.(Fig. 30-32, scale above Fig .3; Fig .33 , scale above Fig .I.) ., cultivated at the University