Bordered Pits in Ray Cells and Axial Parenchyma: the Histology of Conduction, Storage, and Strength in Living Wood Cells

Botanical Journal of the Linnean Society, 2007, 153, 157-168. With 22 figures

Bordered pits in ray cells and axial parenchyma: the histology of conduction, storage, and strength in living wood cells

SHERWIN CARLQUIST FLS*

Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, CA 93105, USA

Received March 2006; accepted for publication September 2006

Bordered pits occur in walls of living ray cells of numerous species of woody dicotyledons. The occurrence of this fea ture has been minimally reported because the pits are relatively small and not easily observed in face view. Bordered pits are illustrated in sectional view with light microscopy and with scanning electron microscopy in face view for dicotyledonous and gnetalean woods. Bordered pits are more numerous and often have prominent borders on tan gential walls of procumbent ray cells, but also occur on radial walls; they are approximately equally abundant on tan gential and horizontal walls of upright cells, suggesting parallels to cell shape in flow pathway design. Axial parenchyma typically has secondary walls thinner than those of ray cells, but bordered pits or large simple pit areas occur on some cross walls of parenchyma strands. There is no apparent correlation between the phylogenetic position of species and the presence of borders in ray cells or axial parenchyma. Bordered pits represent a compromise between maximal mechanical strength and maximal conductive capability. High rates of flow of sugar solutions may occur if starch in ray cells or axial parenchyma is mobilized for sudden osmotic enhancement of the conductive stream or for rapid development of foliage, flowers, or fruits. Measurement of the secondary wall thickness of ray cells may offer simple inferential information about the role that rays play in the mechanical strength of woods. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 157-168.

ADDITIONAL KEYWORDS: mechanical strength - photosynthates - physiological plant anatomy - wood anatomy - xylem.

INTRODUCTION 1997, 1999, 2000, 2001, 2002, 2005) as well as in Gne- tales (Carlquist, 1989b, 1992b). Bordered pits on ray The occurrence of bordered pits between adjacent ray cells where a ray cell contacts a vessel were figured by cells or in cross walls of axial parenchyma strands rep Frost 11931) for Sassafras. Microcasting is a technique resents a phenomenon that has not been widely that offers an excellent potential for demonstrating reported. Pit borders in secondary walls of wood cells borders on pits of parenchyma. Indeed, one can find are usually observed most frequently in face view by borders among ray cells illustrated with this tech means of light microscopy. The wall thickness of nique by Fujii (1993) for Fagus and Quercus and by parenchyma cells in wood, the presence of starch in Peter Kitin (pers. comm.) for Cercidiphyllum and ray cells, and the small diameter of bordered pits in Kalopanax. These authors also illustrated a curious ray cells make borders difficult to observe in this man allied phenomenon in ray cells: borders on blind pits ner. Borders between ray cells are more readily that terminate in intercellular spaces. The interest in observed by light microscopy in sectional view in microcasting has thus far concentrated almost wholly radial sections of woods and have been reported and on tracheary elements, and descriptions of ray cell fea illustrated for a wide range of dicotyledon families tures are few in such studies. Instances can be found (Carlquist, 1981, 1988a, b, 1989a, 1991, 1992a, 1996, in wood anatomical literature in which borders are illustrated but not noticed or even designated as sim ple pits, as in a clear transmission electron micro *E-mail: [email protected] graph of Citrus ray cell walls (Fahn, 1990: 40). The

definition rigorously followed in the present paper is rays of Cecropia of the Moraceae (Carlquist, 1988a). that given by the IAWA Committee on Nomenclature There are also numerous instances of wide rays, in (1964) for a pit border: 'the overarching part of the various stages of fragmentation that contain vascular secondary cell wall'. This is a universally accepted tissue that represents a remnant of vascular continu definition. ity between a stem and a lateral branch. In all In the present paper, scanning electron microscope instances cited in this paragraph, the vascular tissue (SEM) illustrations of bordered pits as seen on tan in rays is dead at maturity, and thus can be differen gential walls of radial cells from tangential wood sec tiated from the living ray cells, which are the focus of tions are presented. Both light microscope and SEM this paper. By 'living' ray and axial parenchyma cell, I photographs have been offered here for Ephedra intend a contrast with cells dead at maturity: trache- (Ephedraceae) and Buddleja (Buddlejaceae, ary elements, ray tracheids, perforated ray cells, and Asteridae). These two genera are illustrated more allied phenomena mentioned above. The precise lon thoroughly by way of showing a wide range of aspects gevity of any given parenchyma cell is not implied preferably seen with both types of microscopy. Hope here. The presence of starch and other photosynthates fully, these appearances may be used as a guide to in ray and axial parenchyma cells suggests prolonged locating bordered pits in ray cells more readily. In longevity, as in Acer, in which these cells function in some species, phenolic compounds or other dark- the renewal of the conductive system at the end of staining materials may fill bordered pits and aid in winter (Sauter, Iten & Zimmermann, 1973). seeing pit contours. Detailed drawings of ray cells The phenomenon of nucleated ray cells with bor by such authors as Braun (1970) and Greguss (1959) dered pits needs consideration in ways other than illustrate only simple pits and are probably not descriptive. On the basis of the observations made in reliable: their drawings of pits in ray cells may reflect preparation for the present paper, bordered pits are the conventional notion that pits among ray cells are present in ray cells of numerous dicotyledons repre nonbordered rather than pit-by-pit analysis. Core, senting the major clades, as well as in Ephedra and Cote & Day (1979) offered a table that lists genera Gnetum. This circumstance suggests that living ray that show vessel-ray pitting ranging from 'simple' to cells with bordered pits should be examined in terms 'bordered'. That table, however, does not indicate of physiology and mechanical strength. Living ray whether the 'bordered' instances represent pitting cells contain enormous quantities of photosynthates, between vessels and rays that is composed of half- the ingress and egress of which in the form of sugars bordered pit pairs or fully bordered pit pairs. should be considered in relation to pitting. Pit fields in Living ray cells that have bordered pits are not to be ray cells with only primary walls represent broad thin confused with ray tracheids. Ray tracheids have only areas that could accommodate photosynthate transfer been reported in conifers (Record, 1934). Ray trache among ray cells. However, ray cells of many species ids are dead at maturity, and have bordered pits like have secondary walls that represent a significant dep those of coniferous tracheids (but smaller); they often osition of wall material in the form of cellulose and occur at ray cell tips, but may occur in other configu other components. This investment in strength-pro rations within rays as well iPinus). Ray tracheids have ducing wall structure must represent an enhancement been reported in Cupressaceae (Chamaecyparis noot- of mechanical strength for the wood. Bordered pits katensis Spach, Cupressus whitleyana Carr., Thuja offer an ideal compromise between optimal mechani plicata D. Don, Sequoia sempervirens (Lamb.) Endl. cal strength and optimal flow: retention of maximal and in many Pinaceae (all species of Larix, Picea, pit membrane areas that favour conduction combined Pinus, and Tsuga; Abies lasiocarpa Nutt., A. lowiana with minimal interruption of wall strength by virtue A. Murr., and most species of Cedrus, and Pseudot- of the relative narrowness of pit apertures (Carlquist, suga; Greguss, 1955; Core et al., 1979). 1988a). Greater pit density on tangential walls (as compared with radial walls) of ray cells, figured in The phenomenon of perforated ray cells is excluded many instances by Braun (1970), is one way of from consideration in the present paper. Perforated expanding the conductive surface, and the bordered ray cells are derived from ray initials, but have the nature of pits on these walls is another. The consider characteristics of vessel elements and represent path ation of ray histology needs to take into account both ways of vessels that traverse rays. mechanical strength and peak flow between cells, pre Vascular tissue embedded in ray tissue is likewise sumably when starch is hydrolysed into sugars that not included in the present paper. Boureau (1957) serve various functions. The mechanical strength of reported this phenomenon in Aeschynomene (vessels rays has been subsumed as a part of the strength fea embedded in massive rays), Cytisus (fibre-tracheids tures of wood, presumably because analysing the rel with helical thickenings in rays), and Monopteryx ative contributions of rays and of 'fibrous tissue' to (fibre strands in rays). These are all genera of mechanical strength is impractical where types of Fabaceae. Radially orientated libriform fibres occur in

© 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153. 157-168 BORDERED PITS IN RAY CELLS AND AXIAL PARENCHYMA 159 strength testing are concerned. In the context of ana tions and their appearances is a prerequisite to exper tomical information, one finds a great deal of litera imental work. The present paper is intended as a ture devoted to the distribution of ray cell types guide to phenomena that have been overlooked to a (upright vs. procumbent) and their phylogenetic sig remarkable extent. This paper surveys these phenom nificance (Carlquist, 1988a). The summary of Metcalfe ena and offers guides to methods of observation. It & Chalk (1983) of work on 'ray structure in relation to does not cover the complete range of pitting types in physiological function' is admirable, but by the small ray or axial parenchyma, nor summarize the system number of papers cited, it shows the rudimentary atic range of occurrence of these features. state of our knowledge. Any consideration of ray structure and function MATERIAL AND METHODS invites concurrent consideration of axial parenchyma. Although the character states of the two parenchyma By extrapolating the sampling of species viewed for systems in wood are analysed separately by wood this study, the number of species representing anatomists for purposes of description, the two sys instances of bordered pits in ray and/or axial paren tems are interrelated histologically and functionally. chyma ranges into thousands. Documenting the spe Braun & Wolkinger (1970) implicated axial paren cies selected at random for observation is not chyma in water flow within wood. That is conceivable practicable. However, the specimens illustrated in the if one considers solutes that may be involved in that present paper are as follows. Ephedraceae: Ephedra flow. Sinnott (1918) found that diffuse porous woods aspera Engelm. ex S. Wats., Carlquist 15828 (RSA); store fat in axial parenchyma in winter, whereas ring E. pedunculata Engelm. ex S. Wats., Carlquist 15819 porous woods store starch in axial parenchyma in win (RSA); E. viridis Coville, cultivated, Santa Barbara ter. The small number of observations on which these Botanic Garden. Asteraceae: Dasyphyllum spinescens conclusions were based suggests that the understand (Less.) Cabrera., MADw-21823. Buddlejaceae: Bud- ing of axial parenchyma physiology, like that of ray dleja bullata H. B & K, MADw-23118. Platanaceae: parenchyma physiology, is still in a rudimentary state. Platanus racemosa Nutt. ex Audubon, cult. Santa Bar Although bordered pits on ray cell walls proved com bara Botanic Garden. Trigoniaceae: Trigoniastrum mon in my observations of many species of dicotyle hypoleucum Miq., KEPw-7925. Winteraceae: Tasman- dons, bordered pits on axial parenchyma are much nia purpurascens Vickery (A. C. Sm.), FPAw-12884. scarcer. One might have thought that the 'pseudotra- Xylarium abbreviations are from Stern (1988). Mate cheids' reported by Lemesle (1956) for Bruniaceae and rial of Ephedraceae and Platanaceae was collected by Lemesle & Duchaigne (1955a, b) were possibly from living specimens. These samples were preserved axial parenchyma cells, but the existence of this cell in 50% aqueous ethanol. The other wood samples were type has not been confirmed (Carlquist, 1978, 1989c). available in dried form, and were boiled in water, then Ray cells in both of these families do have bordered as stored in 50% aqueous ethanol in preparation for sec well as nonbordered pits, and are like those of many tioning. All wood samples are from stems. Sections woody dicotyledons (original data). illustrated by means of light microscopy were pre pared with a sliding microtome and stained with a saf- The nature of nucleated cells referable to the con ranin-fast green combination. This staining method cept of axial parenchyma in Ephedra is explored in the provides colour contrasts that reveal pit contours well. present paper. New evidence is presented to clarify the Liquid preservation in the specimens of Ephedra and nature of these cells. Platanus confirmed the presence of nuclei in all ray Axially orientated cells termed strand tracheids and axial parenchyma cells. Sections studied by have been reported in conifers, but not in angiosperms means of SEM were sectioned with single-edged razor (Record, 1934). Strand tracheids recall axial paren blades. The sections were dried between clean glass chyma in shape, occurrence as strands, and distribu slides to maintain flatness during drying, mounted on tion in conifer woods, but they are dead at maturity aluminium stubs, sputter coated, and examined with a and have small coniferous bordered pits. Strand trac Hitachi S2600N SEM. heids have been well illustrated by Butterfield & Mey- lan (1980) for Pinus radiata D. Don. Strand tracheids, SEM techniques could easily have been used for the like ray tracheids, are excluded from the present entirety of this paper. Light microscope illustrations study. have been included in order to familiarize workers Conclusions concerning the functions of bordered with the appearance of bordered pits as seen with 40 x pits in living parenchyma cells of wood can only be light microscope objectives. SEM offers more dramatic inferential on the basis of the information presented and complete imaging, but light microscopy is entirely here. Experimental work is needed for validation of reliable, in my experience, to establish the presence of concepts concerning physiological and biomechanical borders on pits. The reports of bordered pits in this functions. However, understanding anatomical condi paper do not exclude the possibility that some simple

© 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 157-168 160 S. CARLQUIST pits may be present on any given wall in addition to enlarged are indicated with arrows in Figure 6. In bordered pits. The ray and axial parenchyma cells Figure 6, some cells seem to protrude (bright tones), described here do not differ from these cell types as whereas socket-like pockets (dark) are evident. These observed in innumerable other woods, and should in appearances represent diagonally orientated ray cell no way be interpreted as showing features restricted walls that have been exposed rather than sectioned. to the species selected for illustration. Tangential walls of ray cells (Fig. 7) have numerous Sectional views of ray and axial parenchyma cell bordered pits. The pits are quite varied in diameter. walls as seen with a light microscope have the poten Large bordered pits are seen near the tip of a diago tial disadvantage that pits are usually arranged ran nally orientated ray cell wall (Fig. 8). The pits on domly on a pit wall. One might therefore expect that upright ray cells (Fig. 9, centre right) are small and median views of bordered pits would be difficult to sparse, but also bordered. obtain. In practice, median views of pits in axial and ray parenchyma are as readily obtained as they are in Buddleja sectional views of conifer tracheids. The reader should A light micrograph of a radial section of a B. bullata keep in mind that face views of pits in parenchyma of ray (Fig. 10) shows a tangential wall in sectional view. woods are best rendered in face view with SEM, and in Bordered pits are present on the wall. The same fea sectional view with light microscopy. Alternative tures may be seen for B. bullata rays using SEM modes of viewing these pits (in face view with light (Fig. 11). The tangential walls of upright ray cells have microscopy, in sectional view with SEM) are more dif bordered pits, as seen in sectional (right) and diagonal ficult, but can be successful once one is accustomed to (left, above) view. The double crescents of the pits seen the appearances of these pits in various planes. in diagonal view are indicative of the bordered condi All longisections, whether rendered by light micros tion. An enlarged portion of a wall (Fig. 12) reveals the copy or SEM, have been orientated consistently in this characteristic appearance of several bordered pits as paper (vertical axis orientated vertically on the page), seen in sectional view. A tangential section of a pair of except for Figure 15. The two transactions (Figs 4, 20) ray cells (Fig. 13) at relatively low magnification are represented with the radial axis horizontal on the shows the lumen of the cell on the left, with two crys tals. The cell on the right is very similar, but is viewed page. from the outside surface. This outside surface (Fig. 14) is randomly covered with pits, all of which are bor dered. Two similar outside wall surfaces (Figs 15, 16) RESULTS are presented to show the range of patterns to be RAY CELLS expected. Pits can range from relatively large to minute (Fig. 15). The stripping away of pit membranes Ephedra to reveal pit borders is only partial in Figure 16, but In a radial section of E. pedunculata wood (Fig. 1), the the contours of the borders can been seen vaguely, tangentially orientated walls of ray cells (vertical in even for those pits covered by pit borders. Ray cell the photograph) bear numerous bordered pits. Hori walls that face imperforate tracheary elements in zontal walls have fewer pits, but the pits are also bor Buddleja are thin and bear bordered pits. dered. In this species, the staining combination used tends to make the pit membranes and pit cavities look dark, in contrast to the light tone of the secondary Other angiosperm woods walls. Ray cells of E. pedunculata, like those of other Tasmannia purpurascens show markedly bordered species of Ephedra, often have tangential walls that pits on walls of ray cells (Fig. 17). Most ray cells in are orientated in diagonal rather than vertical direc T. purpurascens are upright. The tangential walls tions in relation to the stem axis (Fig. 2). The number (Fig. 17, upper three-quarters of photograph), the of bordered pits on diagonal walls is, by virtue of this radial transverse (Fig. 17, bottom), and the radial ver angle, greater than the number of pits on more nearly tical walls of the ray cells all bear dense coverings of vertical walls (Fig. 1). Ray cells of Ephedra are alive at bordered pits. maturity, as shown by nuclei (Fig. 3, cell at right; In Dasyphyllum spinescens (Fig. 18), ray cells are Fig. 4). In E. aspera, ray cell walls (Fig. 4) are notably thick walled and bear bordered pits. The pits are thick and bear bordered pits. In all species of Ephedra densely placed not only on tangential walls, but also studied, tracheid to ray pits and vessel to ray inter on transverse and radial vertical walls. Ray cells in faces have bordered pit pairs. the specimens of D. spinescens are square to upright. SEM photographs of E. viridis wood (Figs 6-9) con Ray cells of Platanus racemosa are mostly markedly firm the features described in terms of light micros procumbent (Fig. 19). As seen in radial section, the copy. The tangential section in Figure 6 includes, at 'tangential' walls are mostly markedly diagonal in ori lower power, the cells shown in Figures 7-9; the cells entation. There is a marked disparity in pit density

Figures 1-5. Sections oi Ephedra wood. Figs 1-3. E. pedunculata, views of ray cells from radial sections. Fig. 1. Bordered pits in tangential walls (vertical) are more numerous than in horizontal walls. Fig. 2. Ray cells have tangential walls orientated from near-vertical to diagonal. Fig. 3. Portions of two ray cells (seen at the bottom of Fig. 2) to show the nucleus to the right of the diagonal wall with prominently bordered pits. Fig. 4. E. aspera, wood transaction; ray cells are nucleate and have numerous bordered pits on the strongly oblique walls. Fig. 5. E. pedunculata, cross wall of an axial parenchyma strand in radial section; the cross wall contains bordered pits. Scale bars = 10 }im.

2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 157-168 162 S. CAKLQUIST

Figures 6-9. Scanning electron micrographs of rays from a tangential section of Ephedra viridis wood. Fig. 6. A low-power photograph showing rays; two tracheids on the left. Fig. 7. View of the ray cell surface corresponding to the uppermost arrow in Figure 6; pits are bordered and graded in size down to very small diameters. Fig. 8. Ray cell corresponding to the central arrow in Figure 6; bordered pits at the cell tip are relatively large. Fig. 9. Upright ray cell corresponding to the area near the lowest arrow in Figure 6; a single small bordered pit on the right. Scale bars = 5 um.

2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 157-168 BORDERED PITS IN RAY CELLS AND AXIAL PARENCHYMA 163

Figures 10-16. Wood of Buddleja bullata. Fig. 10. Light micrograph of ray cells from a radial section; borders present on the pits of the wall to the left of the crystal. Figs 11, 12. Scanning electron micrographs of upright ray cells from a radial section. Fig. 11. Bordered pits in sectional view (right) and oblique view (left). Fig. 12. Enlarged portion of Figure 11 to show the bordered nature of pits on a tangential wall. Figs 13-16. Scanning electron micrographs of ray cells from a tangential section. Fig. 13. Two upright ray cells; the lumen of the cell on the left contains two crystals; on the right, the outer surface of the cell. Fig. 14. Portion of the cell on the right in Figure 14 to show the bordered nature of all the pits on the wall. Fig. 15. Another tangential wall surface, showing a wide range in size of the bordered pits. Fig. 16. Another tangential wall surface; the bordered nature of the pits is evident, even in those pits still covered by pit membranes. Figs 10, 11: scale bars = 10 um; Figs 12-16: scale bars = 2 um.

Figures 17-22. Light micrographs of radial (Figs 17-19, 21, 22) and transverse (Fig. 20) sections of angiosperm wood. Fig. 17. Tasmannia purpurascens. Portions of upright ray cells; bordered pits are dense on all walls. Fig. 18. Dasyphyllum spinescens, portions of ray cells showing bordered pits equally dense on horizontal and vertical walls (on the left, a portion of a vessel wall). Figs 19, 20. Platanus racemosa, portions of procumbent ray cells. Fig. 19. Tangential walls (which slant diagonally) are much more densely pitted than the horizontal walls. Fig. 20. Pits on tangential walls (vertical in photo graph) are bordered, whereas pits on the radial walls (horizontal in photograph) are sparse, simple or minimally bordered. Figs 21, 22. Trigoniastrum hypoleucum, axial parenchyma cells. Fig. 21. Axial parenchyma cross walls bear simple pits: one large and some smaller pits in the cross wall on the left, one simple pit in the cross wall on the right (fibre-tracheids in the centre of the photograph). Fig. 22. Axial parenchyma cross walls bear bordered pits; pits are simple or minimally bordered on vertical walls. Scale bars = 10 Jim.

© 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 157-168 BORDERED PITS IN RAY CELLS AND AXIAL PARENCHYMA 165 between tangential and radial horizontal walls. The Bordered pits among living ray cells are clearly com pits are dense and bordered on the tangential walls mon in Gnetales and woody angiosperms. Bordered (Figs 19, 20). The pits on radial horizontal walls pits may be present exclusively, or intermixed with (Fig. 19, walls horizontal in photograph) and radial simple pits, or simple pits may be present exclusively vertical walls (Fig. 20, walls horizontal in photograph) on ray cell walls. When using light microscopy, the are sparse and either simple or minutely bordered. occurrence of pits among ray cells is best seen on Trigoniastrum hypoleucum ray cells (not shown) are radial wood sections, where views of ray cells in sec relatively thick walled and have clearly bordered pits tional view reveal pits more readily. When SEM can be on tangential walls. used, viewing bordered pits in face view is desirable, and tangential sections are advantageous for reveal ing the dense pitting that often characterizes tangen AXIAL PARENCHYMA tial walls of ray cells. Beginning with these methods, one can then readily locate bordered pits in ray cells A diagonal wall between two cells of an axial paren using other ways of viewing ray cell walls. chyma strand is shown for E. pedunculata in Figure 5. The pits on this strand cross wall are bordered, Differential density and the degree of pit border rep although the borders are minimal, and simple pits can resentation are evident when one compares tangential be seen on some strand cross walls in E. pedunculata. walls of ray cells with radial walls. Tangentially ori The pits on vertical walls of axial parenchyma in entated walls are more densely pitted and have more E. pedunculata are mostly inconspicuously bordered prominently bordered pits than radial walls (either (Fig. 5). Ephedra presents a terminological problem, horizontal or vertical) in the case of procumbent cells. because, as seen in E. pedunculata, one can call undi Tangential walls of ray cells are often slanted or diag vided fusiform living wood cells with small minutely onal in orientation, and thus contain more numerous bordered pits either fibre-tracheids or undivided axial pits than vertically orientated tangential walls in a parenchyma cells. given ray cell. Square to upright ray cells tend to have In Buddleja, Tasmannia, Dasyphyllum, and Plata- pits that are approximately equally dense on tangen nus, vertical axial parenchyma walls are thin and bear tial walls as compared with horizontal or radial verti sparse simple pits. Vertical axial parenchyma walls in cal walls. Bordered pits in erect ray cells are common Trigoniastrum (Fig. 21) are a little thicker than in the in many woody species. preceding genera but bear sparse, minutely bordered There is a parallel between the direction of cell elon pits. The cross walls in axial parenchyma of these gen gation, the density of bordered pits, and probable flow. era have relatively large pit areas, as seen in Trigoni Procumbent cells represent a design for radial flow astrum hypoleucum (Fig. 21). The pits may be simple efficiency where flow between living wood cells is con or minutely bordered in Buddleja, Dasyphyllum, and cerned. The elongate nature of conductive tracheary Trigoniastrum, but appear simple in Tasmannia. The elements, as well as sieve elements, by having fewer simple condition is featured in the cross walls of the cross walls per unit length, has less impedance to ver two axial parenchyma strands in Figure 21, whereas tical flow and represents, at a very simple histological the pits in the axial parenchyma strand in Figure 22 level, a parallel to the radial flow design of procum are minutely bordered. bent ray cells. Square to upright cells, seen in this per spective, represent an intermediary flow design: a CONCLUSIONS transition between the procumbent ray cells and the vertical nature of axial parenchyma. HISTOLOGICAL ASPECTS Axial parenchyma cells that have secondary walls The purpose of this study was to open vistas of struc tend to have walls that are relatively thin compared ture and function in living wood cells for further exam with those of ray cell walls and imperforate tracheary ination, and to ask questions that have not been elements in any given species. Indeed, the contrast in explicitly considered before. The patterns presented wall thickness is one of the features used to identify obviously form more of a framework than a completed axial parenchyma in wood transactions. The lateral picture. Without such a framework, interesting struc walls of axial parenchyma are more sparsely pitted ture—function correlations cannot be addressed. than the cross walls. Cross walls may bear a single Although numerous observations were made in large pit. Borders are more commonly present on pits preparation for this study, many more are clearly of the lateral walls than on the cross walls of axial needed. In no way does the present paper differ in parenchyma. These features represent the optimal application of the term 'bordered pit' from those of design for flow within a vertical system of living cells other authors. The occurrence of these structures in in wood. ray and axial parenchyma of wood has been seriously The living axial wood cells of Ephedra form a ques underreported. tion of minor importance. They have bordered pits

© 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007. 153, 157-168 166 S. CARLQUIST much smaller than those of tracheids, and could either ray cells to mechanical strength is greater, judging by be termed fibre-tracheids (Esau, 1965) or axial paren the wall thickness (and by microfibril orientation on chyma. Axial parenchyma cells need not be subdivided the basis of polarized light observations). Burgert into strands, but can occur as nonsubdivided cells, as et al. (2001) offered an interesting insight into the role in Krameriaceae (Carlquist, 2005). However, as in that rays play in the mechanical strength of wood. The E. pedunculata, the axially orientated living wood function of axial and ray cell lumen volume as a space cells of Ephedra may have cross walls (not to be con for the storage of starch and the housing of crystals, fused with septa) or may not be subdivided. Thus, phenolic compounds, etc., sets a boundary above which these cells may be considered a type of axial paren an increase in wall thickness would not be adaptive. chyma. Liquid-preserved materials, which were used Bordered pits represent a way of maximizing wall extensively in the study of Ephedra woods (Carlquist, strength while providing a maximal conductive area 1989b), demonstrate that these cells in Ephedra are between cells (Carlquist, 1988a: 108). If wall thickness always nucleate, which fibre-tracheids rarely are. is minimal, as it is between many parenchyma cells in plants, there is little selective value for bordered pits. Selection for border presence increases with secondary PHYSIOLOGICAL AND MECHANICAL ASPECTS wall thickness. Bordered pit presence is by no means The formation of secondary walls in secondary xylem always correlated with the wall thickness of axial and represents a significant energy expenditure, which ray parenchyma cells. Because the function of bor should be considered in an adaptive light. The func dered pits in nonliving cells of wood has so frequently tion of secondary walls in resisting cavitation in trac- been stressed, one must enquire into how the trade-off heary elements has been stressed (Carlquist, 1975; between mechanical strength and conduction repre Turner & Somerville, 1997; Niklas, 1992, 1997; Hacke sented by bordered pits functions in parenchyma of et al., 2001; Franke et al, 2002). The strength of ray woods. cell and axial parenchyma walls is probably not The function of axial parenchyma and ray cells in involved in that function, but these living cells may woods in the storage of starches and oils has occasion play secondary roles in promoting wood strength. ally been studied (e.g. Sinnott, 1918). Sauter et al. Jacobsen et al. (2005) stated that the 'mechanical (1973) implicated living parenchyma cells in the properties of xylem fibers, as well as other cell types transfer of sugars into vessels of Acer, an action that associated with vessels, including xylem parenchyma renews conduction in vessels by increasing their and, in some species, tracheids, are important to osmotic pressure. Starches and oils are rarely determining resistance to cavitation'. reported in studies of wood anatomy. In part, this is a The presence of secondary walls on tracheary ele methodological matter. Oil and starch can disappear ments is also cited as promoting mechanical support during the drying of wood specimens, and dried wood functions of the stem (Jagels & Visscher, 2006). The samples are used much more often than liquid-pre data of the present paper suggest that ray cells and, to served samples in wood anatomy. The presence of a lesser extent, axial parenchyma cells also contribute starch or oil is not considered a diagnostic feature, and to the mechanical strength of wood. The role of paren thus those interested in wood identification have little chyma cells in wood strength is difficult to measure, reason to consider them. because isolating them from tracheary elements is In fact, sections of living or liquid-preserved woods impractical. There is no reason, however, to consider often reveal the storage of large quantities of starch or the secondary walls of living wood cells as playing no other photosynthates. The accretion of starch is prob role in mechanical support. ably a gradual process, taking place over a period of Wall thicknesses of axial and ray parenchyma cells months. The reverse process, the mobilization of are generally less than those of the walls of tracheary starch into sugars, may be much more rapid. Translo elements with which they are associated, but often not cation rates of sugars to utilization sites have been lit markedly so. The measurement of wall thickness tle studied in xylem. Sudden flushes of growth, rapid potentially offers a simple way of inferring the relative flowering events, or the input of photosynthates to contributions of cell types in wood to the mechanical fruit and seed formation may be involved with high strength of wood (although microfibril orientation is a flow rates of sugars, not merely in phloem, but in potential complicating factor). Axial parenchyma cells, parenchyma cells of the wood. If bordered pits repre because they are relatively thin walled, probably con sent a compromise between conductive efficiency and tribute a small amount to wood strength. Axial paren wall strength, then the dense placement of bordered chyma cells in some woods can lack secondary walls pits on tangential walls of ray cells and on cross walls altogether (Carlquist, 2005), so that secondary walls of of axial parenchyma strands becomes understandable. axial parenchyma cells, where present, must repre Axial parenchyma represents a conduit for the vertical sent some mechanical strength. The contribution of conduction of photosynthates within the wood,

© 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 157-168 BORDERED PITS IN RAY CELLS AND AXIAL PARENCHYMA 167 whereas procumbent ray cells presumably serve for Braun HJ. 1970. Functionelle Histoiogie der sekundaren radial conduction within the wood and into phloem Sprossachsen. 1. Das Holz. Handbuch der Pflanzenanatomie and bark. Upright ray cells can serve for photosyn- 1 (9): 1-190. thate conduction between axial parenchyma and proc Braun HJ, Wolkinger F. 1970. Zur funktionelle Anatomie umbent ray cells. The strong distinction between des axialen Holzparenchyms und Vorschlage zur Reform 'contact cells' (radially elongate ray cells related to seiner Terminologie. Holzforschung 24: 19-26. radial conduction processes) and 'isolation' cells (axi- Burgert I, Bernasconi A, Niklas KJ, Eckstein D. 2001. The ally elongate parenchyma cells) promoted by Braun influence of rays on the transverse elastic anisotropy in (1970) is not confirmed here, although distinctions green woods of deciduous trees. Holzforschung 55: 1-6. between cells representing extremes can be made. Butterfield BG, Meylan BA. 1980. Three-dimensional struc Bordered pits can occur on both tangential and hori ture of wood, 2nd edn. New York: Chapman & Hall. zontal walls of upright ray cells in approximately Carlquist S. 1975. Ecological strategies of xylem evolution. equal numbers (Buddleja, Tasmannia), indicating Berkeley: University of California Press. that radial conduction occurs in upright cells. Bor Carlquist S. 1978. Wood anatomy of Bruniaceae: correlations dered pits may occur on horizontal walls as well as with ecology, phylogeny, and organography. Aliso 9:323-364. tangential walls of procumbent ray cells (Ephedra), Carlquist S. 1981. Wood anatomy of Chloranthaceae (Dicra- indicating that axial conduction between procumbent stylidaceae). Aliso 10: 19-34. ray cells can occur. Carlquist S. 1988a. Comparative wood anatomy. Heidelberg: Springer. The phenomena described above are widely repre Carlquist S. 1988b. Wood anatomy and relationships of sented in woods, but the citation of them to date has Duckeodendraceae and Goetzeaceae. IAWA Bulletin 9: 3-12. been relatively small. From information at hand, Carlquist S. 1989a. Wood anatomy of Tasmannia; summary there is no evident correlation between the phyloge- of wood anatomy of Winteraceae. Aliso 12: 257-275. netic position and presence of bordered pits in axial or Carlquist S. 1989b. Wood and bark anatomy of the New World ray parenchyma cells. The introduction of Ephedra species of Ephedra. Aliso 12: 441-483. into this study should not be taken as an indication Carlquist S. 1989c. Wood and bark anatomy of Degeneria. that bordered pits represent a symplesiomorphic fea Aliso 12: 485-495. ture. Bordered pits in axial and ray parenchyma cells, Carlquist S. 1991. Wood and bark anatomy of Ticodendron: on the basis of present knowledge, occur widely in comments on relationships. Annals of the Missouri Botanical angiosperm clades and show no clear systematic pat Garden 78: 96-104. tern. Species with greater parenchyma cell wall thick Carlquist S. 1992a. Wood anatomy of Solanaceae: a survey. ness may be more likely to have bordered pits in axial Allertonia 6: 279-326. and ray parenchyma. Carlquist S. 1992b. Wood, bark, and pith anatomy of Old World species of Ephedra, and summary for the genus. Aliso Bordered pits are not conspicuous on most axial and 13: 255-295. ray parenchyma cells. This lack of conspicuousness Carlquist S. 1996. Wood anatomy of Akaniaceae and must not be equated with absence, or a lack of inher Bretschneideraceae: a case of near-identity. Systematic ent histological interest. Fibre-tracheids often have Botany 21: 607-616. minute bordered pits, the pit cavities of which are Carlquist S. 1997. Wood anatomy of Buddlejaceae. Aliso 15: often much more difficult to see than those of ray cells, 41-56. but those have often been noted. The methods used in Carlquist S. 1999. Wood and bark anatomy of Schisan- the present study are easily performed and can be draceae: implications for phylogeny, habit, and vessel evolu included in routine studies of wood anatomy. Studies tion. Aliso 18: 45-55. of wood strength in axial and ray parenchyma can be Carlquist S. 2000. Wood and bark anatomy of Takhtajania made, at least at an inferential level, by the calcula (Winteraceae); phylogenetic and ecological implications. tion of the volume of secondary wall material. Exam Annals of the Missouri Botanical Garden 87: 317-322. ination of the photosynthate flow within woods may Carlquist S. 2001. 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