Influence of exogenous ethylene on cambial activity,xylogenesis and ray initiation in young shoots of Leucaena leucocephala (lam.) de Wit

The effect of exogenous ethephon on cambial activity, xylem production and ray population in young shoots of Leucaena leucocephala was investigated anatomically. The application of ethephon showed a diphasic effect on cambial activity and xylogenesis in a dose dependent manner. Lower concentration of ethephon enhanced cambial activity while high concentrations reduced cambial cell divisions and daughter-cell differentiation. High ethephon concentration also resulted in shorter vessel elements, thick walled fibers and phenolic accumulation in ray cells and vessel elements, whereas low concentration allowed elongation of fibers and vessel elements. The density of rays increased significantly with increase in ethylene concentration. The evaluation of longitudinal sections of cambial zone in ethephon treated plants showed high frequency of transformation of fusiform initials into ray initials through divisions and segmentation, resulting in high ray frequency in both xylem and phloem. The present study demonstrates that ethylene plays an important role in regulating secondary vascular tissue composition by reducing the population of fusiform initials in the cambium.     

EFFECT OF ISOLATION OF BARK ON CAMBIAL ACTIVITY AND DEVELOPMENT OF XYLEM AND PHLOEM IN TREMBLING ASPEN

Circular patches of bark were surgically isolated on the sides of trembling aspen (Populus tremuloides Michx.) trees at breast height at various times during the dormant and growing seasons. Subsequently, samples of wood and attached bark were taken from isolated and control sites to determine the effects of isolation of the bark on cambial activity and xylem and phloem development. In control trees cambial activity and xylem and phloem development occurred normally. Isolation of bark during the dormant season (in November, February, or March) did not prevent initiation of cambial activity and of phloem differentiation in spring but continued normal cambial activity and phloem developmented were prevent. Xylem differentiation was essentially prevented by isolation of tissues during the dormant season. The ultimate effect of isolation of the bark on the cambium, either during the dormant season or during the growing season, was subdivision of all fusiform cambial cells into strands of parenchymatous elements; the ultimate effect on the newly formed phloem was early death of the sieve elements. The most conspicuous effect of isolation of the bark after xylem differentiation had begun was the curtailment of secondary wall formation. Shortening of cells of the cambial region was reflected in the length of the vessel members which differentiated from such cells. These results indicate that normal cambial activity and xylem and phloem development require a supply of currently translocated regulatory substances from the shoots.     

INFLUENCE OF PHLOEM BLOCKAGE ON CAMBIAL GROWTH OF SUGAR MAPLE

Circular patches of bark were surgically isolated on the sides of sugar maple (Acer saccharum Marsh.) trees at breast height at various times during the dormant and growing seasons. Subsequently, samples of wood and attached bark were taken from isolated and control sites to determine the effects of isolation of the bark on cambial activity and xylem and phloem development. In control sites cambial activity and xylem and phloem development occurred normally. Isolation of bark during the dormant season (in November, February, or March) prevented initiation of cambial activity and xylem and phloem development in isolated areas of half of the trees. Varying degrees of cambial activity (periclinal divisions) occurred in the remaining isolated areas, but normal cambial activity and xylem and phloem development were prevented. Isolation of bark after initiation of cambial activity and phloem differentiation, but prior to initiation of xylem differentiation, resulted in the formation of very narrow xylem and phloem increments with atypically short vessel members and sieve-tube members, respectively. The xylem increments consisted primarily of parenchyma cells. Isolation of bark after initiation of xylem differentiation resulted in curtailment of secondary wall formation in the last-formed part of many increments. The last-formed vessel members of all these xylem increments were atypically short. Similarly, the last formed sieve-tube members of corresponding phloem increments were atypically short. The atypically short cells in the xylem and phloem of isolated areas reflected the effect of isolation on the cambial region, viz., the subdivision of all fusiform cells into strands of cells. Ultimately, the strands of short fusiform cells lapsed into maturity, leaving only strands of parenchymatous elements between xylem and phloem.     

Modelling for rearrangement of fusiform initials during radial growth of the vascular cambium in Pinus sylvestris L.

In contrast to common belief, recent studies have confirmed that intrusive growth of fusiform cambial initials has a significant role in the rearrangement of the initials, but does not contribute to the cambial circumference increment. We observed a rapid rearrangement of cambial initials on a long series of transverse sections of the vascular cambium and the wood of a 50-year-old pine (Pinus sylvestris L.) tree. A comparison of cell arrangement in consecutive sections, as well as a critical analysis of tangential reconstructions, has confirmed that changes in cell locations in a group of cells on the tangential surface caused no change in the total tangential width of the whole group. Models illustrating changes in locations of the initials have been proposed, assuming that intrusive growth, which makes the growing initials intrude between the neighbouring initials and their immediate derivatives, is localized on the longitudinal edges of cells. We infer that intrusive growth of the cambial initials in P. sylvestris is not involved in the cambial circumference increment, but plays a significant role in the rearrangement of the initials, probably allowing for a relaxation of shearing strains generated during radial growth. The relationship of intrusive growth with the elimination of initials has been discussed with reference to the frequency of anticlinal divisions. It has been proposed that the occurrence of anticlinal divisions in excess over the actual requirement for increase in the cambial circumference could be due to internal shearing strains.     

Mathematical Modeling of Intrusive Growth of Fusiform Initials in Relation to Radial Growth and Expanding Cambial Circumference in Pinus sylvestris L.

This study on the cambium of Pinus sylvestris L. examines the intrusive growth of fusiform cambial initials and its possible contribution to the tangential and radial expansions of the cambial cylinder. The location and extent of intrusive growth of the fusiform initials were determined by microscopic observations and by mathematical modeling. In order to meet the required circumferential expansion of the cambial cylinder, the fusiform initials grow in groups by means of a symplastic rather than intrusive growth, leaving no room for the assumption that intrusive growth of the initials takes place between radial walls and has a direct role in the increase of the cambial circumference. Therefore, it is postulated that the fusiform initials grow intrusively between the tangential walls of the neighboring initials and their immediate derivatives and not between the radial walls of the adjacent initials as per common belief.     

DEVELOPMENTAL CHANGES IN THE VASCULAR CAMBIUM OF POLYGONUM LAPATHIFOLIUM

Developmental changes in the vascular cambium of Polygonum lapathifolium were determined primarily by an analysis of the secondary xylem. The cambium and xylem consist of fascicular and interfascicular regions in this herbaceous dicotyledon. Near the pith vessels are restricted to the fascicular regions of the xylem. During secondary growth vessels are formed in some radial files in the interfascicular regions. Anticlinal divisions are of two types, oblique and lateral. In interfascicular files consisting of fibers only, about two-thirds of the anticlinal divisions are oblique. The oblique partition averages 31% of the length of the dividing initials. In interfascicular files consisting of vessel elements and fibers, there are almost equal numbers of oblique and lateral divisions. The oblique partition averages 37% of the length of the dividing initials in these files. Lateral divisions account for approximately three-fifths of the anticlinal divisions in the fascicular regions, consisting of vessel elements and fibers. The partitions formed in oblique anticlinal divisions average 64% of the length of the dividing cells in the fascicular regions. The frequency of anticlinal division is much higher in files consisting of vessel elements and fibers than in those consisting of fibers only. There is no loss of fusiform initials, except by ray formation. Ray initiation occurs by simple subdivision of fusiform initials. The findings are discussed in relation to the developmental changes in the vascular cambium in plants of different habits.     

Seasonal variations in the cambial anatomy of Tectona grandis (Verbenaceae)

A study of seasonal activity of the cambium in Tectona grandis L. f. has shown that the activity initiates in the first week of June, reaches a peak in July and then slowly declines. The length of fusiform cambial initials undergo considerable variations during the activity and dormancy of the cambium. The initiation of cambial activity is closely associated with the opening of the dormant foliar buds in the first week of May. Cambium is more active with high numbers of immature xylem and phloem elements from July to September when the trees are with mature foliage and flowers and dormant from January to April when leaves dry and defoliation takes place. The differentiation of xylem and phloem starts simultaneously and the number of their immature elements reach the maximum in July.     

VASCULAR CAMBIUM AND WOOD DEVELOPMENT IN CARBONIFEROUS PLANTS. I. LEPIDODENDRALES

Patterns of activity in the vascular cambium of Carboniferous arborescent lycopods (Paralycopodites and Stigmaria) were studied by analysis of serial tangential sections of the secondary xylem. The analysis assumes that cell patterns in the wood accurately reflect those of the corresponding cambium. An evaluation using indirect evidence indicates that the assumption is valid as far as can be determined from comparison with living plants. The tracheids of the secondary xylem enlarge in a centrifugal pattern, suggesting a progressive enlargement of the fusiform initials. There is no evidence of periodic anticlinal division of these initials, and it is proposed that the increase in cambial circumference was accommodated primarily by an increase in fusiform initial size. In some axes with abundant secondary xylem there is evidence that isolated initials or groups of initials sporadically subdivided to form numerous, spindle-shaped meristematic cells. Some of these cells subsequently developed into typical cambial initials. Tissues presumably formed during the cessation of cambial growth in Lepidodendron and Stigmaria are described; the structure of the tissues is suggestive of a postmeristematic parenchymatous sheath. It is concluded that cambial activity in these arborescent cryptogams was clearly different from that of modern seed plants, further attesting to the distinctive nature of this ancient group.     

What do we know about growth of vessel elements of secondary xylem in woody plants?

Despite extensive knowledge about vessel element growth and the determination of the axial course of vessels, these processes are still not fully understood. They are usually explained as resulting primarily from hormonal regulation in stems. This review focuses on an increasingly discussed aspect – mechanical conditions in the vascular cambium. Mechanical conditions in cambial tissue are important for the growth of vessel elements, as well as other cambial derivatives. In relation to the type of stress acting on cambial cells (compressive versus tensile stress) we: (i) discuss the shape of the enlarging vessel elements observed in anatomical sections; (ii) present hypotheses regarding the location of intrusive growth of vessel elements and cambial initials; (iii) explain the relationship between the growth of vessel elements and fibres; and (iv) consider the effect of mechanical stress in determining the course of a vessel. We also highlight the relationship between mechanical stress and transport of the most extensively studied plant hormone – auxin. We conclude that the integration of a biomechanical factor with the commonly acknowledged hormonal regulation could significantly enhance the analysis of the formation of vessel elements as well as entire vessels, which transport water and minerals in numerous plant species.     

The regulation of cambial division and secondary xylem differentiation in xanthium by auxins and gibberellin

Cambial division continued in decapitated Xanthium plants without concomitant xylem fiber differentiation. The application of indoleacetic acid to these plants did not affect the production of cambial derivatives or induce xylem fiber differentiation. When naphthaleneacetic acid was applied either to the second internode or to the stump of a lateral shoot, xylem fiber differentiation was induced in the newly formed cambial derivatives on the xylem side of the cambium in the stem. When naphthaleneacetic acid was applied unilaterally, xylem fiber differentiation was restricted to that side of the stem in the first internode and hypocotyl. Naphthaleneacetic acid also enhanced the production of cambial derivatives. Gibberellic acid enhanced cambial derivative production but did not affect the differentiation of xylem fibers. Similar numbers of cambial derivatives were produced in some naphthaleneacetic acid-treated plants in which xylem fiber differentiation was induced and in gibberellic acid-treated plants which did not differentiate xylem. When naphthaleneacetic acid was applied 72 hours after decapitation, the oldest of the cambial derivatives on the xylem side failed to develop into fibers although younger cells did. These results suggest that auxin has its direct effect on the induction of xylem differentiation rather than the induction of divisions prerequisite to differentiation.     

Variations in the Lengths of Fusiform Cambial Cells and Vessel Elements in Kalopanax pictus

Samples of a mature specimen of Kalopanax pictus, a ring-poroushardwood, were studied to compare the respective lengths offusiform cambial cells and vessel elements in the stem. Thelengths of dormant and reactivated fusiform cambial cells weremeasured with a confocal laser scanning microscope in tissuethat had been macerated by digestion with pectinase and in thicktangential sections. The lengths of early wood and late woodvessel elements were measured in tissues that had been maceratedby Franklin's method. The vessel elements and fusiform cambialcells varied considerably in length within individual samples.The mean length of early wood vessel elements corresponded tothat of fusiform cells in the dormant cambium but not in thereactivated cambium. Significant differences were observed betweenthe mean lengths of dormant and reactivated fusiform cambialcells, between those of reactivated fusiform cambial cells andearly wood vessel elements, between those of reactivated fusiformcambial cells and late wood vessel elements, and between thoseof early wood and late wood vessel elements. The frequency distributionsof lengths of cambial cells were bimodal and differed from thoseof vessel elements, which more closely resembled a normal distribution.The proportion of shorter lengths was higher in the reactivatedcambium than in the dormant cambium, the early wood and thelate wood vessel elements. Our results do not suppot the hypothesisthat the lengths of early wood vessel elements in ring-poroushardwoods change during differentiation. The similar rangesof recorded lengths suggest that short and long vessel elementsmight be derived directly from short and long cambial cells,respectively. Copyright 1999 Annals of Botany Company Kalopanax pictus, cambium, vessel element, confocal laser scanning microscopy, maceration, cell length.     

THE CAMBIUM AND SEASONAL DEVELOPMENT OF THE PHLOEM IN PYRUS MALUS

Evert , R. F. (U. Wisconsin, Madison.) The cambium and seasonal development of the phloem in Pyrus malus. Amer. Jour. Bot. 50(2): 149–159. Illus. 1963.—The cambium in apple consists of several layers of cells at all times, and practically all cambial cells divide periclinally one or more times before undergoing differentiation. The cambial initials do not seem to be in a uniform, uniseriate layer. Judged by collections made during 2 seasons (August, 1958–October, 1960), the seasonal cycle of phloem development is as follows. Early in April, cells in the outer margin of the cambial zone begin to differentiate into sieve elements. At approximately the same time, activity (division) commences throughout the cambial zone. By the end of July or early August, sieve-element differentiation is completed. Cessation of function begins in either late September or in October with the formation of definitive callose on the sieve areas of sieve elements in the outer margin of the functional phloem. By late November, all sieve elements are devoid of contents and most of their companion cells collapsed. Phloem differentiation precedes xylem differentiation by approximately a month and a half; xylem and phloem differentiation cease almost simultaneously; and fiber-sclereid development is coincident with the period of maximal xylem differentiation.     

Seasonal development of phloem in Siberian larch stems

The seasonal development of phloem in the stems of Siberian larch (Larix sibirica Ldb.) was studied over two seasons on 50–60-year-old trees growing in a natural stand in the Siberian forest-steppe zone. Trees at the age of 20–25 years were used to study metabolites in differentiating and mature phloem elements, cambial zone, and radially growing xylem cells in the periods of early and late wood formation. The development of the current-year phloem in the stems of 50–60-year-old trees started, depending on climatic conditions, in the second-third decades of May, 10–20 days before the xylem formation, and ended together with the shoot growth cessation in late July. Monitoring of the seasonal activity of cambium producing phloem sieve cells and the duration of their differentiation compared to the xylem derivatives in the cambium demonstrated that the top production of phloem and xylem cells could coincide or not coincide during the season, while their differentiation activity was always in antiphase. Sieve cells in the early phloem are separated from those in the late phloem by a layer of tannin-containing cells, which are formed in the period when late xylem formation starts. The starch content in the structural elements of phloem depends on the state of annual xylem layer development. The content of low molecular weight carbohydrates, amino acids, organic acids, and phenols in phloem cells, cambial zone, and xylem derivatives of the cambium depends on the cell type and developmental stage as well as on the type of forming wood (early or late) differing by the cell wall parameters and, hence, by the requirement for assimilates. Significant differences in the dynamics of substances per dry weight and cell were observed during cell development.     

THE CAMBIUM AND SEASONAL DEVELOPMENT OF THE PHLOEM IN ROBINIA PSEUDOACACIA

The cambium in black locust consists of several layers of cells at all times. Cambial reactivation (division) is preceded by a decrease in density of cambial cell protoplasts and cell wall thickening but not by cell enlargement. During the resumption of cambial activity, periclinal divisions occur throughout the cambial zone. Early divisions contribute largely to the phloem side. The period of greatest cambial activity coincides with early wood formation. Judged by numerous collections made during two seasons (October, 1960-October, 1962) the seasonal cycle of phloem development is as follows. Phloem differentiation begins in early April, ends in late September. The amount of phloem produced is quite variable (range: 1-10 bands of sieve elements per year). Cessation of function begins with the accumulation of definitive callose in the first-formed sieve elements and spreads to those more recently formed. By late November all but the last-formed sieve elements are collapsed. All sieve elements are collapsed by mid-winter and before the resumption of new phloem production in spring. Phloem differentiation precedes xylem differentiation by at least 1 week, and apparently functional sieve elements are present 3 weeks before new functional vessel elements. Xylem and phloem production ends simultaneously in most trees.     

Cambial activity,annual rhythm of xylem production in relation to phenology and climatic factors and lignification pattern during xylogenesis in drum-stick tree (Moringa oleifera)

The interrelationship among seasonality of cambium, wood formation, cell size variation, lignification, tree phenology and climatic factors has been examined in Moringa oleifera, a tropical evergreen tree. The vascular cambium in Moringa is a storied with a distinct seasonal variation in its structure due to dimensional changes in rays. Though cambium remains active throughout the year it is sensitive to water availability. Peak cambial cell division and rate of xylem differentiation are influenced by average rainfall during the monsoon period. Cambial cell division reaches higher up in the tree trunk when it is supporting a high number of branches and leaves. Statistical analysis of cell size variation and climate factors revealed that xylem cell development is greatly influenced by rainfall and rarely by temperature. Lengths of fusiform initials and vessel elements are positively correlated. The pattern of lignification during xylogenesis shows that the vessels are the first element to develop lignified walls and ray cells are the last elements to become lignified. Fiber cell walls show more syringyl lignin, while the cell walls of other xylem elements are characterized by relatively more guaiacyl lignin units.     

Fluctuations of cambial activity in relation to precipitation result in annual rings and intra-annual growth zones of xylem and phloem in teak (Tectona grandis) in Ivory Coast

Background and Aims Teak forms xylem rings that potentially carry records of carbon sequestration and climate in the tropics. These records are only useful when the structural variations of tree rings and their periodicity of formation are known. Methods The seasonality of ring formation in mature teak trees was examined via correlative analysis of cambial activity, xylem and phloem formation, and climate throughout 1·5 years. Xylem and phloem differentiation were visualized by light microscopy and scanning electron microscopy. Key Results A 3 month dry season resulted in semi-deciduousness, cambial dormancy and formation of annual xylem growth rings (AXGRs). Intra-annual xylem and phloem growth was characterized by variable intensity. Morphometric features of cambium such as cambium thickness and differentiating xylem layers were positively correlated. Cambium thickness was strongly correlated with monthly rainfall (R(2) = 0·7535). In all sampled trees, xylem growth zones (XGZs) were formed within the AXGRs during the seasonal development of new foliage. When trees achieved full leaf, the xylem in the new XGZs appeared completely differentiated and functional for water transport. Two phloem growth rings were formed in one growing season. Conclusions The seasonal formation pattern and microstructure of teak xylem suggest that AXGRs and XGZs can be used as proxies for analyses of the tree history and climate at annual and intra-annual resolution.     

RELATION OF DWARFMISTLETOE (ARCEUTHOBIUM) TO THE XYLEM TISSUE OF CONIFERS. II. EFFECT OF THE PARASITE ON THE XYLEM ANATOMY OF THE HOST12

Srivastava , L. M., and K. Esau , (U. California, Davis.) Relation of dwarfmistletoe (Arceuthobium) to the xylem tissue of conifers. II. Effect of the parasite on the xylem anatomy of the host. Amer. Jour. Bot. (48(3): 209–215. Illus. 1961.—The changes in the xylem anatomy induced by dwarfmistletoe infection were studied in 7 coniferous species. The most pronounced abnormalities are observed in the shape and size of the infected rays. Because of the presence of parasite tissue, the rays assume a hypertrophied appearance; moreover, they fuse to form large composite rays. The union of rays involves intrusive growth of ray cells and displacement of fusiform initials. Some division of fusiform initials also occurs. Rays may increase in number and they may contain more host cells than normal rays. Axial tracheids in infected host woods differ more or less strongly from those of noninfected woods. They may be shorter, wider, and more irregular in shape than the axial tracheids in healthy wood. The samples of xylem from infected pines had a larger number of resin canals than those from healthy trees. Resin canals were also found in infected Tsuga, which normally lacks these structures.     

Developmental anatomy of vascular cambium and periderm ofBotrypus virginianus and its bearing on the systematic position of ophioglossaceae

The developmental anatomy of the vascular cambium and periderm ofBotrypus virginianus was studied, and its bearing on the systematic position of Ophioglossacease is discussed. The cambial zone including cambium is initiated in a procambial ring of the stem before primary vascular tissue is well differentiated. The presumed cambium is composed of fusiform and ray initials. The cambium is extremely unequally bifacial, producing secondary xylem centripetally, and quite a small number of parenchymatous cells but no secondary phloem centrifugally. The cambial activity persists long, although it is very low in the mature part of the stem. It seems that the circumferential increase of the cambium is accommodated by an increase in the number of cambial initials. Secondary xylem is nonstoried and composed of tracheids with circular-bordered pits with evenly thick pit membranes, and uniseriate or partly biseriate radial rays. It makes up the bulk of the stem xylem. Periderm is formed almost entirely around the stem, simultaneous with its increment due to the secondary xylem. The combination of these anatomical features of secondary tissue supports the idea that Ophioglossaceae are living progymnosperms.     

Xylem heterochrony: an unappreciated key to angiosperm origin and diversifications

All angiosperms can be arranged along a spectrum from a preponderance of juvenile traits (cambial activity lost) to one of nearly all adult characters (cambium maximally active, mature patterns realized rapidly early in ontogeny). Angiosperms are unique among seed plants in the width of this spectrum. Xylem patterns are considered here to be indicative of contemporary function, not relictual. Nevertheless, most families of early‐divergent angiosperms exhibit paedomorphic xylem structure, a circumstance that is most plausibly explained by the concept that early angiosperms had sympodial growth forms featuring limited accumulation of secondary xylem. Sympodial habits have been retained in various ways not only in early‐divergent angiosperms, but also among eudicots in Ranunculales. The early angiosperm vessel, relatively marginal in conductive abilities, was improved in various ways, with concurrent redesign of parenchyma and fibre systems to enhance conductive, storage and mechanical capabilities. Flexibility in degree of cambial activity and kinds of juvenile/adult expressions has been basic to diversification in eudicots as a whole. Sympodial growth that lacks cambium, such as in monocots, provides advantages by various features, such as organographic compartmentalization of tracheid and vessel types. Woody monopodial eudicots were able to diversify as a result of production of new solutions to embolism prevention and conductive efficiency, particularly in vessel design, but also in parenchyma histology. Criteria for paedomorphosis in wood include slow decrease in length of fusiform cambial initials, predominance of procumbent ray cells and lesser degrees of cambial activity. Retention of ancestral features in primary xylem (the ‘refugium’ effect) is, in effect, a sort of inverse evidence of acceleration of adult patterns in later formed xylem. Xylem heterochrony is analysed not only for all key groups of angiosperms (including monocots), but also for different growth forms, such as lianas, annuals, various types of perennials, rosette trees and stem succulents. Xylary phenomena that potentially could be confused with heterochrony are discussed. Heterochronous xylem features seem at least as important as other often cited factors (pollination biology) because various degrees of paedomorphic xylem are found in so many growth forms that relate in xylary terms to ecological sites. Xylem heterochrony can probably be accessed during evolution by relatively simple gene changes in a wide range of angiosperms and thus represents a current as well as a past source of variation upon which diversification was based. Results discussed here are compatible with both current molecular‐based phylogenetic analyses and all recent physiological work on conduction in xylem and thus represent an integration of these fields. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 161 , 26–65.     

Structure of the vascular cambium of varying age and its derivative tissues in the stem of Ficus rumphii Blume

AJMAL, S. & IQBAL, M., 1992. Structure of the vascular cambium of varying age and its derivative tissues in the stem of Ficus rumphii Blume. The cambial cells of Ficus rumphii and their derivatives vary in size and number with the age of the tree. The fusiform initials, vessel elements, sieve-tube members, xylem fibres and phloem fibres are relatively short in the terminal part of the stem axis, consistently longer down the stem reaching a maximum, and short again in the main trunk. The width of the respective cells shows a similar variation. The ray cell initials increase both in number and size, and form rays of varying dimensions. Uniseriate and biseriate cambial rays are abundant, while multiseriate rays are especially sparse in the young shoot. The proportion of uni- and biseriate rays falls in the lower part of the stem whereas that of multiseriate rays increases. Likewise, short cambial rays are abundant in young branches but their frequency decreases in the trunk region, leading to a rise of the tall ray population. The rays in the phloem and xylem regions reflect a similar variation pattern with certain fluctuations. The overall proportion of the rays increases in the older part of the axis, reaching a maximum tangential area in the trunk. The transectional area of vessel elements remains more or less constant down to the base after an initial increase in the terminal region, while the sieve-tube proportion tends to be constant in the basal region after a consistent gain from the tree top basewards.