Wood anatomy of Brassicales: new information, new evolutionary concepts.
Comparative wood anatomy offers new opportunities in the era of molecular-based phytogeny. The presence of molecular trees with high degrees of statistical probability means that we can see how, and how rapidly, wood evolves with respect to particular environmental factors. For example, can woodiness change over short periods of time? Can degree of woodiness fluctuate in both woodier and less woody directions in some groups but not in others? Does wood anatomy change rapidly and opportunistically, or is it one of the more conservative features of plant evolution? Which wood features can change more rapidly, and in what ways? Which features are active in countering dry or freezing conditions?
Some orders and families of angiosperms are much better than others for answering such questions. The size of Brassicaceae (3710 species) and Capparaceae (480) and their wide range of ecological distribution provide a much better material for understanding wood evolution than an assemblage of species with stereotyped habitat preferences. The families of Brassicales with fewer species are, in a different way, informative because they represent families that are superficially so different that they were earlier not included in the order tell us how by securing special ecological niches, a clade can survive and be transformed into growth forms as diverse as annuals (Limnanthaceae) or succulent trees (Caricaceae).
At a histological level, Brassicales offer many characters the phytogeny of which can be elucidated by distribution within the order as now conceived. Imperforate tracheary element types and vesturing presence or absence (Fig. 1, columns at right) are two of these. Tracheids in the sense of Bailey & Tupper (1918), Bailey (1936), Carlquist (1961, 1988), IAWA Committee on Nomenclature (1964) and Sano et al. (2011), cells capable of conduction, occur in five families of Brassicales. Implicit in the concepts of Bailey (1944) and the tabular data on bordered pits by Metcalfe & Chalk (1950, xlv) is the idea that the tracheid is the primitive (plesiomorphic) type of imperforate tracheary element in angiosperms, and that it has evolved, in'evcrsibly, into fiber-tracheids in various clades, followed by libriform fibers. Libriform fibers are often thought to be dead at maturity, but in fact, when liquid-preserved materials are studied, they prove to have living contents in an appreciable number of genera. Many workers have relied on the present of septa in libriform fibers (which are then termed septate fibers) as evidence that libriform fibers have prolonged longevity. Brassicales is an ideal group for demonstrating whether or not the tracheid to libriform fiber progression is irreversible, and if it is not, what ecological factors favor occurrence of tracheids in particular clades.
Vestured pits and vestured vessel walls (which may be construed so as to include "warts" on the lumen surface) occur in most Brassicales (Fig. 1) but not all. Are vestured pits apomorphic or plesiomorphic? Are they a constant feature of a species, or can they appear sporadically within a particular species? What is the relationship between vesturing and ecology? Our access to this character (or these characters) depends on access to scanning electron microscopy (SEM). Although vesturing can be detected in many instances with careful light microscopy, availability of SEM has permitted not only much greater certainty about whether vesturing is present in a particular species or not, it can be used to demonstrate that not all vestured pits are alike. Perhaps no order is more important in demonstrating the evolutionary nature and diversity of vesturing than Brassicales, and significant new observations are included in the present study. Examination of vesturing in terms of particular taxa and clades (e.g., Jansen et al. 2001) is required in order to demonstrate the ecological significance of vesturing and its probable phylogenetic status; a broad-brush analysis of vesturing on a global basis (Jansen et al. 2004) cannot provide the selective basis for this feature. Clearly interest in vesturing has peaked in recent years (e.g., Jansen et al. 2001), but much remains to be done, both with SEM and transmission electron microscopy (TEM).
Other wood anatomical features well represented in Brassicales that invite study and interpretation include presence of axial parenchyma as a background cell type. Kribs (1937) gives us the impression that all axial parenchyma is produced by modifications of more plesiomorphic types (e.g., diffuse parenchyma evolving into paratracheal, etc.), but is this an accurate view of axial parenchyma evolution? Brassicales offer some excellent examples of such diverse types as pervasive (axial parenchyma forming the ground tissue of secondary xylem) and bands that occur in latewood, especially as a stem of a "woody herb" senesces (e.g., the Castilleja example: Carlquist, 2015a).
Distinctive successive cambia appear in the group of genera recognized here as Stixaceae (Carlquist et al., 2013), and structural modes referable to successive cambia occur in Capparaceae (Adamson, 1936; Metcalfe & Chalk, 1950) and Brassicaceae (Metcalfe & Chalk, 1950). Interxylary phloem also occurs conspicuously in Brassicaceae (this paper) and Salvadoraceae (Carlquist, 2002). The characteristics of these cambial variants and their probable ecophysiological significance must ultimately be explored with reference to all occurrences, not just those in Brassicales (Carlquist, 2007). However, each instance in which these occur contributes a vital fragment to our understanding of these special conditions, too often neglected by wood anatomists and plant physiologists.
Fiber dimorphism proves to be another neglected phenomenon (Carlquist 1958, 2014). The occurrence of fiber dimorphism in angiosperm woods seems a logical evolutionary development, but, like successive cambia, it characterizes a small number of families and needs further exploration.
Helical sculpture in vessels is quite varied in expression (Carlquist, 1988). The occurrences in Brassicales are intriguing because helical sculpture is less characteristic of genera in the order than it is in some other angiosperm orders (e.g., Asterales). The association of helical sculpture with xeric ecological conditions is notable (Carlquist, 1966), but this feature can be found in genera that are not at all xeric in preferences, but are subject to winter freezing (Acer). Water is minimally available to the plant in either case, but can we differentiate among the two modes of occurrence? Helical sculpture also occurs in various forms, ranging from helical thickenings on the lumen side of a vessel wall to grooves that interconnect pit apertures ("coalescent pit apertures") or widen the inner pit apertures of pits.
Most of the abovementioned features have been shown to have some degree of association with xeric conditions. The habitats of Brassicales are mostly xeric in some way or to some degree, and in this respect, the order is ideal as a template for identifying which features are most indicative for wood xeromorphy, and which seem to be most effective in prevention of cavitation or in restoring the integrity of water columns once embolisms have fonned. Thus, Brassicales can play a key role in understanding of wood xeromorphy as a whole. Of orders of comparable size and ecological distribution, only Asterales and Caryophyllales have offered such excellent materials for the understanding of wood xeromorphy (Carlquist 1966, 2010; Mauseth, 1993).
In the past, many workers have arranged anatomical information about angiosperm woods in a systematic fashion (e.g., Solereder 1885, 1908; Metcalfe and Chalk, 1950). This method is ideal for retrieval of anatomical information, but has the drawback that it might lead us to think of wood characters as purely "taxonomic" in significance, whereas ecology and growth forms are the factors that explain occurrences of wood characters. To be sure, ecological, physiological, and growth form correlations may be difficult to demonstrate, but the adaptive approach is to be preferred to considering that any given feature is "of taxonomic value." This concept does not negate the fact that wood characters have distinctive systematic distributions. The primary selective factor in wood anatomy may be the balance between conductive efficiency and conductive safety (Carlquist, 2012)--a balance that is attained in many different ways. Mechanical strength inevitably also is a prime factor in wood patterns, one that must be analyzed separately from hydraulic capabilities, although there can be overlap.
There has been a tendency to relate the study of wood anatomy to woodier species, based on the economic value of those species. In fact, non-woody species, such as the majority of Brassicales--have secondary xylem, which deserves study equally. One could even say that one cannot understand woody species if one does not also understand the secondary xylem of non-woody or less woody species (Carlquist, 2009a), because early angiosperms probably had secondary xylem but would be classified as "non-woody" if we were to see them in comparison today's angiosperms.
Brassicales as currently recognized includes 19 families (Fig. 1). One could register an objection to inclusion of so many families. In fact, some coalescence has occurred. For example, Akaniaceae and Bretschneideraceae have been merged. One could even add a twentieth family, because the fossil genus Dressiantha belongs to Brassicales but appears in a clade that includes no other families of the order (Gandolfo et al., 1998). The history of Brassicales (sometimes called Capparales) is succinctly offered by Rodman et al. (1998) and by Hall et al., 2002, 2004) and need not be repeated here. There is understandably little impetus to condense Brassicales into one or two families when about half of the families currently included were, in phytogenies proposed before global molecular-based phytogenies emerged, placed in entirely different orders: Tropaeolaceae and Limnanthaceae in Geraniales. Caricaceae was placed in Violales, perhaps close to Passifloraceae. Emblingia was variously claimed to be related to Goodeniaceae, Polygalaceae, or Sapindaceae. Now that these families (and others) have been newly integrated into Brassicales, should we reconsider whether there should be condensation into fewer families? Actually, Brassicales, like Caryophyllales (Carlquist, 2010) proves to be an order in which further aggregation of families would result in umbrella families that cannot be described because of character sets that would be so heterogeneous that the resulting umbrella families would have unifying features too few, too vague, and too rife with exceptions.
For reasons just suggested, descriptions of wood characters of the 19 families of Brassicales are presented here. Without such descriptions, conclusions about wood in relation to phylogeny, ecophysiology, and habit would not be documented. The descriptions of wood of Brassicaceae and Capparaceae could be amplified so as to include variations as yet undescribed, of course, but the generalizations offered seem adequate for the purposes of the present paper.
The topology of the tree offered in Fig. 1 is likely to be modified somewhat as more molecular data becomes available. I have tried to place the families according to the most recent trees, but no molecular tree presently offered includes all 19 families. The tree in Fig. 1 is essentially an attempt to integrate the phylogeny of Su et al. (2012) with that of Hall et al. (2004). The familial unit Stixaceae Doweld has been hesitantly adopted because of the amazing similarities in wood anatomy between wood of Forchhammeria and Stixis. It includes Forchhammeria, Neothorelia, Stixis, and Urania, genera once included in Capparaceae (Pax & Hoffmann, 1936). The tree offered in Fig. 1, although based on those of Hall et al. (2004) and Su et al. (2012), is not the same as those or other trees because all trees offered to date do not include all 19 families of Brassicales. The wood descriptions and illustrations of the present paper are organized according to the tree of Fig. 1, reading from bottom to top. The descriptions of wood of the families are offered because if available elsewhere, they are scattered through the literature, and are not organized consistently to each other. In addition, the recent alterations in content of some of the families has now been altered (e.g., Capparaceae now excludes a number of genera, notably those of Cleomaceae, included by Pax & Hoffmann, 1936), so that older descriptions of wood of Capparaceae are no longer valid. The descriptions serve as a convenient data source when discussing the systematic distribution and evolutionary changes of any of the wood characters of Brassicales.
Materials and Methods
This paper is based on a series of my monographs of brassicalean families, cited at the beginnings of each of the family descriptions, plus monographs by others that cover more than one or two species. Much new original data is presented here, as are original illustrations (particularly SEM images). The bibliography of Gregory (1994) contains references to non-monographic sources of information on small numbers of species in larger families and genera. Documentation of collections is offered in the monographs cited, as well as in the captions of figures. Authors of binomials are given in the captions; for species not illustrated, binomial authors are provided in the running text. Species numbers and geographical distributions are from Stevens (2015). Terminology is in accord with Carlquist (1988, 2001), which in turn is essentially that of IAWA Committee on Nomenclature (1964), plus modifications necessitated by insights from physiology (Sano et al., 2011) and ontogeny (Carlquist, 2007).
Most studies were based on dried wood samples, but liquid-preserved materials proved valuable where soft tissues were concerned in families such as Caricaceae, Moringaceae, Salvadoraceae (interxylary phloem) and Stixaceae (successive cambia in Forchhammeria). Both sliding microtome sectioning and paraffin sectioning following softening with ethylene diamine (Carlquist, 1982) were employed. For SEM studies, sections made by hand with a single-edged razor blade proved entirely satisfactory. These hand sections were made from material boiled in water and stored in 50 % ethanol. The sections were rinsed in distilled water and then flattened by drying between clean glass slides under pressure. The thickness of such sections, which would be disadvantageous in light microscopy, was valuable because three-dimensional images of cells, rather than slices of cells, could be obtained. Some permanent slides made with sliding microtome sectioning and a Canada balsam mounting medium were used for light microscopy, but replicates of some of these slides, especially those from earlier years, were soaked in xylene and the sections recovered. Once cleansed with several changes of xylene and dried between clean glass slides, these sections were entirely satisfactory for SEM work. The majority of the SEM work was done with a Hitachi S2600N.
Quantitative data included here as indicators of relative mesomorphy or xeromorphy of particular woods, and as a way of documenting presence of protracted juvenilism (paedomorphosis). More extensive measurements can be found in the papers cited for each family. Quantitative data (vessel diameter, ray height, etc.) vary with portion of plant, size of sample, position within a growth ring, etc., and without knowing these factors, quantitative data reveal only generalized information. The present paper contains original qualitative, as well as observations in such resources as Solereder (1908) and Metcalfe & Chalk (1950). Wood terminology follows Carlquist (1988, 2001). The sequence of characters discussed in each family begins with transactional plan of wood and storying, if any, as seen in low power transverse and tangential sections respectively. The sequence of features that follow are vessel elements, then imperforate tracheary elements, axial parenchyma, rays, and crystals. An attempt has been made to cover the diversity within each of these categories, but in the larger families, the coverage is necessarily incomplete.
1. Akaniaceae (Carlquist 1996; Heimsch, 1942).
Bretschneidera (Fig. 2).
Wood plan. Growth rings moderately well developed (Fig. 2a), latewood vessels about half the diameter of earlywood vessels.
Storying. Wood non-storied (Fig. 2b).
Vessels. Vessels in small groups, groups larger in latewood (Fig. 2a). Perforation plates predominantly simple; variously scalariform plates occasional (Fig. 2e). Lateral wall pitting (both vessel to vessel and vessel to septate fiber) alternate or opposite. Vessel to ray pitting scalariform, opposite, or transitional. Vessel pits non-vestured. Helical thickenings on the vessel lumen surface, sometimes anastomosing, often fading into the vessel wall (Fig. 2f).
Imperforate tracheary elements. All imperforate tracheary elements are moderately thin-walled septate fibers, with small simple or vestigially bordered pits (Fig. 2c and d).
Axial parenchyma. Axial parenchyma is scanty paratracheal (vasicentric) and terminal. Axial parenchyma in strands of two to seven cells. The cross-walls between the cells of the strands have bordered pits (Fig. 2d).
Rays. Multiseriate rays (Fig. 2b) relatively wide (5.8 cells at widest point). Uniseriate rays also present. The ray type is Heterogeneous Type II of Kribs. Tip cells and sheathing cells of the multiseriate rays upright, the multiseriate ray cells otherwise procumbent.
Crystals not observed.
Akania (Fig. 3).
Wood plan. Growth rings very inconspicuous (Fig. 3a).
Storying. Wood not storied (Fig. 3b).
Vessels. Vessels solitary or in small groups (Fig. 3a). Perforation plates mostly simple. A few multiperforate perforations (modified scalariform, with variously shaped perforations) present. Lateral wall pits of vessels circular (Fig. 3e). Pits non-vestured. Vessel-to-vessel pits alternate (Fig. 3e), vessel-to-axial parenchyma and vessel-to-ray pitting scalariform to transitional. Lateral walls of vessels with varied helical sculpturing, grooves interconnecting pit apertures or helical thickenings present (Fig. 3f).
Imperforate tracheary elements. Septate fibers present (Fig. 3c). Pits of the septate fibers are simple or with narrow borders (Fig. 3d). Septate fibers rather thick walled.
Axial parenchyma in strands of four to eight (mostly five) cells. Pits on the cross-walls of the axial parenchyma strand often with perceptible borders. Axial parenchyma vasicentric scanty plus bands in latewood.
Rays. Rays Heterogeneous Type IIA of Kribs (1935), but rays of two distinct sizes; the wider rays average 7.1 cells across at widest point as seen in a tangential section (Fig. 3b). Multiseriate rays have an upright cell at upper and lower tip, and upright sheathing cells, but otherwise are composed of procumbent cells. Uniseriate rays are composed of upright cells.
Crystals. Occasional solitary rhomboidal crystals in ray cells.
Comments: The number of resemblances between wood of Akania and that of Bretschneidera is so compelling that one is amazed that these two genera had been relegated to widely-separated parts of the angiospenn tree (Carlquist, 1996). One also is amazed that two woods so similar could occur in such a disjunct pattern. To be sure, not all of the similarities may be synapomorphies. We have as yet no picture of the woods ancestral to Brassicales, although Akaniaceae gives us a good guideline of what to look for in Sapindales, Malvales, and related clades.
The list of similarities between the two genera includes: vessels solitary or in small groups; helical sculpture on walls of vessels; vessel to vessel pitting alternate; vessel to axial parenchyma and vessel to ray pitting scalariform to transitional; pits non-vestured; scalariform perforation plates or modifications of them scarce, but present throughout the secondary xylem; imperforate tracheary elements are septate fibers with simple or vestigially bordered pits; axial parenchyma scanty vasicentric and terminal; bordered pits present on cross-walls of the strands of axial parenchyma; multiseriate rays wide, with tip and sheath cells upright but ray cells otherwise procumbent; uniseriate rays composed of upright cells.
The few differences (septate fiber walls thicker in Akania; growth rings more apparent in Bretschneidera; crystals not observed in Bretschneidera) are those one might expect of congeneric species. The gross morphology of the flowers of the two genera, however, shows some conspicuous (but perhaps unimportant) differences.
2. Tropaeolaceae (Carlquist & Donald, 1996). (Fig. 4)
The information on wood anatomy of Tropaeolaceae is restricted to root and stem material of a single species, the commonly-cultivated Tropaeolum majus. One should keep in mind that this species is a scandent annual herb.
Wood plan (Fig. 4a). Growth rings absent. Vessels prominently dimorphic, the smaller ones grading into vasicentric tracheids.
Storying. Wood non-storied (Fig. 4b).
Vessels. Wide vessels (ca. 80 pm in diameter) solitary or in pairs; narrow vessels (ca. 28 pm in diameter) present (together with vasicentric trachcids) in large numbers. Perforation plates mostly simple, but occasionally scalariform or scalarifonn-like, both in wide and narrow vessels (Fig. 4e and f). Lateral wall pitting of vessels and vasicentric tracheids covered with alternate bordered pits (Fig. 4c). Pits not vestured (Fig 4d).
Imperforate tracheary elements. In transection (Fig. 4a), wood of Tropaeolum appears to have fibers as a background cell type, but these cells, when seen in longitudinal section and in macerations, prove to be both narrow vessel elements and vasicentric tracheids. Libriform fibers are also present. The libriform fibers are alive at maturity and contain globular starch grains.
Axial parenchyma. Axial parenchyma is sparse, vasicentric (Fig. 4a), with primary walls. Axial parenchyma in strands of two cells.
Rays. Multiseriate only in the stem, more than 1 cm in average height. Roots with a few uniseriates in addition to the multiseriates. Ray cells mostly upright, with a few square to procumbent cells in central portions of rays.
Crystals. No crystals observed.
Comments: Molecular data places Tropaeolaceae adjacent to the two genera of Akaniaceae (Gadek et al., 1992; Rodman et al., 1998; Hall et al., 2004; Soltis et al., 2011). The wood of Tropaeolaceae can, in this context, be seen to be essentially an herbaceous version of wood of Akaniaceae, modified for the vining habit (vessels dimorphic: Carlquist, 1985a).
3. Moringaceae (Olson, 2001, 2002, 2007; Olson & Carlquist, 2001). (Fig. 5)
Our knowledge of wood anatomy of Moringaceae is definitive, thanks to the field work of Mark Olson, especially in northeastern Africa. The resulting wood monograph (Olson & Carlquist, 2001), groups the species of the single genus, Moringa, according to habit, which also closely follows the phytogeny of the genus (Olson, 2001): bottle trees, sarcorhizal trees (thick roots with soft wood), slender trees (e.g., the widely-cultivated M. oleifera), and tuberous shrubs (stems of a finite duration, borne atop a large underground tuber). The species illustrated here (Fig. 5) include bottle trees (M. drouhardii, M. hildebrandtii, M. stenopetala) and the multi-stemmed tree M. oleifera (Fig. 5f).
Wood plan: Olson & Carlquist (2001) recognize four groupings of species. The anatomical differences can be seen most clearly in transverse sections.
(a) Bottle trees. Wood background of stems composed of thin-walled wide libriform fibers (Fig. 5a) with occasional bands of narrow fibers (Fig. 5b). Degeneration of some wide fibers in young stems (Fig. 5a) can result in formation of air spaces. Wide bands of paratrachcal axial parenchyma yield, often abruptly, to libriform fibers. Wood of roots is similar, with wider bands of axial parenchyma, narrower bands of libriform fibers.
(b) Sarcorhizal trees. Stem wood of M. arborea Verde, as in the bottle trees, wood of M. ruspoliana Engl, with wide bands of wide water-storing libriform fibers rather than wide bands of axial parenchyma. Axial parenchyma vasicentric, a single layer thick, in M. ruspoliana. Roots have wider bands of irregular diameter axial parenchyma cells and occasional bands of libriform fibers.
(c) Slender trees. Stem background tissue consists of a preponderance of libriform fibers, with variable amounts of earlywood axial parenchyma in the wet season. Roots have alternating bands of libriform fibers and paratracheal axial parenchyma bands (much as in stems of the bottle trees).
(d) Tuberous shrubs. Stems slender, like those of the slender trees, with a similar preponderance of libriform fibers. Root secondary xylem background tissue is composed almost entirely of thin-walled axial parenchyma, rich in starch, as in stems of Caricaceae.
Storying. Larger, less juvenile stems have storied libriform fibers and axial parenchyma (Fig. 5c). Rays are storied only in older stems that have the most pronounced storying of axial elements.
Vessels. Vessel elements are circular in transverse section (Fig. 5a and b), solitary or in small groupings, widest in diameter in the bottle trees (148 [micro]m), intermediate in slender trees (129 pm) and narrowest in stems of tuberous shrubs (40-80 [micro]m mean diameter). Vessel diameter is wider in roots than in stems in any given species (Olson & Carlquist, 2001). Perforation plates all simple (Fig. 5d). Lateral wall pitting of vessels alternate, from circular to oval (Fig. 5d and e), somewhat laterally elongate to pseudoscalariform. Shallow helical grooves present in vessels of M. rivae stems. Vessel-to-vessel pitting in my material of M. oleifera non-vestured (Fig. 5d and e), but vestured pits reported and figured for that species by Jansen et al. (2001). Jansen's report of vesturing is enclosed in parentheses, his symbol for indicating that not all collections or portions show vesturing in a species. When present, vestured pits in vessels are like those shown here for Setchellanthus (Fig. 8e), with vestures present only at the edges of the pit aperture. Vessel density greatest in stems of tuberous shrubs (32 per [mm.sup.2]), intermediate in sarcorhizal trees (20), and slender trees (13), and least in bottle trees (7).
Imperforate tracheary elements. These can all be categorized as libriform fibers, but range from rectangular with blunt ends to fusiform, with some intermediate fibers cuboidal but with fusiform tips at either end (Fig. 71-75 in Carlquist & Olson, 2001). The acicular libriform fibers are more common in the slender trees. Wider libriform fibers are thought to be related to water storage. Criteria for distinguishing between axial parenchyma and libriform fibers include subdivision in axial parenchyma cells, slit-like pits on librifonn fibers (as opposed to small circular pits on parenchyma walls), and greater density of pits on walls of axial parenchyma cells.
Axial parenchyma. Some axial parenchyma cells may be non-subdivided, but their co-occurrence with subdivided (strands of) parenchyma, as well as the above criteria permits discrimination between the two cell types. Axial parenchyma strands can be up to four cells in length; the proportions of undivided and subdivided axial parenchyma vary according to species (see radial sections of wood in Olson and Carlquist, 2001).
Rays. The rays are Heterogeneous Type IIB of Kribs (1935), with the proportion of upright to procumbent cells varying according to degrees of wood juvenilism. Tall non-storied rays composed chiefly or entirely of upright cells are indicative of paedomorphosis in wood of the genus. The opposite non-juvenilistic condition is illustrated here for M. stenopetala (Fig. 5c).
Crystals. Both druses (Fig. 5f) and solitary rhomboidal crystals have been reported in ray cells of Moringa, depending on species (Olson & Carlquist, 2001). Crystals are more common in pith and secondary phloem than in wood.
Starch storage. Starch is abundant in axial parenchyma of the tuberous species.
4. Caricaceae (Carlquist, 1998a; Fisher, 1980) (Fig. 6).
Wood plan. The secondary xylem of Caricaceae consists of vessels and bands of laticifers embedded in a tissue composed of axial parenchyma (Fig. 6a-d). The laticifers are articulated and anastomosing (Fig. 6c). The bands of laticifers sometimes cross rays rather than being confined to fascicular xylem. No fibers have been reported in the secondary xylem, but the secondary phloem is rich in fibers (accounting for the fact that some xylarium samples consist entirely or mostly of secondary phloem).
Storying. The vascular cambium is storied, and occasional axial parenchyma cells are storied (Fisher, 1980).
Vessels. Vessels are often grouped (Fig. 6a), although solitary vessels (Fig. 6d) are not uncommon. The average number of vessels per group for the family, 1.6, can be interpreted as indicating that the secondary xylem is mesomorphic, if only by means of succulence (Carlquist, 1998a). The vessel diameter and low vessel density (Fig. 6d) are also indicative of mesomorphy. Perforation plates are simple. Vessel-to-vessel lateral wall pitting consists of circular alternate pits (Fig. 6e and f) scalariform pitting (horizontal length of pits conforming to facet width) or pseudoscalarifonn pitting (elongate pits extending over more than one facet) is common on vessel to axial parenchyma and vessel to ray contacts of vessels. Pits are non-vestured (Fig. 6e f).
Axial parenchyma. Strands consist of one, two, or three cells. The axial parenchyma distribution type can be termed pervasive (Carlquist, 1988). Axial parenchyma proliferation occurs in the form of divisions that increase the stem thickness (Arnold & Baas Becking 1949). Radial and tangential cell expansion, as well as divisions, participate in this process. Some lignification occurs in a few parenchyma cells adjacent to vessels (Fisher, 1980).
Rays. Vascular rays are multiseriate; a few uniseriate rays are present (Fig. 6b, to the left of the laticifers). Radial sections show that a few upright cells are present in rays, but the majority of the ray cells are procumbent. The procumbent nature of ray cells is achieved, in part, by radial stem expansion. Increase in tangential width of multiseriate rays by radial divisions was reported by Fisher (1980).
Crystals. Crystals in ray cells of Caricaceae have been reported only in Cylicomorpha, where druses occur in some ray cells (Carlquist, 1998a). Druses are abundant in the cortex of Carica but absent in the secondary xylem (Fisher, 1980).
Starch. Starch is present in axial parenchyma, especially near vessel elements (Carlquist, 1998a) and at the margins of rays (Fisher, 1980).
Comments. The secondary xylem of Caricaceae, despite its parenchymatization and succulence, is not really juvenilistic in most features, showing that paedomorphosis and succulence do not necessarily go together. Caricaceae could be called "woody succulents," provided that one understands that the axial and ray parenchyma cells, which have primary walls only, may function in any of several ways not studied experimentally as yet, such as photosynthate storage and retrieval.
Olson (2002) has noted that Cylicomorpha may be sister to the remainder of Caricaceae. In this regard, occurrence of druses in the rays of some Moringa species and in the rays of Cylicomorpha is an interesting point of resemblance between Cylicomorpha and Moringaceae. Cylicomorpha is African, as are most Moringa species (M. oleifera, native to India, is an exception). When viewed as the sister family of Moringaceae, Caricaceae appear to have developed a curious division of labor between strength in secondary phloem fibers and storage of water and photosynthates in axial parenchyma and rays, with their thin primary walls. In this regard, there may be a correlation between the large fleshy fruits of Caricaceae, compared to the follicular fruits of Moringaceae.
The presence of laticifers in Caricaceae is clearly an apomorphy; laticifers are unknown elsewhere in Brassicales. Other apomorphies and synapomorphies of Caricaceae and Moringaceae are listed by Olson (2002).
5. Limnanthaceae (Carlquist & Donald, 1996) (Fig. 7).
The information here is based on a single species of Limnanthes. The family is small (two genera, eight species), and is not diverse with respect to habit. All are annuals that occur in places moist during winter, but drying out during warmer months.
Wood plan: The basal stem of Limnanthes consists of a circle of bundles. Secondary growth is mostly limited to these areas (Fig. 7a), so that a complete cylinder does not typically form. Some of these units are tangentially wide, up to a third of the stem circumference. Zones of primary rays with little secondary growth are interpolated among these units. The wood consists wholly of vessels and parenchyma (Fig. 7a and b).
Storying. No storying has been observed (Fig. 7a and b). One would not expect storying in a species with so little secondary xylem accumulation.
Vessels. Vessel diameter increases from protoxylem into secondary xylem, then decreases again as secondary growth proceeds to cessation. Vessels are mainly in radial groupings, Vessel elements are very short (average = 72 [micro]m) and have simple perforation plates (Fig. 7c and d). Vessel-to-vessel pits are alternate (Fig. 7c), but vessel-to-axial parenchyma and vessel-to-ray pitting tends to be scalariform (Fig. 7d). Vessel diameter is narrow, about 28 pm in the specimen studied. Vessels are angular, square to polygonal in transection (Fig. 7a).
Axial parenchyma. Axial parenchyma is scattered among the vessels, a pattern termed intervascular by Carlquist (1988, 2001). Axial parenchyma is not subdivided into strands (Fig. 7c, lower right; Fig. 7d, an axial parenchyma cell lies between the two vessels). Axial parenchyma has primary walls only.
Rays. Uniseriate and biseriate rays observed (Fig. 7b). These are difficult to discern in tangential sections because ray cells and axial parenchyma appear similar. Ray cells have primary walls only.
Stands on primary xylem helices. On the insides of primary xylem vessels, SEM studies reveal distinctive strands of wall material, possibly pectic in nature, stretching between the helices and the primary wall (Fig. 7e and f). These are most apparent close to the gyres (Fig. 7e), but may also form a webbing superimposed on the primary wall (Fig. 7f). Reports of such structures in protoxylem of angiosperms other than Limnanthes are not evident to me, although protoxylem is little studied by means of SEM.
Comments. Limnanthes wood seems clearly related to the annual habit. However, there are many kinds of annual habits in angiosperms, and these have been little explored. The stems of annual Brassicaceae and Tropaeolaceae reveal wood plans different from that of Limnanthaceae.
6. Setchellanthaceae (Carlquist & Miller, 1999) (Fig. 8).
The single species of Setchellanthus is a shrub, branched from the base, up to one meter in height. It is native to arid hillsides in restricted areas of northern and central Mexico.
Wood plan. Growth rings are only minimally developed, evident in terms of fluctuation more in terms of imperforate tracheary element diameter than in vessel diameter (Fig. 8a). Because vessels are mostly solitary, and vary in diameter, precise delimitation of earlywood and latewood is difficult.
Storying. Neither axial xylem nor rays are storied (Fig. 8b).
Vessels. Vessels are circular in outline (Fig. 8a). Most vessels are solitary (vessels per group from 1.03 to 1.12), which would correlate with the presence of vasicentric tracheids (see Carlquist, 1984). Vessel density is low, 35-113 pm (Fig. 8c). Vessel diameter of several collections studied ranges from 28 to 39 pm. Vessel element length ranges from 148 to 198 um. Vessel-to-vessel pitting is alternate, the circular pits small (2 pm in diameter) with vesturing along the edges of pit apertures (Fig. 8e). Vesturing was not reported earlier (Carlquist and Miller 1999). Helical sculpture (grooves and ridges) is present on some vessel walls (Fig. 8f). Yellowish deposits are present in some vessels (Fig. 8c).
Imperforate tracheary elements. Fiber-tracheids (Fig. 8c) and vasicentric tracheids (Fig. 8d) are present. There are occasional vestigial vestures on pits of the vasicentric tracheids. Imperforate tracheary element lengths (292-230 [micro]m) as well as vessel element length decrease as the stem widens.
Axial parenchyma. Axial parenchyma consists of single cells rather than strands. Axial parenchyma is in the form of diffuse cells, diffuse-in-aggregates, and vasicentric scanty. These were apparent only in preparations that had been counterstained, permitting axial parenchyma cells to be distinguished in transection from imperforate tracheary elements.
Rays. Rays are all uniseriate, mostly one cell in height, but occasionally two. Rays are so slender tangentially that they are not readily apparent in Fig. 8b. Rays clearly belong to Paedomorphic Type III of Carlquist (1988), a type seen in small shrubs such as Empetrum. Ray cells have secondary walls.
Crystals. No crystals were observed.
Comments. The decrease in vessel element length and imperforate tracheary element length over time, as well as the exclusively upright cells of rays, are hallmarks of paedomorphosis (Carlquist, 1962). Setchellanthus wood is distinctive in Brassicales in having vasicentric tracheids, and these may play an important role in conductive safety is this arid-land shrub.
7. Koeberliniaceae (Gibson, 1979 and original data below) (Fig. 9).
Earlier descriptions of wood of Koeberliniaceae include Canotia (e.g., Metcalfe and Chalk, 1950), but Canotia has been excluded and placed in Celastraceae). Koeberlinia is monogenetic, and had been thought to consist of the single species K. spinosa, which occurs in summer-wet desert areas of southern California to Texas and adjacent portions of Mexico. In 2008, a second species, K. holacantha W. C. Holmes, K. L. Yip, & Rushing was discovered in similar areas of Bolivia.
Wood plan. Growth rings are well developed (Fig. 9a), with earlywood vessels ca. 50 pm in diameter. Latewood vessels occupy the bulk of each growth ring and are 25 pm or less in diameter. A few patches of vessel-free wood seen in the first growth ring of one specimen.
Storying. Tangential sections show storying in all of the fascicular elements but not in the rays (Fig. 9b).
Vessels. Vessels are solitary (Fig. 9a), circular in outline. Vessel elements are short (average = 77 pm). Perforation plates are simple. Vessel-to-vessel pits are circular and alternate. Vessel to ray pits are circular. Vessel pits are vestured (suspected by Gibson, 1979, but not observed by Jansen ct al., 2001); the vestures form a series of minute knobs along the edges of the pit aperture (Fig. 9d). Helical thickenings on inner surfaces of vessels (Fig. 9e), most conspicuously in latewood.
Imperforate tracheary elements. Tracheids with fully bordered pits form the background cell type of fascicular secondary xylem (Fig. 9c). The bordered pits are vestured. Mean tracheid length = 83 [micro]m).
Axial parenchyma. Diffuse axial parenchyma is present. It is sparse and randomly scattered. Because of the high vessel density, one could say that vasicentric scanty parenchyma is in part present, because axial parenchyma--vessel contacts are inevitable. Parenchyma mostly in strands of two cells.
Rays. Rays are Heterogeneous Type IIB of Kribs (1935). Uniseriates are relatively few. Multiseriate rays are composed mostly of procumbent cells; a few square and upright cells may be found as sheathing cells. Multiseriate rays average 320 pm in height, but are quite variable in dimensions, and up to eight cells wide at widest point. Ray cells with secondary walls, some with bordered pits. Uniseriates are composed mostly of square to upright cells.
Crystals. Druses are present in ray cells of secondary xylem near the pith (Fig. 9f), but rhomboidal crystals predominate in secondary xylem. Rhomboidal crystals are borne one per ray cell and vary greatly in size.
Comments: Presence of tracheids in wood of Koeberliniaceae is unusual among Brassicales, but probably accords with the desert habitat. Tracheids are present in wood of such desert genera as Krameria and Prunus. The presence of tracheids is associated with solitary vessels, a correlation proposed for angiosperm woods at large earlier (Carlquist, 1984).
8. Bataceae (McLaughlin, 1959; Carlquist, 1978) (Fig. 10).
Bataceae consists of two subtropical to tropical species of Batis, B. argillicola van Royen (South Pacific) and B. maritima (New World). Both tend to occupy muddy saline flats near seacoasts and to be shrubs up to a meter in height with succulent leaves. The data here are based on materials of B. maritima.
Wood plan. Little fluctuation in vessel or imperforate tracheary element diameter, growth rings are lacking (Fig. 10a shows increase in vessel diameter distal to the pith).
Storying. Storying is evident in larger stems, in which the storied pattern of axial parenchyma and imperforate tracheary elements agrees with the storying of vessel elements (Carlquist, 1978).
Vessels. Vessels are circular in transverse section (Fig. 10a). About a third of the vessels are solitary; the others are grouped in radial multiples. Vessel-to-vessel and vessel-to-ray pitting consists of circular alternate pits. The vessel-to-vessel pits are vestured (Fig. 10d), as reported by Jansen et al. (2001). Mean vessel diameter=40 pm; mean vessel element length = 104 pm; mean number of vessels per sq. mm in transverse section, 126. Faint helical striations visible with SEM on vessels walls (Fig. 10e).
Imperforate tracheary elements. Minute borders are present on pits of imperforate tracheary elements, which are therefore termed fiber-tracheids here (Fig. 10c). These fiber-tracheid pits are vestured (Fig. 10c).
Axial parenchyma. Axial parenchyma is paratracheal, one to several cells thick around vessels or vessel groups; also present in the form of short apotracheal bands. Axial parenchyma in strands of two cells near vessels, but undivided distal to the vessels.
Rays. All rays multiseriate, composed of upright and procumbent cells (Fig. 10b); procumbent cells more common in wood of wider stems. Multiseriate rays two to eight cells wide.
Crystals. Crystals were not reported earlier, but druses are present in some ray cells (Fig. 101).
Comments. The degree of juvenilism in wood of Batis is not markedly pronounced, and the wood does not qualify as genuinely paedomorphic.
9. Salvadoraceae (den Outer & van Veenendal 1981; Fahn et al., 1986; Carlquist, 2002) (Figs. 11 and 12).
Salvadoraceae consist of three genera of shrubs of warm and dry and somewhat alkaline habitats.
Wood plan. Interxylary phloem strands present (Fig. 12a and e), markedly contrasting with thick-walled fibers and variously arranged axial parenchyma bands (Figs. 11 and 12a and e).
Storying. Axial parenchyma is clearly storied in accordance with the vessel elements, but storying less evident in the libriform fibers (Figs, lib and 12b and f).
Vessels. Vessels mostly grouped (Azima, 4.2 vessels per group; Dobera, 2.0; Salvadora, 5.2: Figs. 1 la and 12a and c). Vessels circular in transection, narrow (vessel diameter in Azima, 25 [micro]m; Dobera, 15 [micro]m; Salvadora, 12 [micro]m). Vessel elements short (Azima, 229 pm; Dobera, 134 um; Salvadora, 175 [micro]m). Vessel-to-vessel and vessel-to-ray pits are alternate and circular to polygonal. Vestured pits not reported earlier for Salvadoraceae (Jansen et al., 2001; Carlquist, 2002), but newly reported here for Azima (Fig. 1 Id). The vestures of Azima tetracantha are distinctively large and few per pit. In addition to the vestured pits, sphaeroidal vestures are present on flanges that parallel the perforations plates in Azima tetracantha (Fig. 11e and f). The perforations are either bordered (Fig. 11e) or non-bordered, but the vestures, similar in size to those on the vessel pits, are not on the borders on the perforation plates themselves, but on flanges separated from the perforation borders by a groove. These vestures may be crowded (Fig. 11e) or somewhat sparse (Fig. 11f). This represents a new kind of vesturing in angiosperms. Helical striations were observed on vessel walls in Salvadora (Fig. 12e).
Imperforate tracheary elements. Libriform fibers are present, mostly quite thick-walled (Figs. 11a and 12a and e), and with simple pits. The libriform fibers are about four times the length of vessel elements in the various species, an evidence of intrusive growth that lessens the storying evident in vessels and axial parenchyma.
Axial parenchyma. Axial parenchyma strands are composed chiefly of two cells in Azima (Fig. lib), but undivided cells are more common in Salvadora (Fig. 12b). As seen in transection (Figs. 11a and 12a and e), axial parenchyma takes the form of tangential bands that are usually paratracheal or in contact with vessels (Fig. 11a). A few diffuse cells are present (Fig. 11a).
Rays. Vascular rays are mostly multiseriate in Azima (Fig. 11a) and Dobera (Fig. 12e), but mostly uniseriate to triseriate in Salvadora (Fig. 12b). Most ray cells are square to procumbent; the proportion of procumbent cells increases with increase in stem diameter. The rays qualify as Heterogeneous Type IIB of Kribs (1937), perhaps transitional to Homogeneous Type II, in which all cells are procumbent and all rays are multi seriate.
Crystals. Crystals are very common (in almost every ray cell) in Azima and Dobera, less so in Salvadora. Crystals are rhomboidal to polyhedral (Figs, lie and 12d). The crystals are encapsulated, and a variously thin layer of cell wall material can be seen (Figs. 11c and 12d). Crystals that are smaller than those in rays may be found occasionally in axial parenchyma.
Starch. Starch grains are common both in axial parenchyma and in ray cells, and may be seen in liquid-preserved material (Salvadora). Starch grains may be single or borne in mutually-compressed pairs.
Comments. Interxylary phloem strands are present in all three genera (den Outer and van Veenendal 1981; Carlquist, 2002). Interxylary phloem is otherwise present in Brassicales only in Brassicaceae; the term "included phloem" is an umbrella term that fails to distinguish between Interxylary phloem and successive cambia and must be discontinued. Delayed onset of production of interxylary phloem strands was observed in Azima tetracantha (Carlquist, 2002), in which the first cm. of secondary xylem was observed not to have any such strands. Strands vary in size from very small (Fig. 12a, left) to moderately large (Fig. 12a, right). Interxylary phloem strands are embedded within axial parenchyma (Fig. 12a). Cambial activity within the strands leading to the formation of new secondary phloem cells (Carlquist 2013). In this way, longevity of the phloem strands is assured.
The wood of Salvadoraceae is notable for xeromorphy: narrow grouped vessels, short vessel elements, conspicuous storying, thick-walled libriform fibers, presence of Interxylary phloem strands, and large rhomboidal to polyhedral encapsulated crystals. The wood of Salvadoraceae cannot be characterized as paedomorphic.
10 Emblingiaceae. (Fig. 13).
There is a small amount of data provided by Metcalfe based on an Emblingia twig in Erdtman et al. (1969). The present account is new information, and is based on a somewhat larger twig. Study of a more mature wood sample of the single species is needed. Emblingia is a relatively small shrub native to limited sandy coastal areas of southwestern Australia. The placement of Emblingia as sister to core Brassicales was proposed by Hall et al. (2004). Prior to availability of molecular data, Emblingia was placed in various families, including Goodeniaceae, Polygalaceae, and Sapindaceae (Erdtman et al., 1969).
Wood plan. Growth rings are evident on the basis of wider vessels in earlywood (Fig. 13a).
Storying. Storying is not present in the tangential section of Fig. 13b, although that is only a three-year old stem.
Vessels. Vessels are circular in transection, and solitary (Fig. 13a). Vessel elements are caudate, and about 240 pm long in the material studied. Lateral wall pits are alternate (Fig. 13e--f), with narrow slit-like pit apertures (Fig. 13e). Pits are not vestured (Fig. 13e and f). Inner surfaces of vessels are faintly striate as seen with SEM (Fig. 13e). Perforation plates are simple and bordered (Fig. 13e).
Imperforate tracheary elements. All imperforate tracheary elements are tracheids, with bordered, non-vestured pits (Fig. 12c and d). Tracheids are about 420 pm in length.
Axial parenchyma. Axial parenchyma is scanty vasicentric, with wall thickness similar to that of the tracheids. Axial parenchyma is in strands of two to four cells.
Rays. Rays are uniseriate and biseriate (Fig. 13b). Uniseriate rays look rather similar to axial parenchyma strands in tangential sections. The ray type is Paedomorphic Type I of Carlquist (1988). Ray cells have secondary walls.
Crystals. No crystals were observed.
Comments. Emblingia is distinctive in having tracheids, which are associated with solitary vessels, in agreement with the scheme of Carlquist (1984). The wood of the material studied was paedomorphic in ray structure, a condition that very likely persists beyond the third year.
11. Tovariaceae (Carlquist, 1985b) (Fig. 14a-e).
There are two species of the neotropical genus Tovaria, the sole genus of the family. They are short-lived shrubs of unstable scree. The only study to date, based on a collection of T. pendula from montane cloud forests of Peru, represents the mature wood pattern of this species.
Wood plan. The secondary xylem is uniform with no growth ring activity (Fig. 14a).
Storying. No storying is present in the tangential section (Fig. 14b).
Vessels. Vessels are grouped (1.77 vessels per group, Fig. 14a), and are circular in transverse section. Vessels arc rather wide for the order (68 [micro]m). Mean vessel element length is 306 [micro]m). The number of vessels per square mm is 2.40. Pit apertures are narrow and slit-like (Fig. 14d). Although some texturing appears in SEM micrographs of pit cavities (Fig. 14e), this does not seem referable to vesturing, although further studies of other specimens should be undertaken. No helical sculpturing appears on the vessel walls (Fig. 14d).
Imperforate tracheary elements. Pits are very small and circular and apparently non-bordered (Fig. 14c). Libriform fibers are thus present.
Axial parenchyma. The parenchyma type is vasicentric scanty, in strands of two or three cells.
Rays. Both uniseriate and multiseriate rays are present, the latter more common. Ray cells are mostly upright, with a few procumbent cells in central portions of multiseriate rays (Paedomorphic Type I of Carlquist, 1988). Mean height of multiseriate rays is 536 pm; mean width at widest point is 3.8 cells. Ray cells have thin but lignified walls, like the walls of libriform fibers and vessels.
Crystals. No crystals observed.
Comments. The wood of Tovaria, based on stems of maximal diameter that were available, can be termed paedomorphic according to the concepts provided earlier (Carlquist, 1962).
12. Pentadiplandraceae (data original) (Fig. 14f and g).
No description of wood anatomy of Pentadiplandra has been provided prior to the present essay. The sole species of the family, P. brazzeana, is a scandent shrub native to Cameroons. The data herewith were derived from SEM studies of a small-diameter stem with about 2 mm thickness of secondary growth. This is not sufficient to provide reliable data on most characters, but the data on vestures seem decisive.
Vessels. The vessels are circular in outline and are mostly solitary. Lateral wall pits are alternate and circular. Vestures are present, and best seen from the inside of vessels (Fig. 14f), although they are also visible in pits as seen from the outer surfaces of vessels. The vestures are restricted to the edges of pit apertures.
Imperforate tracheary elements. Tracheids are present, based on two criteria: the prominent borders on pits (Fig. 14g) and the fact that vessels are not grouped (Carlquist, 1984). Vestures are present on the margins of the pit apertures (Fig. 14g. Tracheid wall thickness, about 2.5 um.
Rays. In the limited material available, all rays were multiseriate, rays three to four cells wide at their widest point. Ray cells were all observed to be upright, which is to be expected in wood of a young stem or branch.
Crystals. Single rhomboidal crystals seen in pith cells and in ray cells close to the pith.
13. Gyrostemonaceae (Carlquist, 1978) (Fig. 15; Fig. 16a-d).
Wood plan. As seen in transverse sections, tangential bands of axial parenchyma I which vessels are embedded are conspicuous (Figs. 15a and d, 16a). These bands are mostly not annual. The boundary between latewood and earlywood is illustrated for Tersonia brevipes (Fig. 16a).
Storying. Storying is visible in vessel elements and axial parenchyma, but only to a limited degree in imperforate tracheary elements (Figs. 15b and e, 16b) because of the intrusive nature of the imperforate tracheary elements. Tangential sections of bark show that fusiform cambial initials in the family are storied in samples old enough to show mature wood patterns for the genera.
Vessels. Vessels are circular in outline, less often oval in Codonocarpus (Fig. 15a) and Tersonia (Fig. 16a), but conspicuously grouped in Gyrostemon (Fig. 15d) and Didymotheca. Mean vessel diameter ranges from about 50 to 100 [micro]m (main stem, branches, and root included). Number of vessels per sq. mm ranges from 14 (e.g., Codonocarpus, Fig. 15a) to almost 100 (G. subnudus, Fig. 15d). Mean vessel element length ranges from 148 to 321 pm in the collections of G. ramulosus Desf., a range as great as that of the entirety of species investigated (Carlquist, 1978). Perforation plates are simple. Lateral wall pits of vessels are circular and alternate and non-vestured (Fig. 16c).
Imperforate tracheary elements. Nature of pitting suggests that there is a range from fiber tracheids to tracheids in these cells, depending on the density of the bordered pits (Fig. 16d). The imperforate tracheary elements are mostly tracheid-like in Tersonia, but fiber-tracheids with much-reduced pit borders in Codonocarpiis. Wall thickness is about 3 [micro]m.
Axial parenchyma. Axial parenchyma is in bands associated with the largest vessels, but elsewhere is diffuse or, more commonly, diffuse-in-aggregates. Axial parenchyma characteristically occurs in strands of two cells, with relatively thin but lignified walls.
Rays. Rays are mostly multiseriate, but a few uniseriates are also seen in all species studied. Procumbent cells are present almost exclusively (Fig. 15c). Upright and square cells are present at tips of rays and occasionally as sheathing cells along the sides of rays (Fig. 15e). Rays are thus Heterogeneous Type II transitional to Homogeneous Type II. Mean multiseriate ray height ranges from 391 [micro]m in Codonocarpus to 408 [micro]m in G. subnudus (Fig. 15e) to 1538 [micro]m in Tersonia brevipes (Fig. 16b).
Crystals. No crystals were observed.
Comments. The relatively tall rays of Tersonia suggest greater juvenilism, but other common indicators of protracted juvenilism (abundance of upright ray cells, absence of storying) are not present. Gyrostemonaceae are woodier than one might think on the basis of its lack of study in earlier wood anatomy literature, but Gyrostemon and Codonocarpus qualify as trees. Tersonia is a sprawling subshrub. Succulent leaves and stems are present in the genera other than Codonocarpus.
Starch is present in rays (especially in Didymotheca), but is less common in axial parenchyma.
14. Borthwickiaceae (data original) (Fig. 16e and f).
The single species of Borthwickia now constitutes its own family (Su et al., 2012). There is no description of the wood anatomy of this species prior to the present essay. The data herewith were derived from a stem about 4 mm in diameter from a single herbarium specimen, and thus represent incomplete material. Hopefully, a larger-diameter stem will be collected for study so that the nature of secondary xylem can be more accurately described. Borthwickia is a shrub or small tree from subtropical regions of China (Su et al., 2012).
Wood plan. There is no evidence of growth rings in the material studied.
Storying. There is no storying in the specimen at hand (Fig. 16e); storying would not be expected in a relatively young stem.
Vessels. Vessels are circular, mostly solitary. Vessels have alternate circular pits. There is no evidence of vesturing in the pits (Fig. 16f).
Imperforate tracheary elements. Pits with vestigial borders were seen (with SEM). These could be termed either libriform fibers or fiber-tracheids, depending on the cell viewed.
Axial parenchyma. Axial parenchyma is vasicentric; strands of two cells were observed.
Rays. Multiseriate rays composed of upright cells are present in the young stem examined (Fig. 16e).
Crystals. No crystals were observed.
Comments. The rays in this specimen are clearly juvenile, with only upright ray cells. This may well change as a stem increases in diameter, so more material must be examined before we can say whether or not such juvenilism is present in larger stems as well.
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|Title Annotation:||p. 24-55|
|Publication:||The Botanical Review|
|Date:||Mar 1, 2016|
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