Wood anatomy of selected Cucurbitaceae and its relationship to habit and systematics

Qualitive and quantitative data are presented on wood anatomy and cambial conformations of four species of Cucurbitaceae. Although all woody to various degrees, the four species were selected to show a wide range of habits and therefore to discern possible correlations between habit and wood anatomy. Vessels are widely spaced and libriform fibers are minimal where storage of water and carbohydrates is prominent. Axial parenchyma is dimorphic: lignified thick-walled paratracheal may lend strength to the vessel elements (which are wider than long), whereas thin-walled apotracheal parenchyma may lend flexibility to stems (especially in lianoid species) and serve for storage. Rays are multiseriate only and alter little from primary rays (but large multiseriate rays originate suddently in fascicular areas of one species). Distribution and abundance of libriform fibers relate to habit: most abundant in the shrubby Acanthosicyos , least in the storage-oriented lower stems of Apondanthera ). Vasicentric tracheids extend radially and interconnect vessels, potentially providing a subsidiary conductive system that would maintain the conductive pathways of the large vessels if some of those vessels were to be disabled or deactivated. Cucurbitaceae are characterized by septate fibers, vasicentric tracheids, and storied wood structure. Each of these features is found in at least half of the families now commonly included in Violales, to which Cucurbitaceae are thought to belong.     

DEVELOPMENTALLY VARIABLE,POLYMORPHIC WOODS IN CACTI

Sixteen genera of cacti were discovered to have polymorphic wood, that is, the plants produce one type of wood while young but a different type when older. The polymorphisms are: fibrous wood (with vessels and scanty paratracheal parenchyma) followed by parenchymatous wood (with vessels but few or no fibers) (Hylocereus venezuelensis, Dendrocereus nudiflorus, Borzicactus humboldtii, Haageocereus australis, Morawetzia sericata, Stephanocereus leucostele, Trichocereus schickendantzii); WBT wood (with wide-band tracheids, vessels, and apotracheal parenchyma but few or no fibers) followed by fibrous wood (Buiningia aurea, Oreocereus celsianus, Vatricania guentheri); WBT wood followed by parenchymatous wood (Echinopsis tubiflora, Gymnocalycium marsoneri, G. oenanthemum, Notocactus warasii, Parodia maassii); trimorphic wood in which WBT wood is followed by fibrous wood, which is followed by parenchymatous wood (Melocactus intortus, Arrojadoa braunii). The different phases within each plant may differ in vessel cluster size, percentage of the vessels that are solitary, diameter of vessels, and lignification of ray cells. Several of these genera are not closely related to the others, so wood polymorphism may have arisen several times.     

Water-storing and Cavitation-preventing Adaptations in Wood of Cacti

Ancestral cacti presumably had abundant, fibrous, heavily lignifiedwood, similar to that present in the relictual, leaf-bearinggenus Pereskia. During the evolutionary radiation of the subfamilyCactoideae, diverse types of bodies and woods arose. Severalevolutionary lines have retained an abundant, fibrous wood:all wood cells, even ray cells, have thick lignified walls,and axial parenchyma is only scanty paratracheal. Aside froma diversity of vessel diameters, there seems to be little protectionagainst cavitation during water-stress, and little water-storagecapacity. This strong wood permits the plants to be tall andto compete for light in their tree-shaded semi-arid habitats.In other evolutionary lines, the wood lacks fibres, and almostall cells have thin, unlignified walls. Vessels occur in anextensive matrix of water-storing parenchyma, and tracheidsare also abundant, constituting over half the axial tissue insome species. There is excellent protection against cavitation,but little mechanical support for the plant body; however, theseplants are short and occur in extremely arid, unshaded sites.Scandent, vinelike plants of two genera produce a dimorphicwood—while their shoots are extending without externalsupport, they produce fibrous, lignified wood, but after leaningagainst a host branch, they produce a parenchymatous, unlignifiedwood.Copyright 1993, 1999 Academic Press Cactaceae, cactus, water-stress, wood, evolution, xylem     

8种木本旱生植物的木质部解剖学特性研究

从生态学角度对8种旱生植物木材的次生木质部结构进行了对比研究.结果表明,8树种次生木质部结构的共同特点为:生长轮分界明显(霸王和四合木的年轮有时不连续),半环孔材,单穿孔,导管壁间纹孔互列,木纤维无横隔膜,联合傍管轴向薄壁组织丰富,导管壁上呈现螺纹加厚,都具有异型射线;它们主要差异在于射线和导管的数量化指标:四合木异型射线1~2列,霸王、白刺、沙冬青和胡枝子射线2~5列,柽柳、柠条和花棒大型射线4~10列;导管分子长度和纤维长度在树种间和同一树种个体间差异较明显.四合木和沙冬青导管频率较大,导管管孔较小,因此Vulnerability和Mesomorphy值也较小,更能适应干旱环境.     

Relationships within balsaminoid Ericales: a wood anatomical approach

Wood samples of 49 specimens representing 31 species and 11 genera of woody balsaminoids, i.e., Balsaminaceae, Marcgraviaceae, Pellicieraceae, and Tetrameristaceae, were investigated using light microscopy and scanning electron microscopy. The wood structure of Marcgraviaceae, Pellicieraceae, and Tetrameristaceae is characterized by radial vessel multiples with simple perforation plates, alternate vessel pitting, apotracheal and paratracheal parenchyma, septate libriform fibers, and the presence of raphides in ray cells. Tetrameristaceae and Pellicieraceae are found to be closely related based on the occurrence of unilaterally compound vessel-ray pitting and multiseriate rays with long uniseriate ends. The narrow rays in Pelliciera are characteristic of this genus, but a broader concept of Tetrameristaceae including Pelliciera is favored. Within Marcgraviaceae, wide rays (more than five-seriate) are typical of the genus Marcgravia. Furthermore, there is evidence that the impact of altitude and habit plays an important role in the wood structure of this family. The wood structure of Balsaminaceae cannot be compared systematically with other balsaminoids because of their secondary woodiness. Balsaminaceae wood strongly differs due to the presence of exclusively upright ray cells in Impatiens niamniamensis, the absence of rays in Impatiens arguta, and the occurrence of several additional paedomorphic features in both species.     

广义锦葵科火绳树属4个种的枝条木材解剖及其分类学意义

研究了广义锦葵科火绳树属4个种枝条的木材解剖。火绳树属枝条为散孔至半环孔材,管孔主要为单管孔和2～3个管孔组成的径列复管孔;导管间纹孔式和射线导管间纹孔式互列、小;侵填体和螺纹加厚缺如。射线主要为单列射线,2～3列射线较多;射线细胞多为方形,射线组织主要为异型,边缘直立细胞常1行;射线组织稀为同型;鞘细胞和瓦形细胞缺如。轴向薄壁组织傍管和离管型,主要为带状。晶体丰富,主要在射线、纤维和薄壁组织中。研究的4个种可以通过枝条木材解剖特征加以区分。     

Wood anatomy of Rauvolfioideae (Apocynaceae): a search for meaningful non-DNA characters at the tribal level

Wood anatomical studies in the economically important Apocynaceae or dogbane family are fragmentary. This study represents a first attempt to unravel the phylogenetic significance and major evolutionary trends in the wood of the family, using existing and new microscopic wood observations within the large subfamily Rauvolfioideae. On the basis of LM and SEM observations of 91 species representing all 10 currently recognized tribes, we found that most of the tribes are characterized by a unique combination of wood characters, such as vessel grouping, vessel element length, fiber type, frequency of uniseriate rays, and fused multiseriate rays. Climbing rauvolfioid taxa can generally be distinguished from erect species by their wider vessels, tendency to form paratracheal axial parenchyma, presence of tracheids, and occurrence of laticifers in rays. With respect to the entire family, there is a general phylogenetic trend toward shorter vessel elements, a higher proportion of vessels in multiples and more vessels per multiple, higher tracheid abundance, more paratracheal parenchyma, and fewer cells per axial parenchyma strand in the more derived Apocynaceae. Most of these evolutionary trends are likely to be triggered by drier environmental conditions and/or shifts from an erect to a climbing habit.     

Evolution of extreme xeromorphic characters in wood: a study of nine evolutionary lines in Cactaceae

Vessels of xeric-adapted woods have been predicted to be narrower than those of mesic-adapted woods, to occur at higher densities, to occur in larger clusters, and to have a greater percentage of them in clusters. These predictions were tested by comparing wood structure of several evolutionary lines of xeric-adapted cacti to that of mesic-adapted Pereskia, which probably resembles the ancestral cacti. Although derived cacti occur in habitats with water stress ranging from mild (rain forests) to severe (open deserts with little vegetation other than cacti), as long as plants retain wood with an ordinary fibrous matrix, wood characters are remarkably uniform and not correlated with habitat aridity. However, in several evolutionary lines, novel wood types occur with characters that fulfill the predictions for xeric-adapted woods listed above. However, conductive area (fraction of wood transverse-sectional area occupied by conduits) and estimated specific conductance (conductance per square millimetre) are correlated with shoot height (the need for mechanical support from xylary fibers) rather than with habitat aridity: tall plants transport water through relatively few, wide vessels, permitting much of the wood volume to consist of fibers. Small plants with little wood use large numbers of narrow vessels rather than small numbers of wide ones, thereby achieving conductive safety.     

Relictual vegetative anatomical characters in Cactaceae: the genusPereskia

The genusPereskia, which contains numerous morphological features considered relictual in the Cactaceae, has numerous anatomical features that we consider to be relictual also. These were studied to establish a basis for determining the ways that morphogenic mechanisms and anatomical characters diversified as the family evolved. ThesePereskia features may be relictual in the family: epidermis predominantly unistratose and lacking crystals; hypodermis absent or of about three layers of weakly collenchymatous cells with druses; cortex thin and predominantly parenchyma with druses and mucilage cells but lacking cortical bundles; secondary phloem without early differentiation of sclerenchyma but with secondary sclereids developing later, either idioblastically or in clusters; ergastic substances lacking from old secondary phloem; wood with a matrix of libriform fibers (mostly septate and nucleate), scanty paratracheal parenchyma, vessels solitary or in small clusters, perforations simple, pitting circular, oval or very broad; wide-band tracheids absent; ray cells slightly thick-walled, lignified, upright, isodiametric or procumbent; all primary rays narrow; pith without medullary bundles; leaves lacking hypodermis, with only weak development of palisade mesophyll; veins of four orders, strongly distinct in size, none with fibers; vessels in leaves narrower than those in stems.     

Wood Chemical Composition in Species of Cactaceae: The Relationship between Lignification and Stem Morphology

In Cactaceae, wood anatomy is related to stem morphology in terms of the conferred support. In species of cacti with dimorphic wood, a unique process occurs in which the cambium stops producing wide-band tracheids (WBTs) and produces fibers; this is associated with the aging of individuals and increases in size. Stem support and lignification have only been studied in fibrous tree-like species, and studies in species with WBTs or dimorphic wood are lacking. In this study, we approach this process with a chemical focus, emphasizing the role of wood lignification. We hypothesized that the degree of wood lignification in Cactaceae increases with height of the species and that its chemical composition varies with wood anatomy. To test this, we studied the chemical composition (cellulose, hemicellulose, and lignin content) in 13 species (2 WBTs wood, 3 dimorphic, and 8 fibrous) with contrasting growth forms. We also analyzed lignification in dimorphic and fibrous species to determine the chemical features of WBTs and fibers and their relationship with stem support. The lignin contents were characterized by Fourier transform infrared spectroscopy and high performance liquid chromatography. We found that 11 species have a higher percentage (>35%) of lignin in their wood than other angiosperms or gymnosperms. The lignin chemical composition in fibrous species is similar to that of other dicots, but it is markedly heterogeneous in non-fibrous species where WBTs are abundant. The lignification in WBTs is associated with the resistance to high water pressure within cells rather than the contribution to mechanical support. Dimorphic wood species are usually richer in syringyl lignin, and tree-like species with lignified rays have more guaiacyl lignin. The results suggest that wood anatomy and lignin distribution play an important role in the chemical composition of wood, and further research is needed at the cellular level.     

Distribution of starch and lipids in reaction wood of some angiosperm trees

Distribution of starch and lipids has been studied in upper and lower parts of tilt branches and vertical branches of Azadirachta indica A. Juss., Mangifera indica L. and Polyalthia longifolia (Sonn.) Thw. Starch grains in tiltwood are smaller than in vertical wood. Their accumulation is considerably lower in upper tiltwood than in lower tiltwood and vertical wood. The rays and axial parenchyma contiguous to vessels show the highest lipid content in the upper tiltwood of A. indica and P. longifolia. The accumulation of lipids in axial and ray parenchyma of upper tiltwood is smaller than in lower tiltwood and vertical wood. Since we found that not only the upper wood but also the lower part of the tilt wood is different from normal wood, we would like to suggest that the entire new wood of a leaning stem should be considered to be reaction wood.     

Living Cells in Wood 3. Overview; Functional Anatomy of the Parenchyma Network

The very different evolutionary pathways of conifers and angiosperms are very informative precisely because their wood anatomy is so different. New information from anatomy, comparative wood physiology, and comparative ultrastructure can be combined to provide evidence for the role of axial and ray parenchyma in the two groups. Gnetales, which are essentially conifers with vessels, have evolved parallel to angiosperms and show us the value of multiseriate rays and axial parenchyma in a vessel-bearing wood. Gnetales also force us to re-examine optimum anatomical solutions to conduction in vesselless gymnosperms. Axial parenchyma in vessel-bearing woods has diversified to take prominent roles in storage of water and carbohydrates as well as maintenance of conduction in vessels. Axial parenchyma, along with other modifications, has superseded scalariform perforation plates as a safety mechanism and permitted angiosperms to succeed in more seasonal habitats. This diversification has required connection to rays, which have concomitantly become larger and more diverse, acting as pathways for photosynthate passage and storage. Modes of growth such as rapid flushing, vernal leafing-out, drought deciduousness and support of large leaf surfaces become possible, advantaging angiosperms over conifers in various ways. Prominent tracheid-ray pitting (conifers) and axial parenchyma/ray pitting to vessels (angiosperms) are evidence of release of photosynthates into conductive cells; in angiosperms, this system has permitted vessels to survive hydrologic stresses and function in more seasonal habitats. Flow in ray and axial parenchyma cells, suggested by greater length/width ratios of component cells, is confirmed by pitting on end walls of elongate cells: pits are greater in area, more densely placed, and are often bordered. Bordered pit areas and densities on living cells, like those on tracheids and vessels, represent maximal contact areas between cells while minimizing loss of wall strength. Storage cells in rays can be distinguished from flow cells by size and shape, by fewer and smaller pits and by contents. By lacking secondary walls, the entire surfaces of phloem ray and axial phloem parenchyma become conducting areas across which sugars can be translocated. The intercontinuous network of axial parenchyma and ray parenchyma in woods is confirmed; there are no “isolated” living cells in wood when three-dimensional studies are made. Water storage in living cells is reported anatomically and also in the form of percentile quantitative data which reveal degrees and kinds of succulence in angiosperm woods, and norms for “typically woody” species. The diversity in angiosperm axial and ray parenchyma is presented as a series of probable optimal solutions to diverse types of ecology, growth form, and physiology. The numerous homoplasies in these anatomical modes are seen as the informative results of natural experiments and should be considered as evidence along with experimental evidence. Elliptical shape of rays seems governed by mechanical considerations; unusually long (vertically) rays represent a tradeoff in favor of flexibility versus strength. Protracted juvenilism (paedomorphosis) features redirection of flow from horizontal to vertical by means of rays composed predominantly or wholly of upright cells, and the reasons for this anatomical strategy are sought. Protracted juvenilism, still little appreciated, occurs in a sizeable proportion of the world’s plants and is a major source of angiosperm diversification.     

The first record of fossil Vitaceae wood from the Southern Hemisphere,a new combination for Vitaceoxylon ramunculiformis,and reappraisal of the fossil record of the grape family (Vitaceae) from the Cenozoic of Australia

Austrovideira dettmannaegen. & sp. nov. from the early Oligocene Capella Flora in central Queensland is the first fossil Vitaceae wood described from the Southern Hemisphere. A new combination, Stafylioxylon ramunculiformis (Poole & Wilkinson) Pace & Rozefelds for a Northern Hemisphere fossil wood is also proposed. Austrovideira and Stafylioxylon share with Vitaceoxylon secondary xylem with two diameter classes of vessels, wide vessels usually solitary, narrow vessels forming radial chains, very wide and tall rays, scanty paratracheal parenchyma and septate fibres. Austrovideira differs from Vitaceoxylon in having scalariform intervessel pits and homocellular rays composed exclusively of procumbent cells. This combination of features is seen in the Ampelocissus‐Vitis clade, and a clearly stratified phloem with fibre bands alternating with all other axial elements and phloem rays rapidly dilating towards the periderm is restricted to Parthenocissus and Vitis. Stafylioxylon shares with Austrovideira the presence of scalariform intervessel pits but it differs from that genus in both ray composition and bark anatomy, as it lacks a stratified phloem. These fossil wood genera demonstrate that the lianescent habit in the Vitaceae was established by the Eocene in the Northern Hemisphere and by the Oligocene in the Southern Hemisphere. The pollen and seed fossil record shows that the Vitaceae were in Australia by the Eocene and fossil seeds suggest that the family had radiated by this time. The Oligocene Capella flora with two seed taxa and fossil wood (Austrovideira) provides further evidence of an Australian radiation. The fossil evidence, suggests a significant Gondwanic history for the family.     

Comparative wood anatomy of epacrids (Styphelioideae,Ericaceae s.L.)

The wood anatomy of 16 of the 37 genera within the epacrids (Styphelioideae, Ericaceae s.l.) is investigated by light and scanning electron microscopy. Several features in the secondary xylem occur consistently at the tribal level: arrangement of vessel-ray pits, distribution of axial parenchyma, ray width, and the presence and location of crystals. The primitive nature of Prionoteae and Archerieae is supported by the presence of scalariform perforation plates with many bars and scalariform to opposite vessel pitting. The wood structure of Oligarrheneae is similar to that of Styphelieae, but the very narrow vessel elements, exclusively uniseriate rays and the lack of prismatic crystals in Oligarrheneae distinguish these two tribes. The secondary xylem of Monotoca tamariscina indicates that it does not fit in Styphelieae; a position within Oligarrheneae is possible. Like most Cosmelieae, all Richeeae are characterized by exclusively scalariform perforation plates with many bars, a very high vessel density and paratracheal parenchyma, although they clearly differ in ray width (exclusively uniseriate rays in Cosmelieae vs. uniseriate and wide multiseriate rays in Richeeae). Several wood anatomical features confirm the inclusion of epacrids in Ericaceae s.l. Furthermore, there are significant ecological implications. The small vessel diameter and high vessel frequency in many epacrids are indicative of a high conductive safety to avoid embolism caused by freeze-thaw cycles, while the replacement of scalariform by simple vessel perforation plates and an increase in vessel diameter would suggest an increased conductive efficiency, which is especially found in mesic temperate or tropical Styphelieae.     

Pollen morphology and wood anatomy of the Crudia group (Leguminosae, Caesalpinioideae, Detarieae)

Pollen from all 12 and wood from 11 genera in the Crudia group have been examined using light, scanning electron and transmission electron microscopy. This group is currently of systematic interest because it is part of a tribe undergoing taxonomic revision. The pollen grains fall into four groups: (1) Oxystigma, Kingiodendmn, Gossweilerodendron, Bathiaea,.Neoapaloxylon, Stemonocoleus, Guibourlia and Prioria have pollen of a widespread and generalized cacsalpinioid type that are small to medium sized, spheroidal to prolate, tricolporate and with a perforate exine, with some variation in surface ornamentation, aperture margins and ultrastructure. (2) Crudia pollen is tricolporate, coarsely striate with a coarsely scabrate to vcrmiculate aperture membrane. (3) Augouardia is tricolporate and coarsely reticulate. (4) Hardwickia and Colophosperrnum are pantoporate and reticulate or microreticulate-rugulate. The wood of Prioria, Oxystigma, Kingiodendmn and Gossweilerodendron has diffusely arranged axial canals, and these are four genera that have recently been merged into Prioria. Bathiaea has tangentially arranged axial canals. The other genera lack normal axial canals. Crudia is distinct, with banded parenchyma and variably storied short rays, Augouardia has much less abundant axial parenchyma that is mainly scanty paratracheal and vasiccntric, Guibourtia has mainly aliform parenchyma and rays variable in height and width, and Colophosperrnum and Hardwickia have similar paratracheal parenchyma patterns, although the rays tend to be wider in the latter. Our conclusion is that the Crudia group is not monophyletic.     

Ontogenetic mechanisms and the evolution of Cactaceae

Abstract

The ancestors of cacti were leafy trees that had hard, woody trunks. The development of the cactus body is controlled by ontogenetic mechanisms that have evolved, and now they produce a body that is leafless, succulent and has a photosynthetic cortex. Specific changes include: bark formation is postponed and the epidermis and stomata function for many years; the outer cortex is a palisade cortex with intercellular spaces; there are cortical bundles that resemble leaf veins but which have secondary xylem and phloem. Wood development has changed dramatically such that water storage is maximized (increased ray parenchyma) and danger of water stress is minimized (increased paratracheal parenchyma, loss of fibers). Several genera have polymorphic wood: the plants produce one type of wood for several years, then later they produce a different type. It is possible that the extensive evolutionary changes have resulted from mutations in the controller regions of genes, not in the structural regions.     

A comparison of the wood anatomy of 11 species from two cerrado habitats (cerrado s.s. and adjacent gallery forest)

A comparative study of the secondary xylem (wood) anatomy of 11 species (38 specimens) occurring in cerrado s.s. and the adjacent gallery forest (both cerrado s.l. habitat) was made with the aim of identifying the anatomical characteristics of ecological value and correlating them with the environmental conditions. The anatomical features that vary, in general, between the two habitats are: growth ring distinctness (well or poorly defined); tyloses and deposits (more abundant in cerrado specimens); gelatinous fibres (more evident in cerrado specimens and in different patterns between habitats); variation in paratracheal and banded parenchyma (more abundant in cerrado); and more cells per parenchyma strand in cerrado. In general, gallery forest specimens have wider vessels, fewer vessels per square millimetre and larger intervessel pits, indicating more efficient water conduction, whereas cerrado s.s. specimens are the opposite, with low vulnerability and mesomorphy indices, demonstrating greater safety under conditions of water stress. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, ?? , ??–??.     

壳斗科木材识别和分类

从106种壳斗科木材构造特征.作为本科木材识别和分类依据,概括于下: 散孔材,管孔细,单个密布,在生长轮末端一狭带无管孔,宽射线为多列射线,较狭而数较多为水青冈属。辐射孔材,管孔成串径列,宽射线聚合型,在横面反光弱为主,分布不均,促使材身槽棱不整齐,槽底平而宽长的为稠属。辐射孔材,管孔成串径、斜列为主,宽射线聚合型(常见)和栎型,在横面反光强,分布均匀,促使材身槽棱整齐,槽底尖而分隔为青冈属。半环孔材或环孔材,早材管孔沿生长轮不连续排列,在肉眼下可见至明显,晚材管孔成串径列,弯曲或火焰状,宽射线聚合型,常数少且几条聚近分布,大部分树种无宽射线的为锥属。环孔材,早材管孔沿生长轮连续排列,肉眼下明显,侵填体丰富,晚材管孔成串径列,弯曲或火焰状,宽射线栎型为栎属。环孔材,早材管孔沿生长轮连续排列,肉眼下明显,具侵填体,晚材管孔成串径列,弯曲或火焰状,射线细而肉眼下不见为栗属。 各属共同特征,薄壁组织离管带状,似环管(实际为环管管胞),生长轮不同程度波状。     

Effects of flooding on wood and bark anatomy of four species in a mangrove forest community

Variation in the wood and bark anatomy of the dominant species of a mangrove forest community in Mexico was evaluated in relation to some environmental factors, and their physiological adaptations to salinity and flooding period are discussed. The forest is characterized by three zones according to the presence of dominant tree species and flooding periodicity. Vessel arrangement and wood and bark ray height are strongly associated with flooding zones where trees are growing. Variance analyses revealed significant differences among zones for these anatomical characteristics. Soil texture and water salinity were the most useful parameters for the prediction of values of anatomical characteristics. More abundant vessels in radial multiples in a shorter flooding period suggest a functional advantage of multiple vessel groups. Taller wood and bark rays in response to prolonged flooding period can be attributed to anoxic conditions. Among zones, significant differences in the vulnerability index of the species were detected, but not with respect to relative conductivity. Significant differences among zones exist for wood and bark characteristics involved in vertical and horizontal water transport, photosynthates and gas exchange.     

The anatomy and grain pattern in forks of hazel (Corylus avellana L.) and other tree species

Key message

This paper provides a new anatomical model for how branches are joined together at forks in trees, using evidence from MicroXCT scanning, ESEM output and visual observation.

Abstract

Wood grain arrangements at the forks and other junctions within a tree must be arranged to mechanically join together the two or more branches, yet not adversely restrict sap flow. The grain orientation at junctions therefore represents a trade-off in xylem performance between the functions of efficient sap conductance and the provision of adequate load-bearing capacity. Initial observations of wood grain orientation were made on the surfaces of several dozen debarked and fractured bifurcations of a wide range of tree species, both by eye and using a scanning electron microscope. Subsequently, small volumes of wood were sampled from two locations within the junctions of hazel, at the junction apex and on the outer section of join. Wood was imaged in 3D using high-resolution X-ray tomography, and the scanned volumes were analysed for their wood grain patterns. It was found that the wood at the junctions of hazel forks contained only 37 % of the number of vessels contained in wood within the adjacent stem. The vessel elements formed at the junctions were only 32.5 % the length of those in the stem, had a mean diameter only 50.5 % of the stem vessels and consequently only 26.3 % of their lumen volume. The passage of the vessels through the fork wood deviated from a straight line (Euclidean) distance by more than 14 times more than the stem wood vessels did. The interweaving of vessels in the fork wood was over 22 times greater than in the stem wood. A survey of rays showed them to be 58 % more abundant in fork wood but only 62 % of the height of rays in the stem wood. These results suggest that where two branches of similar diameter join to form a tree fork, an interlocking wood grain pattern is formed at the apex of the bifurcation, which provides higher tensile strength to the junction. Breaking of the hazel fork requires wood fibres to be stretched axially and broken across, which requires greater stress than breaking normal wood radially or tangentially.