Secondary Vascular System Research Articles

A cytoskeletal basis for wood formation in angiosperm trees: the involvement of microfilaments.

The cortical microfilament (MF) component of the cytoskeleton within axial elements of the secondary vascular system of the angiosperm tree, Aesculus hippocastanum L. (horse-chestnut) was studied using transmission electron microscopy of ultrathin sections and indirect immunofluorescence microscopy of actin in thick sections. As seen by electron microscopy, MF bundles have a net axial orientation within fusiform cambial cells and their secondary vascular derivatives (i.e. in the axial xylem and phloem parenchyma, xylem fibres, vessel and sieve elements, and companion cells). Immunofluorescence studies, however, reveal that this axial orientation can be more accurately described as a helix of extremely high pitch; it is a persistent feature of all axial secondary vascular elements during their development. Helical MF arrays are the only arrangement seen in secondary phloem cells. However, in addition to helices, other MF arrays are seen in secondary xylem cells. For example, fibres possess ellipses of MFs associated with simple-pit formation, and vessel elements possess circular arrays of MFs that associate with the developing inter-vessel bordered pits, ray-vessel contact pits, and with the perforation plate. Linear MF arrays are seen co-oriented with the developing tertiary wall-thickenings in vessel elements. The possible roles of MFs during the cytodifferentiation of secondary vascular cells is discussed, and compared with that of microtubules.

Um Novo Lenho Gimnospérmico da Formação Rio Bonito, SC, Brasil - Aterradoxylon solidum gen. et sp.nov.

The morphoanatomic study of a taphoflora from the Rio Bonito Formation, Pouso Redondo County, Santa Catarina State, Brazil, resulted in the determination of new gymnospermous genera, among them a fossil wood that, by being represented by 9 of the 25 samples studied, constitutes the second form with significant representation in the assemblage (36%). By virtue of the solid and heterocelular pith, the centrifugal differentiation of the primary vascular system and the large pits in the cross-field of the secondary vascular system, the wood in question is similar to the morfogenus Megaporoxylon KRÄUSEL, 1956 from the Permian of South Africa. However, the presence of singular characters, like the nature of the secretory system and the outline of pith as well as the configuration of the bordered pits on radial walls of the secondary xylem cells, evidences the necessity of its segregation in new taxon which is named Aterradoxylon solidum gen. et sp. nov.

Physiological role of the secondary circulatory system in fish

Most studies of the secondary circulation or the “secondary vascular system” (Vogel 1981; Vogel and Claviez 1981) have concentrated on descriptions of morphology, rather than function, among various species (see Steffensen and Lomholt 1992). The scarce available evidence at this time characterizes the secondary vascular system in fishes as being a pool of blood which is low in pressure, low in red cell content, absent from the muscle tissues, associated with both the inner and outer surfaces of body including the gills, and of a volume that approaches or exceeds that of the primary circulation. Vogel (1985a) demonstrated the occurrence of an extremely dense capillary network of the secondary system over the scales of trout and tilapia. Satchell (1991) pointed out that similar vascularization on the scales had been observed for both freshwater and seawater teleosts in old anatomical literatures (Jakubowski 1960a,b, 1989; Kaczmarski 1966; Tyszkiewicz 1969), while it is not always clear whether such capillary networks belong to either the primary or secondary vascular system. A similar subepithelial vascular network has also been observed in the Atlantic hagfish Myxine glutinosa (Lametschwandtner et al. 1989). Vogel (1985a) also stated that secondary capillary networks are distributed in the mucous membrane of the mouth and the peritoneum, and possibly in the intestinal wall. The central venous sinus (CVS) which is located within the primary lamella of the gill is currently considered to be part of the secondary vascular system (Vogel 1985a).

Anatomy and relationships of the secondary vascular system of fish

Anatomy and relationships of the secondary vascular system of fish

Cambium: old challenges – new opportunities

Trees represent a, probably the, major component of the biosphere and have a unique place in the history of Mankind. One of their most fascinating features is the process of secondary growth which is effected principally by the secondary vascular system, the developmental continuum of secondary phloem, vascular cambium, and secondary xylem. However, for too long assumptions about the developmental biology of trees have had to be based upon studies of primary growth systems within annual, herbaceous species because study of the secondary vascular system had been largely ignored. Even when attempts are made to understand some of the most fundamental features of the secondary vascular system, such as xylogenesis, the current model system, isolated Zinnia mesophyll cells, is not entirely appropriate to the situation in the intact tree. Some deficiencies of the Zinnia system are discussed, and the advantages of the genus Populus as a model for study of the hardwood secondary vascular system are considered. Some of the new approaches which are poised to lead to significant advances in our knowledge of the cell bio-logy of the secondary vascular system of trees – spe-cifically of the cell wall, the plasmalemma, and the cytoskeleton – are discussed. The value of one of these new techniques – immunocytochemistry – is demonstrated by a consideration of recent work on the role of the cytoskeleton in the hardwood secondary vascular system.

Visualization of the cytoskeleton within the secondary vascular system of hardwood species.

A single fixation technique has been devised to demonstrate localization of alpha-tubulin (for microtubules) and F-actin (for microfilaments) within the secondary vascular system of hardwood trees by indirect immunofluorescence microscopy using butyl-methylmethacrylate-embedded material. Application of this technique to problems of cytomorphogenesis during secondary growth and its versatility are demonstrated with the hardwood species Aesculus hippocastanum L., Salix viminalis L., S. burjatica Nazarov x S. viminalis L., Hedera helix L., Acer platanoides L., Platanus sp., Quercus ilex L. and Liriodendron tulipifera L., and in the softwood Pinus pinea L. The methods employed have considerable scope for advancing knowledge of the role of the cytoskeleton in differentiation within the secondary vascular system of woody species.

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.

Anomalous Root Structure on Woody Plants in vitro

Abstract The anatomical structure of in vitro-generated roots contrasted sharply with ex vitro roots produced on parallel treatments of micropropagated woody plant clones. In vitro roots contained enlarged, irregular, frequently pigmented cortical cells with numerous intercellular spaces, and only primary vascular system 4–5 weeks after initiation, while ex vitro roots of the same age were comprised of smaller, uniformly arranged cell layers and were developmentally further advanced (exhibited secondary vascular system development). Light exposure was not a major influence on anomalous structure of in vitro roots, other than pigmented cell frequency. Ex vitro rooting hormone stimulated production of thicker, more frequent root initials, although root structure beyond the point of emergence from the stem quickly transformed to the slender, compact character typical of ex vitro production.

The course of root differentiation from root primordia in poplar stems

Upon rooting of poplar stem cuttiags the total inductive stimulation does not take place, but the individual root primordia maintain a relative independence. This becomes evident by various time parameters of their differentiation. Under experimental conditions, in dormant cuttings from one year individuals of the hybrid I 214 the activation of the first root primordia occurs after 24 h, the pre-emergent development of the roots formed was completed after 72 h.The activated root primordium is divided into two regions. In the distal region with the predominating cell division the root apex with histogens is formed by the action of initials. From peripheral cell layers of the distal region the so-called “Wurzeltasche” develops which covers the root cap. Due to cell elongation in the proximal region the root apex is pushed up towards the stem surface. The beginning differentiation of the connective vascular tissue is a preparative step for the connection of the vascular system of the developing root with the secondary vascular system of the maternal stem. Following the penetration of the root through the peripheral stem tissues this connection is realized with progressing development. In the developed root the protoxylem elements differentiate continuously and acropetally in direct continuity with tracheids of the basal connective region.