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New observations of radial sieve tubes in the secondary xylem of two genera and four species of agarwood — Aquilaria sinensis, A. crasna, A. malaccensis and Gyrinops versteeghii (Thymelaeaceae) — are presented in this study. The earliest radial sieve tubes in Gyrinops are formed in the secondary xylem adjacent to the pith. The radial sieve tubes originate from the vascular cambium and develop in both uniseriate and multiseriate ray tissue. In addition to sieve plates in lateral and end walls, scattered or clustered minute sieve pores are localized in the lateral wall of radial sieve tubes. There is a direct connection between radial sieve tubes in ray tissue and axial sieve tubes in interxylary phloem strands (IP), such as (i) connection by bending of radial sieve tube strands, (ii) connection of two IP strands by an oblique bridge, and (iii) connection of two IP strands at a right angle. The average number of radial sieve tubes and interxylary phloem was found to be 1.7 per mm3 and 9.1 per mm2 in the secondary xylem. Considering the higher frequency of radial sieve tubes with the increasing thickness of the secondary xylem, the direct connections between radial and axial sieve tubes could play a significant role in assisting the translocation of metabolites in Aquilaria and Gyrinops.

Our study investigated the effect of stem temperature increase on xylem formation in Robinia pseudoacacia tree-trunks, caused by direct exposure to solar radiation. It is important to determine factors which may improve the concentricity of deposited wood tissue and intensify xylogenesis because a strong irregularity of wood tissue deposited in the radial direction in mature trees of R. pseudoacacia reduces the commercial value of the wood. Samples of vascular cambium along with adjacent tissues were collected from the southern (illuminated) and northern (shaded) side of tree-trunks growing in the inner and peripheral (thus exposed to direct sunlight) zones of the research plot. Sampling was performed several times during the growing season. The collected material was examined by epifluorescence microscopy and the thickness of deposited tissue comprising cambial xylem derivatives was measured. Deposition of a markedly greater amount of xylem on the southern side of tree-trunks in the peripheral zone of the plot was observed before full leaf development. Instrumental climatic data confirmed that in the early stage of the growing season, temperature on the southern side of the peripheral zone tree-trunk was higher than on the northern side. No clear response in terms of directional deposition of xylem was noticed in the inner zone trees and in peripheral zone trees after full leaf development. This study highlights the importance of temperature increase, caused by solar radiation, for R. pseudoacacia xylogenesis, which may be considered as a factor that affects the course of the radial growth before full leaf development.

All cell types of the secondary xylem arise from the meristematic cells (initials) of the vascular cambium and grow under mechanical constraints emerging from the circular-symmetrical geometry that characterises many tree trunks. The course of intrusive growth of cambial initials has been elucidated, but is yet to be described in the case of xylem fibres. This study explains the geometry of intrusive growth of the secondary xylem fibres in the trunk of Robinia pseudoacacia. Long series of serial semi-thin sections of the vascular cambium and the differentiating secondary xylem were analysed. Since fibres grow in close vicinity to expanding cells of the derivatives of the vascular cambium, we assumed that they have similar growth conditions. Dealing with the cylindrical tissue of the vascular cambium in a previous study, we used a circularly symmetrical equation for describing the growth mechanism of cambial initials. Like the cambial initials, some of the cambial derivatives differentiating into the various cell types composing the secondary xylem also exhibit intrusive growth between the tangential walls of adjacent cells. As seen in cross sections of the cambium, intrusively growing initials form slanted walls by a gradual transformation of tangential (periclinal) walls into radial (anticlinal) walls. Similarly, the intrusive growth of xylem fibres manifests initially as slants, which are formed due to axial growth of the growing cell tips along the tangential walls of adjacent cells. During this process, the tangential walls of adjacent cells are partly separated and dislocated from the tangential plane. The final shape of xylem fibres, or that of vessel elements and axial parenchyma cells, depends upon the ratio of their intrusive versus symplastic growths in the axial, circumferential and radial directions.

The structure and development of interxylary phloem (IP) and external phloem in Aquilaria sinensis were investigated using light and scanning electron microscopy. Complete IP strands were isolated, measuring 14 ± 4 mm in length and 417 ± 124 μm in width. The outer margin of IP was composed of two to three layers of fusiform parenchyma cells. The development of IP can be divided into five stages: 1) Locally IP starts its differentiation within a small segment of a broad cambial zone, at the cost of xylem differentiation. 2) Inward growth of IP advances, and fibres and sieve tubes differentiate. 3) IP is constricted by the encroachment of immature xylem cells between cambium and immature IP. 4) IP is isolated from the cambium and surrounded by immature, non-lignified xylem tissue. 5) IP is surrounded by lignified xylem tissue, and the fibres within IP become lignified.In all the phloem islands in a ten-year-old stem, sieve elements showed positive staining of callose with aniline blue. However, no staining of callose was observed in the external secondary phloem of agarwood trees collected from two different sites. No sieve tubes or sieve pores were detected by SEM observation of numerous serial cross and radial sections of the external phloem. We therefore conclude that sieve tubes are absent from the external phloem or extremely rare and that the transport of photosynthetic products in the stem of A. sinensis takes place in the interxylary phloem.

Stem ontogeny and structure of two neotropical twining vines of the genus Callaeum are described. Secondary growth in Callaeum begins with a typical regular cambium that gradually becomes lobed as a result of variation in xylem and phloem production rates in certain portions of the stem aligned with stem orthostichies. As development progresses, lignified ray cells of the initially formed secondary xylem detach on one side from the adjacent tissues, forming a natural fracture that induces the proliferation of both ray and axial nonlignified parenchyma. At the same time, parenchyma proliferation takes place around the pith margin and generates a ring of radially arranged parenchyma cells. The parenchyma generated in this process (here termed disruptive parenchyma) keeps dividing throughout stem development. As growth continues, the parenchyma finally cleaves the lignified axial parts of the vascular system into several isolated fragments of different sizes. Each fragment consists of xylem, phloem and vascular cambium and is immersed in a ground matrix of disruptive parenchyma. The cambium present in each fragment divides anticlinally to almost encircle each entire fragment and maintains its regular activity by producing xylem to the centre of the fragment and phloem to the periphery. Additionally, new cambia arise within the disruptive parenchyma and produce xylem and phloem in various polarities, such as xylem to the inside and phloem to the outside of the stem, or perpendicularly to the original cambium. Unlike the very distinctive stem anatomical architecture resulting from this cambial variant in Callaeum, its secondary xylem and phloem exhibit features typical of lianas. These features include very wide conducting cells, abundant axial parenchyma, high and heterocellular rays and gelatinous fibres.

The activity of the vascular cambium, which plays an important role in plant growth and development, is well known to respond to environmental changes such as drought. Although plant death is suggested to be tightly associated with the death of cambium and other meristematic tissues, direct observations of the cambium ultrastructure during drought-induced mortality remain poorly documented. Here, seedlings of Corylus avellana were drought stressed over various days in autumn to reach xylem water potentials (Ψx, -MPa) between -1 and -5.2 MPa. Light and transmission electron microscopy was applied to visualize the ultrastructure of the root cambial cells, while stem vulnerability curves were obtained to quantify embolism resistance. Seedlings that were moderately water stressed with Ψx less negative than P50 (-2.0 MPa, i.e., the xylem water potential at 50% loss of hydraulic conductivity) showed intact cambium cells. In contrast, the cell membrane of fusiform cambial cells became detached, the tonoplast was shrunken or damaged and cell organelles started to disintegrate in severely water stressed plants with Ψx more negative than P50. Ray cambial cells, however, remained intact and alive for a longer period than fusiform cells. Death of the cambial fusiform cells corresponded more or less to the lethal water potential of this species (Ψlethal = -4.83 MPa). These observations suggest that drought-induced mortality of C. avellana is closely associated with cambial death.

The stem anatomy of Passiflora edulis, P. foetida, P. suberosa, P. subpeltata, and P. vesicaria was studied in samples collected in Durban (KwaZulu-Natal Province, South Africa) and Baroda (Vadodara, Gujarat State, India). Radial stem growth in all the species is realized by a single, bidirectional vascular cambium. However, unequal activity in small segments of the cambial cylinder results in a lobed stem outline in P. foetida var. ellisonii, and a furrowed xylem cylinder in P. edulis f. edulis and P. vesicaria var. vesicaria. In P. subpeltata and P. edulis f. flavicarpa the xylem remains cylindrical in outline. In all the species investigated, secondary xylem is diffuse-porous with growth rings indistinct or absent. In transverse view, vessels are round to oval with different diameter categories, including very narrow fibriform vessels intermixed. In P. edulis f. edulis, stems are lobed due to the unidirectional activity of the cambium in small segments. Rays are mostly both narrow (1–3-seriate) and wide (multiseriate). The latter often become aggregate at some distance from the pith.

The responses of the vascular cambium and tracheid differentiation to extreme drought in Aleppo pine (Pinus halepensis Mill.) were investigated. The research focused on the drought year of 2005, in the primary study area at Maigmo (MAI) in southeastern Spain, with comparisons in Jarafuel (JAL) and Guardamar (GUA). The climate in this region is typically warm and dry with hot summers. Wood formation throughout the 2005 growing season was studied in transverse microtome sections and integrated with a retrospective dendrochronological analysis of crossdated increment cores collected in 2009. For most anatomical sections collected throughout the growing season at MAI, the vascular cambium appeared to be dormant as indicated by the low number of cells per radial file. Occasionally, immature xylem derivatives were observed during the growing season but without production of an annual ring. In increment cores collected at MAI, the 2005 position in the annual ring series contained either a narrow ring of both earlywood and latewood (47% of samples), a narrow ring of apparent latewood with no earlywood (13%), or a missing ring (50%). We introduce the term “dark ring” to refer to those annual rings of apparent latewood with no earlywood. For trees at JAL, the 2005 ring had below-average width and contained both earlywood and latewood. At GUA, the trees produced the widest 2005 ring of all three sites and mainly contained an intra-annual density fluctuation (IADF). The IADF was formed after cambial reactivation in the autumn. Although dark rings, IADFs, and especially missing rings complicate dendrochronological analysis, these anatomical features may provide an additional proxy record from which to infer climate variability and change in the past.

Successive cambia are often associated with the climbing or shrub habit, and is less common in trees. We studied formation of successive cambia and structure of secondary xylem in young stems of Cocculus laurifolius DC., a tree species of Menispermaceae. Cell division in the vascular cambium ceased in pencil-thick stems. Subsequently, parenchyma cells located outside the perivascular fibre cap re-differentiated and gave rise to several small segments of meristematic cells, of which the central cells divided repeatedly to initiate the first successive cambium which produces secondary xylem centripetally and phloem centrifugally. Cells located on the inner side of the newly initiated cambium differentiated into conjunctive tissue while cells on the outer side of it divided further and differentiated into sclereids. Xylem was diffuse porous and composed of vessels, fibre tracheids and ray parenchyma cells, and only differed in vessel diameter from wide-vessel climbing relatives.

Peduncles of Couroupita guianensis Aubl. undergo extensive secondary growth, which is a rare and unexplored feature so far. In the present investigation seasonal behaviour of vascular cambium was studied in fruit-bearing peduncles and compared with the vegetative branches of similar diameter. In peduncles, the cambium remained active throughout the year. The number of cambium cells and differentiating xylem cells increased from May and reached a maximum in July-August. Although cambial growth occurred throughout the year, it was relatively sluggish in February despite the development of new leaves and ongoing extension growth. In contrast, cambial cell division in young branches initiated in February, peaked in the same months as peduncle cambium while cambial cell division and differentiation of xylem remained suspended from October to January. Cessation of cambial cell division in the branches during this period may be correlated with the presence of mature leaves. In both (branches and peduncle), rapid cell division and increase in the number of differentiating xylem elements in April-May is positively correlated with the development of flower buds and new leaves. The present anatomical investigation revealed that cambial activity in both peduncle and vegetative branches are independent of phenology and climatic conditions. In conclusion, we believe that variations in the number of differentiating cambium derivatives in peduncles benefits from a dual source of growth hormone supply, i.e. from developing new leaves and flower buds.

This study, aimed at elucidating cambial activity and wood formation in Acacia tortilis subsp. tortilis growing in the dry region of Al Baha in Saudi Arabia, reveals that the vascular cambium was active for almost the whole year, producing phloem in two flushes (February to March and then June to August), and xylem for about 11 months. The maximum number of cell layers in the cambial zone (10–13) was observed during June at high temperature, high leaf water deficit and lowest relative humidity (52%) and rainfall. A close relationship existed between the emergence of new leaves and the initiation/acceleration of cambial activity and tissue differentiation. The mean and maximum monthly temperature of the site showed positive and highly significant correlations (p < 0.01) with the width of the cambial zone. The mean monthly relative humidity and mean monthly rainfall were negatively but significantly correlated with cambial activity (p < 0.05). Leaf water deficit had a favorable effect on the cambial activity (r = 0.894, p < 0.01). Acacia tortilis subsp. tortilis has thick-walled vessels of medium diameter and frequency, thick-walled fibers occupying a high tissue proportion, high wood density (0.905 g cm-3) and tannin-like deposits in the vestured vessel pits. The apparent drought tolerance of this species is hypothesized to be due to its deep root system, allowing optimal cambial activity during extreme drought and extremely high day temperatures.

The development of internal phloem in the Jerusalem cherry, Solanum pseudocapsicum L. (Solanaceae), was studied in young and mature stems. The early presence of primary internal phloem is succeeded by the development of secondary internal phloem from an internal cambium situated between the protoxylem and primary internal phloem. In the second and third visible internodes of the young stem, procambial derivatives begin to differentiate as discrete strands of internal protophloem in a perimedullary position prior to the differentiation of protoxylem and external protophloem. In 6–8 mm diameter stems, sieve elements of the internal phloem become non-conducting, begin to collapse, and undergo obliteration. In 15–20 mm diameter stems internal cambium is initiated from the parenchyma cells situated between the protoxylem and primary internal phloem. The development of internal phloem and an internal cambium in S. pseudocapsicum is compared with that in other taxa. There seems to be a gradual variation in the origin of an internal cambium from either remnants of the procambium or dedifferentiation of peripheral pith cells across dicotyledons with an internal cambium.

Anatomical changes involved in traumatic phloem resin duct (TPRD) formation in western redcedar (Thuja plicata) roots were examined following abiotic wounding and fungal invasion by Armillaria ostoyae. Following necrophylactic periderm formation, hyperplasia and expansion of a band of phloem parenchyma cells occurred in close proximity to the vascular cambium and schizogenous and lysigenous separation of its derivatives resulted in a series of longitudinal resin ducts in the inner to mid-phloem region. Fungal invasion appeared to amplify traumatic resin duct formation in the phloem. While traumatic cavities in the phloem have been reported for other Cupressaceae, this is the first report documenting TPRD formation in western redcedar.

The present study focuses on the origin, secondary growth, and anatomical characteristics of the intraxylary phloem compared to the regular phloem in Combretum rotundifolium Rich. The intraxylary phloem first originates in the primary stem from the perimedullary procambium and, later, from the internal cambium that develops by differentiation of the perimedullary procambium and dedifferentiation of perimedullary parenchyma cells. Tangential sclerenchyma bands, irregular stratification of the cellular elements, and uniseriate rays are present in the regular phloem, but not in the intraxylary phloem. We are the first to report the presence of an unusual bidirectional internal cambium that produces parenchymatous cells centrifugally and intraxylary phloem centripetally.

Many tropical tree species produce growth rings in response to seasonal environmental factors that influence the activity of the vascular cambium. We applied the following methods to analyze the annual nature of treering formation of 24 tree species from a seasonal semi-deciduous forest of southeast Brazil: describing wood anatomy and phenology, counting tree rings after cambium markings, and using permanent dendrometer bands. After 7 years of systematic observations and measurements, we found the following: the trees lost their leaves during the dry season and grew new leaves at the end of the same season; trunk increment dynamics corresponded to seasonal changes in precipitation, with higher increment (active period) during the rainy season (October–April) and lower increment (dormant period) during the dry season (May–September); the number of tree rings formed after injuries to the cambium coincided with the number of years since the extraction of the wood samples. As a result of these observations, it was concluded that most study trees formed one growth ring per year. This suggests that tree species from the seasonal semi-deciduous forests of Brazil have an annual cycle of wood formation. Therefore, these trees have potential for use in future studies of tree age and radial growth rates, as well as to infer ecological and regional climatic conditions. These future studies can provide important information for the management and conservation of these endangered forests.

Morphological variability in wood fibres of Lonchocarpus sericeus (Poir.) Kunth ex DC. (Leguminosae), a tropical hardwood tree with doublestoreyed cambium, was examined in thin tangential and transverse sections as well as in macerations of wood tissue. Occurrence of characteristic protrusions (lateral expansions) was detected on the extended part of the main fibre body. Distance between the adjacent protrusions corresponded to the height of a storey (horizontal tier) of the cambial initials. Rays were shorter in height than the neighbouring fusiform initials and therefore unable to reach the boundary of the storey. This situation facilitated the lateral expansion of the adjoining fibres during their apical elongation by intrusive growth. The presence of the characteristic protrusions on the fibre body thus indicated that the given fibre was associated with a double-storeyed cambium having rays shorter than the length of fusiform initials. The ultimate shape of fibres was thus correlated to the height of storeys and the height and width of rays.

The anatomy of an unusual canker on Fraxinus spp. was investigated with elastomer microcasts. The canker was caused by a new fungal disease, 'Coin Canker of Ash', affecting nursery stock in Northeastern North America. Cankers on bark surfaces were remarkably round and coppercolored. Diseased areas of stems had non-parallel orientations of axial parenchyma tissue as well as of the rays, vessel elements and fibers in contrast to healthy areas. Bark in diseased areas of stems was eroded beneath the surface and a callus was produced along the margins of damage that filled the cavity of eroded tissue. Diseased areas had large aggregates of sclereids compared to healthy areas. Ray initials in diseased areas of the stem were shorter, multiseriate and 3–10 or more cells in width compared to the longer uni- and biseriate initials in healthy areas. Wood in diseased areas had circular vessels in tangential view due to a change in the shapes of individual vessel elements, compared to parallel and straight vessels in healthy areas. Individual elements became spindleshaped and gave rise to zigzag vessels. The fungal pathogen, Neofabraea alba, appeared to alter the way in which cambial cells differentiated.

This paper contains a description of the only known non-banded bark in the geologic record. The bark is attached to a branch of the Araucarioxylon arizonicum tree that was found recently in the Late Triassic Chinle Formation (~ 220 Mya) in Arizona. In the fossil the vascular cylinder is almost totally enclosed in rough bark 2–11 mm in thickness. The inner bark is less than 1 mm thick and comprises a very narrow zone of presumed phloem containing infrequent fibers. The outer bark varies in thickness up to 10 mm, and consists of curving lenses of sequent periderm free of both dilation tissue and resin canals but is quite unlike the banded bark of Araucaria heterophylla. Wide zones of very thin-walled rectangular parenchyma cells arranged in radial files and groups of thin-walled spongy parenchyma, and small clusters as well as solitary sclerenchyma occur in the rhytidome. The vascular cambium contains 1–4 fusiform cells per radial file. Uniseriate rays are common in the secondary xylem and extend to the cambial zone. Small (1 mm in diameter), simple tunnels in the bark and wood indicate that the branch was attacked by phytophagous insects. A possible petrified beetle larva similar in form and size to Anobium is associated with these tunnels.

Development of cambial variant and xylem structure were studied in the stem of Cocculus hirsutus (Menispermaceae). In the early stages of stem development several collateral vascular bundles are joined by interfascicular cambium resulting in the formation of a complete cambial cylinder. After functioning for two to three years the cambial ring ceases its activity. Subsequently a second ring of cambium is formed from the innermost cortical parenchyma cells. These parenchyma cells undergo periclinal divisions to give rise to cells that become lignified, abaxially, and cambial cells, adaxially. The cambial cells divide periclinally giving rise to individual vascular bundles with xylem and phloem. Later the cambium in each bundle is joined by interfascicular cambium. Subsequent cambia develop similarly resulting in the formation of successive rings of xylem and phloem. During the leafless condition, all the cambial rings are dormant, and flanked by mature xylem and phloem elements. With the sprouting of new leaves, either the existing outermost cambium reactivates or an entire new ring of cambium develops. The xylem is diffuseporous with indistinct growth rings. It is composed of fibre-tracheids, tracheids, vessel elements, libriform fibres, and parenchyma cells. Xylem rays are multiseriate, compound and heterocellular. Deformed libriform fibres and vessel elements commonly occur among the ray cells in all the successive rings of xylem. The length of fibres and the height and width of xylem rays increase gradually from the centre towards the periphery of the stem.

Kino veins and/or pockets are a characteristic defect of some Eucalyptus species and have long been recognized as sources of degrade in eucalypt timber. In a comparative study, the course of both histochemical and anatomical responses of three juvenile Eucalyptus species (E. nitens, E. globulus and E. obliqua) to treatment with an ethylene-releasing compound, 2-chloroethyl phosphonic acid (CEPA), were examined. Kino veins were induced in both E. obliqua and E. globulus. The hormone treatment failed to induce kino vein formation in E. nitens. The traumatic parenchyma (that later differentiated into kino veins) was not always derived from the initials of the vascular cambium as reported in previous studies of E. globulus. Instead, the traumatic parenchyma appeared to be produced from the meristematic activity of the inner phloem parenchyma. In addition, kino veins found in 2- to 3-year-old E. globulus were mostly included in the phloem and not in the xylem as generally observed in mature E. globulus.

The inflorescence stem of Arabidopsis thaliana is characterized by a wavy ring of lignified fibers inward to the cortex. In young stems, primary fibers develop from the ground meristem, forming the outer part of the pith, while in mature stems the fiber system is comprised both of these primary fibers as well as secondary ones that differentiate from the cambium. Typically, the cambium formed in the inflorescence stem is not always continuous. Therefore, the secondary xylem and the secondary parts of the wavy band of fibers are usually formed only in sectors at the circumference. Submergence of the developing inflorescence stem arrests both the lignification of the wavy band of primary fibers and the development of the cambium. The arrest of cambial ontogeny and fiber lignification by environmental conditions offers new avenues to study the cascade of genes involved in cambial initiation and fiber lignification.

Secondary growth in Achyranthes aspera, Alternanthera polygamous, A. pungens, A. sessilis, and A. triandra was achieved by the development of a cambial variant resulting in successive rings of xylem and phloem. Each new cambium was developed at a distance about two to three cells external to the phloem produced by the previous cambium. The development of phloem was not synchronous in the species studied. Phloem development started either simultaneously with xylem or after the formation of a few xylem derivatives. In Achyranthes, xylem production started first followed by the development of phloem. Phloem mother cells differentiated into sieve tube elements, companion cells and axial parenchyma. Xylem was storied and exclusively composed of axial elements. Radial elements were absent in all the xylem rings of the stem. Vessels were angular and mostly solitary, but radial and tangential multiples were also observed occasionally. Xylem fibres were nonseptate and nucleated. Development of phloem and the rayless nature of the xylem is discussed.

Growth periodicity was followed for two consecutive annual cycles to reveal the pattern of wood fonnation in plantation-grown teak at three different localities in India. Rainfall and age were the two important factors that influenced cambial activity. Cambial reactivation occurred during March-April in both years. The pre-monsoon showers broke the cambial donnancy at all three localities. Almost a month's interval was observed between bud break and initiation of radial growth . Irrespective of age and locality, a peak period of cambial activity occurred during June-July. Dormancy began during October-December, depending on the age of the trees and locality. Juvenile trees and those grown in relatively high rainfall areas had a prolonged cambial activity and retained foliage throughout the year. They produced wider rings with higher proportions of latewood. Irrigation of 5-year-old trees led to the loss of typical ring porosity of teak wood; their first three growth rings were more or less diffuse-porous. This is attributed to uninterrupted cambial activity resulting in production of rather uniform-sized vessels.

Seasonal behaviour of vascular cambium in Tectona grandis L. f. growing in Moist Deciduous Forests (MDF) and Dry Deciduous Forests (DDF) of Gujarat State in Western India was studied for one annual cycle. In both the forests active cambial cell division and simultaneous differentiation of xylem and phloem started in June when the dormant shoot buds opened. In MDF cambial cell activity reached its peak in August-September and ceased in October; in DDF it ceased in November after reaching a peak in July-August. Maximum radial growth in trees of both forests occurred during the monsoon period. In both forests, phloem differentiation ceased before xylem differentiation. During dry months and the leafless periods the cambium remained dormant. Xylem mother cells next to the mature xylem in MDF underwent differentiation into xylem elements following the onset of periclinal divisions in March. In both forests, the seasonal anatomical changes associated with the cambium closely followed the phenology of the tree and local climatic conditions.

Botrychium dissectum Sprengel rhizomes were examined at monthly intervals from February 1993 through December 1994. Sampies taken ranged from those with an inactive cambium and only mature tracheids to those having an active cambium and immature tracheids. The vascular cambium became activated in the early fall prior to maturation of the leaf and fertile spike complex. Intertracheid pit membranes with tori were present in all sampies, although the morphology of the torus varied. The presence of tori was first observed in a tracheid that had just initiated its secondary wall formation. As the pit membrane matured, matrix material was hydrolyzed first from the margo area, then from the torus, and eventually the pit membrane was represented only by a very thin network of microfibrils. In addition, studies confirmed that tracheids bordering parenchyma cells developed a torus thickening, aIthough no thickening of the parenchyma cell wall occurred. Torus ontogeny in B. dissectum combined features previously described for angiosperms and gymnosperms.

Structural variations in cambium, xylem and phloem collected from main trunks of Sterculia colorata, S. alata, S. villosa, S. urens and S. foetida growing in the South Dangs forests were studied. In all five species, the cambium was storied with variations in the length of fusiform cambial cells. Compared to other species S. foetida had the longest and S. urens the shortest fusiform cambial cells. Cambial rays in all the species were compound (tall) and heterocellular with sheath cells. Their height and width were maximal in S. foetida and in S. villosa respectively. In all the species the storied nature of fusiform cambial cells was maintained in derivative cells that developed into sieve tube elements; vessel elements and axial parenchyma of both phloem and xylem. However, fibres of phloem and xylem were nonstoried. The dimensions of elements in phloem and xylem varied among the species. The variation in the mean length of sieve tube elements and vessel members coincided with that of fusiform cambial cells.

The rate and duration of the phases of secondary xylem fibre development were quantified at 10 and 60% of tree height from the base in three trees of Eucalyptus globulus. At 60% of tree height the number of cells undergoing division, en1argement and secondary wall development was fewer than at 10% of tree height (P < 0.0001). A very strong positive correlation (r = 0.968, df = 4, P< 0.01) also existed between the rate of radial cell production and the number of cells in the cambial zone. At 60% of tree height cambial derivatives differentiating into fibre tracheids required an average of 5.3 days to complete enlargement and 25.2 days to complete secondary wall development. At 10% of tree height the average duration of these phases was 6.0 and 32.8 days respectively. Although not statistically significant these differences suggest that the duration of fibre differentiation increases basipetally. The average rate of fibre elongation was 66 µm d-1 at 60% of tree height and 99 µm d-1 at 10% of tree height. The duration of the differentiation of vessel elements and cells associated with vessels was shorter than for fibres, indicating that the different cell types in the developing secondary xylem have their own characteristic rate and duration for the processes of differentiation. Considering theories that suggest vessel development is promoted by auxin, this observation is regarded as indirect evidence that auxin reduces the duration of the differentiation process.

Anatomy and development of vascular tissues in the annual stems of Croton glandulosus var. septentrionalis are described. In primary stages of growth the stem possesses a eustele of bicollateral bundles; internal phloem is notably more extensive than the external. In addition to a vascular cambium and secondary xylem that form in the usual fashion, additional cambia add cells to the internal phloem portion of the bicollateral bundles, forming well-marked medullary bundles at the perimeter of the pith. At first, the perimedullary cambial strands produce only internal secondary phloem; later, internal secondary xylem is also formed in some stems. When internal secondary xylem is present, the medullary bundles have an inverted orientation, i.e., phloem innermost (towards centre of pith) and xylem outermost (near protoxylem). Cells of the medullary bundles include sieve tube elements, vessel ekments, and fibres. Normal (external) secondary phloem is weakly developed. Normal secondary xylem contains short vessel elements with simple perforation plates and alternate intervascular pits, libriform fibres, narrow heterocellular rays, and lacks axial parenchyma.

In Ficus religiosa (Moraceae) extension and radial growth occurs in late July and early August, respectively, under the local climate of Aligarh. The derivative tissue differentiates into xylem and phloem simultaneously in August. The phloem production stops late in August, restarts early October and then continues up to November. The xylogenesis continues up to November without interruption. Formation of the precursor phloem is observed in March. Cell size and the relative proportion of fusiform and ray initials vary with season.

The wood of Dalbergia paniculata is unique as it consists of concentric layers of broad xylem, alternating with bands of narrow phloem. This anomaly results from the periodic formation of successive cambia in the secondary phloem. Some phloem parenchyma cells dedifferentiate to form a discontinuous ring of cambium. Such parenchyma cells have higher succinate dehydrogenase activity than the neighbouring cells of secondary phloem. The newly differentiated cambial layer functions bidirectionally, and its products give rise to xylem internally and phloem externally. The phloem along with cambium present internal to the newly formed xylem becomes included.The wood is diffuse-porous and the intervessel pits are vestured. The phloem has welldifferentiated sieve tube members and companion cells.

A single 1.41-m-long trunk of Cycas micronesica K.D. Hill was found growing horizontally in natural habitat on the island of Guam. Transverse sections of the trunk showed enhancement of secondary vascular tissues on the lower side by the production of more arcs of accessory vascular cambia and more parenchyma as compared to the upper side. Xylem tracheids were similar in upper and lower sides. There was no evidence of compression wood tracheids as seen in conifers.

The annual rhythms of cambial and phellogen activity in Pinus halepensis and P pinea were investigated. Under natural conditions the cambium of P halepensis begins its activity in autumn, enters a quiescent period during midwinter, resurnes activity towards spring and enters a second rest period in summer. The ring border is formed during summer. Irrigated plants growing outdoors were active almost all the year round.The cambium of P pinea is active between April and November and enters a true winter dormancy.The duration of xylem production exceeded that of the phloem. More xylem than phloem cells were formed. The phellogen was active during a short period only.Pinus halepensis seems to follow the Mediterranean climate patterns whereas P pinea follows the pattern of a colder climate.

Primary anatomy and secondary development is described for two root types from the Fremouw Peak locality (Transantarctic Mts, Antarctica) of early to middle Triassic age. Roots of Antarcticycas have a bilayered cortex with thick surface cuticle, diarch xylem, and a clearIy defined endodermis surrounded by a single cell layer possessing phi thickenings. Secondary development begins with phellern and phelloderm production from the out er primary phloem position, and is followed bya bifacial vascular cambium next to the primary xylem that pro duces sieve cells and ray parenchyma to the outside. Young roots of Antarcticoxylon are similar to those of Antarcticycas, but may possess 2-3 cell layers with phi thickenings. Secondary development from a bifacial vascular cambium produces alternating bands of sieve cells and phloem parenchyma cells in the secondary phloem and wood with uniseriate rays and scattered axial parenchyma. The presence of phi thickenings and an epidermal cutieie in both roots suggests environmental stress related to water regulation. The occurrence of phi thickenings in the roots of some conifers, angiosperms, a fossil cycad and a probable seed fern suggests this character is of ecological rather than phylogenetic significance.

In the course of the righting movement in young spruce trees (Picea jezoensis Carr. var. hondoensis Rehd.) inclined at 45°, the occurrence of compression wood associated with the development of vascular cambium in the shoot was observed. In shoots, the recovery first took place at the mid point, a few days after inclination. The observations of serial cross sections taken from the apex downward revealed no appreciable difference in the development of the procambium-cambium continuum between the upper- and underside of the shoot. The formation and structure of primary tracheary elements were similar, irrespective of the site of the procambium in the shoot. No compression wood cells occurred before the vascular cambium cylinder was complete. The stimulus of compression wood formation is received only by the differentiating secondary xylem tissues derived from the cambium cylinder.

Developmental changes in the xylem were studied in a stem of Bocconia vulcanica Donn. Smith with a xylem radius of 3.0–4.5 cm. Growth rings are absent. The vascular cambium is nonstoried with fusiform initials averaging 282 µm long. The specialised vessel members are short, with oblique to transverse end walls, simple perforations, and alternate intervascular pitting. Vessels are relatively uniform in diameter and arrangement throughout the wood. Fibres have moderately thin walls and do not increase in length from the primary xylem to the cambium. Axial parenchyma is paratracheal, scanty to vasicentric. Rays are exclusively multiseriate, tall, and heterocellular with a predominance of erect and square cells. Sheath cells occur along the sides. There are no fibres in the secondary phloem and a periderm is not present. The xylem and bark are similar in many respects to that formed in some groups of dicotyledons that are basically herbaceous with evolution toward woodiness.

The secondary phloem of nine species in five genera of Winteraceae was examined with regard to features that could serve for taxonomic and phylogenetic evaluation of the family. The species examined were as follows: Bubbia pauciflora, B. semecarpoides, Drimys lanceolata, D. winteri, Exospermum stipitatum, Pseudo wintera axillaris, Zygogynum baillonii, Z. bicolor, and Z. vinkii. The nine species showed the following common characteristics: 1) origin from nonstoried vascular cambium with long fusiform initials; 2) ray system consisting of high multiseriate and high uniseriate rays; 3) occurrence of secondary partitioning in the differentiating phloem so that the sieve elements are much shorter than the tracheids; 4) lack of sharp differentiation between lateral sieve areas and those of the sieve plates; 5) predominance of compound sieve plates; 6) short companion cells, often single in a given sieve element; 7) phloem parenchyma cells in strands; 8) lack of specialised fibres (bast fibres) in the secondary phloem; 9) presence of nondispersing protein body in the sieve element protoplast. Features numbered 1, 2, 4-6 are considered to be indications of low evolutionary level. The significance of the other three features (3, 7-9) requires further evaluation. Among these three is the secondary partitioning the occurrence of which seems to imply that in some taxa the well known sequence of evolutionary shortening of cambial initials and their derivatives may be accelerated on the phloem side.

Laticifers are recorded both penetrating from primary tissues into the vascular cambium and secondary xylem, and traversing the secondary xylem in young stems of various Croton spp. These observations are unique for Euphorbiaceae, and highly unusual in other families. They indicate that laticifers in some instances enter the secondary xylem from the cortex, and in others become enveloped in secondary xylem following secondary meristematic activity by the vascular cambium.

Kino veins, generally referred to as gum veins, are formed in Eucalyptus spp. in response to injury. Although they are retained as defects in the wood of many species, in some they become inc1uded in the phloem, then the rhytidome and are eventually shed. Ninety-three Eucalyptus spp. were sampled to determine whether they had xylem or phloem (bark) veins. The species which exhibited kino veins in the phloem were all members of three sections of the subgenus Symphyomyrtus. All species examined of the subgenera Monocalyptus and Corymbia had xylem veins. Kino veins are formed by the lysigenous breakdown of parenchyma bands produced by the cambium. The difference which results in the veins either being shed in the bark or inc1uded in the xylem is determined at the time of their initiation. The parenchyma bands may be produced on either the xylem or on the phloem side of the cambium. The fate of the veins becomes apparent once normal cambial divisions resurne the production of xylem and phloem.

Under favourable growth season and by suitable technical means, regeneration and continuous growth of new bark after girdling has been observed in many trees. Differentiation of the secondary xylem varies after arteficial treatment. Thus , the authors consider that (1) under appropriate conditions most trees could be girdled on a large scale with subsequent new bark regeneration and continued growth, (2) after removal of the phloem the living cells of the secondary xylem, i.e., wood parenchyma cells, may function in transporting nutrients from the treecrown downwards, and (3) finally, after girdling or when cultured in vitro, both immature xylem and phloem can dedifferentiate into meristematic tissue that further develops vascular tissue.

Summary Seasonal variations in the activity of vascular cambium have been investigated in Gmelina ar­ borea from January 1978 to December 1978. The cambium is nonstoried and comprises elon­ gated fusifonn initials and isodiametric ray ini­ tials. Cambial activity initiates in the first week of March, reaches a peak in August, then de­ clines and ceases in November. The cambial zone shows marked periodical ·changes and be­ comes 4-9-layered during the dormant period (January - February; November- December) and 4-20-layered during the active period (March-October). The initiation of cambial activity is closely associated with the emer­ gence of new crops of leaves occurring in two distinct flushes. The cambium is most active in August when the tree is with mature foliage and rainfall is highest. Dormant cambium is noticed when defoliation takes place during the dry months. The differentiation of xylem and phloem starts simultaneously and the number of their immature elements reaches a maximum in August. The variations in the mean length of fusiform initials show a close relationship with that of xylem fibres.

Cytochemical techniques and mild extractions were used at the electron microscope level for the study of the cambial zone of several hardwoods and one softwood. The maturation processes of the primary radial and tangential cell walls involve a progressive disappearance of their initial heterogeneity. The buttress-like zone joining these walls appears to be the starting point for a characteristic sequence of changes and intra-wall rearrangement. Topochemical results have suggested an alternative to the 'emboxing concept' of cell wall development.

The central cylinder of xylem plus pith was removed from the main stem of 1- to 2- year-old Broussonetia papyrifera (L.) Vent. treelets, leaving only the bark, cambial zone cells and a few immature xylem cells. The immature phloem and xylem cells and original cambium zone cells rapidly produced callus, then the original cambial zone started to produce xylem and phloem. A flat meristem was initiated intermittently inside the callus. Eventually, these islands connected laterally to form a ring of new cambium that began to produce normal xylem inward and phloem outward. About one month later, a concave oblate trunk had formed, which connected distally and proximally with the original trunk.