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.     

Cortical microtubules rearrange during differentiation of vascular cambial derivatives, microfilaments do not

The plant cytoskeleton has been implicated in a variety of morphogenetic events in higher plants. Most of this work, however, has concentrated on epidermal cells or primary tissues. We have investigated the cortical microtubular (CMT) and microfilament (MF) components of the cytoskeleton in a secondary tissue  –  active vascular cambium of Aesculus hippocastanum L. (horse-chestnut)  –  and followed the changes in these components during the early stages of differentiation of fusiform cambial derivatives to axial elements of the secondary vascular system. A correlative approach was used employing indirect immunofluorescence microscopy of α-tubulin on 6 μm sections, and transmission electron microscopy of 60 nm sections. The study has demonstrated a rearrangement of the CMT cytoskeleton, from random to helical, as fusiform vascular cambial cells begin to differentiate as secondary phloem vascular tissue. A similar CMT rearrangement is seen as fusiform cambial cells begin to differentiate as secondary xylem fibres. This rearrangement is interpreted as evidence of determination of cambial derivatives towards vascular development. Axially-oriented MF bundles are present in fusiform cambial cells and their axial orientation is retained in the vascular derivatives at early stages of their development even though the CMTs have become rearranged. Received: 5 August 1996 /  Accepted: 23 September 1996     

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 anatomy of the chi-chi of <Emphasis Type="Italic">Ginkgo biloba</Emphasis> suggests a mode of elongation growth that is an alternative to growth driven by an apical meristem

The chi-chi of Ginkgo biloba L. are cylindrical woody structures that grow downwards from the branches and trunks of old trees, eventually entering the soil where they give rise to adventitious shoots and roots. Examination of segments of young chi-chi taken from a mature ginkgo tree revealed an internal woody portion with irregular growth rings of tracheid-containing secondary xylem covered by a vascular cambium and bark. The cambium was composed of both fusiform cells and parenchymatous ray cells. Near the tip of the chi-chi, these two types of cambial cells had orientations ranging between axial, radial and circumferential with respect to the cylindrical form of the chi-chi. The xylem rays and tracheids that derived from the cambium showed correspondingly variable orientations. Towards the base of the chi-chi, the fusiform cells and young tracheids were aligned parallel to the axis, indicating that the orientation of the cambial cells in basal regions of the chi-chi gradually became normalised as the tip of the chi-chi extended forwards. Nevertheless, in such basal sites, tracheids near the centre of the chi-chi showed variable orientations in accordance with their mode of formation during the early stages of chi-chi development. The initiation of a chi-chi is proposed to derive from a localised hyperactivity of vascular cambial-cell production in the supporting stem. The chi-chi elongates by tip growth, but it does so in a manner different from organ growth driven by an apical meristem. It is suggested that the chi-chi of Ginkgo is an “evolutionary experiment” that makes use of the vascular cambium, not only for its widening growth but also for its elongation.     

A mathematical model of pattern formation in the vascular cambium of trees

The beautiful patterns apparent in wood grain have their origin in the alignment of fusiform initial cells in the vascular cambium of trees. We develop a mathematical model to describe the orientation of fusiform initial cells, and their interaction with the plant hormone indole-3-acetic acid (auxin). The model incorporates the following four assumptions: (1) auxin is actively transported parallel to the long axis of the initials, (2) auxin diffuses perpendicular to the long axis of the initials, (3) the initials tend to orient parallel to the flux of auxin through the cambium, and (4) adjacent initials tend to orient parallel to one another. Each assumption is justified on the basis of available evidence and cast in mathematical form. Our main result is a pair of nonlinear differential equations that describe the coupling between the distribution of auxin in the cambium and the orientation of fusiform initials. Numerical solutions to the equations show qualitative resemblance to the wood grain patterns observed at branch junctions, wounds and knots, and topological defects.     

Cambial reactivation in locally heated stems of the evergreen conifer Abies sachalinensis (Schmidt) Masters

A study was made of cambial activity, the localization of storage starch around the cambium, and the localization and occurrence of microtubules in cambial cells from dormancy to reactivation in locally heated (22–26 °C) stems of the evergreen conifer Abies sachalinensis. Heating induced localized reactivation of the cambium in the heated portions of the stem. Erect ray cambial cells resumed cell division 1 d prior to the reactivation of fusiform cambial cells and procumbent ray cambial cells. The re-initiation of the division of fusiform cambial cells occurred first on the phloem side. During the heat treatment, the amount of storage starch decreased in procumbent ray cambial cells and in the phloem parenchyma adjacent to the cambium but increased in fusiform cambial cells. Preprophase bands of microtubules, spindle microtubules and phragmoplast microtubules were observed both in erect ray cambial cells and in procumbent ray cambial cells. By contrast, no evidence of the presence of such preprophase bands of microtubules was detected in fusiform cambial cells. The results suggest that the localized heating of stems of evergreen conifers might provide a useful experimental model system for studies of the dynamics of cambial reactivation in intact trees. Received: 25 May 2000 / Accepted: 12 July 2000     

Ethylene evolution changes in the stems of<Emphasis Type="Italic"> Metasequoia glyptostroboides</Emphasis> and<Emphasis Type="Italic"> Aesculus turbinata</Emphasis> seedlings in relation to gravity-induced reaction wood formation

Two-year-old Metasequoia glyptostroboides and 3-month-old Aesculus turbinata seedlings were tilted at a 45° angle to induce compression wood formation on the lower side of the former species and tension wood on the upper side of the latter. Two weeks later, the seedlings were tilted in an opposite direction at 45° so that the upper and lower sides changed to each other. This reverse tilting was kept for 7 weeks for M. glyptostroboides and 6 weeks for A. turbinata. The seedlings were sampled and analyzed at intervals throughout each experimental period so that an ethylene evolution kinetic was monitored. Ethylene evolution from the cambial region of the upper and lower sides of tilted stems was measured separately by gas chromatography with a flame ionization detector. Xylem production expressed as wood area during each experimental period was microscopically determined. In both tilting and reverse tilting periods, the rates of ethylene evolution from the lower side of M. glyptostroboides and the upper side of A. turbinata, where xylem production was accelerated and compression or tension wood formation was induced, had increased to high levels, whereas those from the opposite sides had either remained low (in tilting period) or rapidly recovered to low levels (in reverse tilting period). The cambial activity quantified by wood formation, including reaction wood, in both species showed the same tendency as ethylene evolution. The stem side with vigorous ethylene evolution, xylem development and reaction wood formation reversed with the reversal of tilting orientation. The roles of accelerated ethylene evolution in reaction wood formation in the tilted seedlings of gymnosperm and angiosperm trees are compared and discussed.This work was presented at the 5th Pacific Region Wood Anatomy Conference, Yogyakarta, Indonesia, 9–14 September 2002     

MIGRATING CAMBIAL DOMAINS AND THE ORIGIN OF WAVY GRAIN IN XYLEM OF BROADLEAVED TREES

Anatomical analysis of wavy-grained xylem samples of Fraxinus, Acer, and Betula reveals a strong underlying pattern of orientational domains within the cambium. Domains are local areas in the cambium within which the orientation of pseudotransverse divisions of fusiform initial cells is predominately unidirectional rather than random. The orientation of ray splitting and ray uniting also tends to be unidirectional within domains. Because their boundaries migrate with time, domains are not fixed in shape or position. A specific group of initial cells may first be located in the interior of a domain, later near a boundary, and still later in another domain of opposite orientational prevalence. Domain patterns and their movements were delineated by classifying and mapping ray splitting and uniting events in temporal series of tangential sections of terminal late xylem. The domain patterns move slowly upward, preceding the similarly outlined, but noncoincident, grain pattern. A wavy grain pattern can be interpreted as being the result of a domain pattern of similar geometry. The efficacy of a domain pattern in eliciting a grain pattern is probably indirectly dependent upon the frequency of occurrence of pseudotransverse divisions and of ray splitting and uniting. The frequency is low in cambium producing straight-grained wood. Evidence of 10-fold and greater increases in frequency of ray splitting and uniting was found in straight-grained to wavy-grained transitional xylem.     

Elastic anisotropy of bone lamellae as a function of fibril orientation pattern

In this study, the homogenized anisotropic elastic properties of single bone lamellae are computed using a finite element unit cell method. The resulting stiffness tensor is utilized to calculate indentation moduli for multiple indentation directions in the lamella plane which are then related to nanoindentation experiments. The model accounts for different fibril orientation patterns in the lamellae—the twisted and orthogonal plywood pattern, a 5-sublayer pattern and an X-ray diffraction-based pattern. Three-dimensional sectional views of each pattern facilitate the comparison to transmission electron (TEM) images of real lamella cuts. The model results indicate, that the 5-sublayer- and the X-ray diffraction-based patterns cause the lamellae to have a stiffness maximum between 0° and 45° to the osteon axis. Their in-plane stiffness characteristics are qualitatively matching the experimental findings that report a higher stiffness in the osteon axis than in the circumferential direction. In contrast, lamellae owning the orthogonal or twisted plywood fibril orientation patterns have no preferred stiffness alignment. This work shows that the variety of fibril orientation patterns leads to qualitative and quantitative differences in the lamella elastic mechanical behavior. The study is a step toward a deeper understanding of the structure—mechanical function relationship of bone lamellae.     

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.     

Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.     

Differentiation of the ray system in woody plants

The regulation of vascular ray differentiation has received limited attention, despite the fact that vascular rays constitute an important part of the secondary body of plants. In this paper we review developmental aspects of the ray system and suggest a general hypothesis for the regulation of ray differentiation and evolution. In studies of ray differentiation, two basic factors should be taken into consideration: 1) the normal gradual increase in ray size in relation to age, distance from the pith, and distance from the young leaves; and 2) the influence of wound effects on the size, structure, and spacing of rays. The relationships between the rate of cambial activity and secondary xylem differentiation are not clearly understood. There are contrasting results on the relationships between ray number and rate of radial growth. The rate of radial growth (= rate of cambial activity) is not the regulating mechanism of ray characteristics. Bünning (1952, 1965) proposed that rays are distributed regularly in the tissue, as the outcome of an inhibitory influence expressed by them. However, Bünning’s hypothesis contradicts a basic feature of the vascular ray system, namely, fusion of rays. Detailed histological studies of the secondary xylem revealed that proximity to and contact with rays plays a major role in the survival of fusiform initials in the cambium (Bannan, 1951, 1953). Such evidence led Ziegler (1964) to suggest that since the cambium is supplied predominantly via the rays, this is an effective feedback regulative system for an equidistant arrangement of the rays. The hypothesis that rays are induced and controlled by a radial signal flow seems to be the best explanation for the structure and spacing of rays. The formation of a polycentric ray—a special case of “ray” initiation inside a vascular ray—supports the idea that radial signal flow occurs within the rays (Lev-Yadun & Aloni, 1991a). This idea is also supported by findings fromQuercus species in which aggregate rays in the xylem disperse naturally in branch junctions and, following partial girdling, leave a longitudinal narrow bridge of cambium and bark as a result of enhanced axial signal flow (of auxin and other growth regulators) (Lev-Yadun & Aloni, 1991b). The longitudinally elongated shape of rays is their response to axial signal flows (mainly the polar auxin flow). Two methods have been used to study the evolution of the ray system: 1) statistical studies of the relationships between vessel and ray characteristics in many species, when vessel characteristics were the evolutionary standard, and 2) comparison of ray characteristics in fossils originating from several geological eras. We suggest that evolution of the ray system reflects changes in the relations between radial and axial signal flows.     

Within-tree variation in cambial anatomy and xylem cell differentiation in Eucalyptus globulus

Summary Microscopic examination of the vascular cambium of Eucalyptus globulus, from four trees, at nine height levels, revealed a non-storied cambial structure with both ray and fusiform initials. Average fusiform initial length increased from 443 m at 70% of tree height from the base to a maximum of 484 m at 10% of tree height. At 2.5% of tree height average fusiform initial length sharply declined to 438 m. A strong positive correlation (r = 0.757, df = 7, P <0.05) existed between average fusiform initial length and average fibre length at each height, giving quantitative support to the notion that variation in fusiform initial length is an important mechanism influencing fibre length in hardwoods. However, 62% of the longitudinal variation in average fibre length was explained by the extent of elongation during differentiation, which increased linearly down the stem. At 70% of tree height, the average length of fibres was 1.84 times that of the fusiform initials, compared to 2.11 times at 2.5% of tree height. Concomitant with this longitudinal gradient of elongation was a gradient of increasing tangential area of the cambium occupied by cambial rays. This was accounted for by dimensional increases in the ray initial cells with decreasing height. Possible mechanisms that regulate the elongation of developing fibres and the proportion of cambial rays are discussed.     

The origin and development of vascular cambium in girdled stems ofEucommia ulmoides Oliv.

We conducted anatomical studies of girdled stems ofEucommia ulmoides at various developmental stages to elucidate the origin and development of callus and the vascular cambium. In the transverse view, ray initial cells in the cambial zone began to divide both periclinally and anticlinally 2 d after girdling. Fusiform initial cells started to enlarge at 3 d, then gradually proliferated via periclinal divisions. Thus, the callus was derived from the ray initial cells of the cambial zone as well as from fusiform initial cells. In the tangential view, callus cells derived from ray initial cells were short while those from fusiform initial cells were long, thereby producing a heterogeneous structure. However, the fusiform initial cells underwent transverse divisions 10 d after girdling, which resulted in shorter cells and a homogeneous callus structure. Afterward, some short cells divided transversely while others elongated, so that a heterogeneous form was regained. Finally, the vascular meristem that was girdled early in its development redifferentiated from short and long cells in the callus. The long cells developed into fusiform initials, with the short ones becoming ray initials.     

Function and dynamics of auxin and carbohydrates during earlywood/latewood transition in scots pine

In temperate regions the annual pattern of wood development is characterized by the formation of radially narrow and thick walled latewood cells. This takes place at the later part of the growing season when cambial cell division declines. To gain new insight into the regulation of this process, micro-analytical techniques were used to visualize the distribution of indole-3-acetic acid (IAA), soluble carbohydrates, and activities of sucrose (Suc)-metabolizing enzymes across the cambial region tissues in Scots pine (Pinus sylvestris). The total amount of IAA in the cambial region did not change with latewood initiation. But its radial distribution pattern was altered, resulting in an increased concentration in the cambial meristem and its recent derivatives. Thus, initiation of latewood formation and cessation of cambial cell division is not a consequence of decreased IAA concentrations in dividing and expanding cells. Rather, IAA most likely has a role in defining the altered developmental pattern associated with latewood formation. Carbohydrates and enzyme activities showed distinctive radial distribution patterns. Suc peaked in the phloem and decreased sharply to low levels across the cambial zone, whereas fructose and glucose reached their highest levels in the maturing tracheids. Suc synthase was the dominating Suc cleaving enzyme with a peak in the secondary wall-forming tracheids and in the phloem. Soluble acid invertase peaked in dividing and expanding cells. Suc-phosphate synthase had its highest activities in the phloem. Activities of cell wall bound invertase were low. The absence of major seasonal variations indicates that carbohydrate availability is not a trigger for latewood initiation. However, steep concentration gradients of the sugars suggest a role for sugar signaling in vascular development.     

Critical nuclear DNA size and distribution associated with S phase initiation

Using HeLa S-3 cells synchronized by selective detachment, in this paper we report a parallel study of nuclear morphology and autoradiography grain patterns between middle G₁ and middle S phases: Our results show two distinct [³H]-thymidine labeling patterns. The first “peripheral” labeling pattern has a characteristic nuclear size distribution, in contrast to the heterogeneous and varying size distributions of Feulgen-stained nuclei, and apparently is characteristic of very early S phase. The sizes of the second labeling pattern—homogeneous or inhomogeneous grain distribution throughout the nucleus—are equal or larger than the first and vary with S phase progression. Together, the corresponding nuclear sizes of the labeled nuclei represent the larger extreme of nuclear areas, and the labeling index closely parallels the fraction of nuclei with areas larger than the minimum size of the labeled nuclei. These results suggest a characteristic nuclear size (reflecting unique intranuclear DNA distribution) as a necessary, if not sufficient, requirement for S phase initiation. Parallel experimentation with rat liver cells—synchronized in vivo by partial hepatectomy and analyzed by thin section autoradiography—confirms the existence of a peripheral labeling pattern in both the very early part and the very late part of S phase, which reconciles our data with previous results and points to the fact that both initiation and termination sites for DNA replication are near the nuclear periphery.     

A phase dynamic model of systematic error in simple copying tasks

A crucial insight into handwriting dynamics is embodied in the idea that stable, robust handwriting movements correspond to attractors of an oscillatory dynamical system. We present a phase dynamic model of visuomotor performance involved in copying simple oriented lines. Our studies on human performance in copying oriented lines revealed a systematic error pattern in orientation of drawn lines, i.e., lines at certain orientation are drawn more accurately than at other values. Furthermore, human subjects exhibit “flips” in direction at certain characteristic orientations. It is argued that this flipping behavior has its roots in the fact that copying process is inherently ambiguous—a line of given orientation may be drawn in two different (mutually opposite) directions producing the same end result. The systematic error patterns seen in human copying performance is probably a result of the attempt of our visuomotor system to cope with this ambiguity and still be able to produce accurate copying movements. The proposed nonlinear phase-dynamic model explains the experimentally observed copying error pattern and also the flipping behavior with remarkable accuracy.     

Developmental and structural features of secretory canals in root and shoot wood of Copaifera langsdorffii Desf. (Leguminosae–Caesalpinioideae)

Copaifera langsdorffii Desf., popularly known as copaíba, is an oleoresin-producing tree has been overexploited in Brazil by the pharmaceutical, cosmetic and varnish industries. Despite the long history of the use of this species, the structural knowledge on the resin-producing sites remains inadequate and is limited to the trunk. The aim of this study was to describe the origin, structure and developmental features of secretory spaces present in shoot and root wood of C. langsdorffii, based on the usual techniques of wood anatomy studies. Both root and shoot wood present secretory canals distributed along the marginal parenchyma bands which delimit growth layers. Secretory canals are constituted by a locally biseriate epithelium and lume that contains terpenes. They are arisen in the cambial zone from fusiform cambial initial cells and, eventually, ray initials. The origin of lumen is schizogenous. The epithelial cells have meristematic potential and are responsible for the locally biseriate epithelium. Mature secretory canals are wider and are separated between themselves only by a row of radial cells. The fusion of these adjacent secretory canals is frequent and results from the radial cell collapse. The finding of an interconnected system of resin canals in C. langsdorffii might be of value in terms of sustainable extraction of the resin and realistically assess its sustainable harvest potential.     

Fibre length and gibberellins A1 and A20 are decreased in Eucalyptus globules by acylcyclohexanedione injected into the stem

Trinexapacethyl (TriEt), an acylcyclohexanedionetype inhibitor of gibberellin (GA) biosynthesis, was applied to 3-year-old Eucalyptus globules saplings by localised injection near the base of each stem. The objective was to alter cambial region GA levels and to study the effects on secondary xylem fibre development. Seven weeks later wood samples, with bark and cambial region intact, were removed 10 and 30 cm above the point of injection. Fusiform cambial cell dimensions were compared with those of fibre-tracheids in the most recently formed 100 um of secondary xylem. Increasing TriEt applications from 5 to 5 000 mg active ingredient significantly reduced average fibre length, and to a lesser extent average fusiform cambial cell length. Also reduced was the number of cells in the cambial zone and the number of differentiating fibres with primary walls. However, no trends were evident for changes in fibre diameter, the proportion of vessel elements or the ratio of cambial ray cells to fusiform cambial cells. Two gibberellins (GA₁ and GA₂₀), indole-3-acetic acid (IAA) and abscisic acid (ABA) were quantified in cambial region tissues by gas chromatographymass spectrometry using stable isotope labelled internal standards. Increasing TriEt application reduced both GA₁ and GA₂₀ levels. Effects on IAA and ABA were not significant, although their levels tended to be lower at the highest TriEt application rate. The elongation of secondary xylem fibres was positively correlated with higher levels of endogenous GA₁ (r_s= 0.74, P < 0.01) and GA₂₀ (r_s= 0.72, P < 0.01). These results support a causal role for GA₁ in cambial cell division. They are also consistent with the hypothesis that the elongation of differentiating secondary xylem fibres in woody an–giosperms is dependent on GA₁ levels in the cambial region.     

Indole-3-acetic acid level in wood,bark and cambial sap of apple rootstocks differing in growth vigour

Free and conjugated IAA levels were determined in wood, bark and cambial sap of M.9, M.26 and MM.106 apple rootstock genotypes differing in growth vigour. The measurements were done on May 15^th, June 15^th and July 15^th. The level of free IAA in bark and wood of the tested trees varied from 27.0 to 52.7 ng·g⁻¹ f.w. while the conjugated hormone content averaged 3–5 times higher. In the bark and wood samples, the differences in auxin content between rootstock genotypes and the time of stem harvesting were insignificant. The level of free IAA in cambial sap was on average 10 to 20 times higher than in both bark and wood tissues, while the conjugated hormone level varied from none (below detection limit) to 37 ng·g⁻¹ f.w. Content of free IAA level in cambial sap from dwarf M.9 rootstock was significantly lower than that in either of the more vigorous genotypes. In both vigorous rootstocks IAA level in cambial sap remained at a similar level at all sampling dates but M.9 cambial sap showed a trend towards decreasing auxin content later in the growing season.