Vascular strand

APase in onion roots. Enzyme activity was mainly extracellular with the heaviest concentration in corner spaces between the epidermal and hypo-dermal layers. He suggested the possibility of a subcutaneous pore through which the enzyme could be released to the root surface. Bieleski and co-workers (Reid Bieleski, 1970 Bieleski Johnson, 1972) studied the psi induction and location of APase in duckweed (Spirodela oligorrhiza). APase in control plants was located primarily in and around the vascular strands. In P-deficient plants psi-APase activity was 10-20 times the control value. Enzyme activity was primarily located in the epidermis of the root and undersurface of the frond, the tissue locations most likely to provide access to phosphate esters in the medium. These workers further demonstrated that hydrolysis of organic phosphates occurred in the external medium and/or the apoplast followed by Pi uptake into the cell. 

SoLtellor epithelium Scdtellum Vascular strand r Coleoptile ( shoot cap "... 

About 380 million years ago, plants with vascular tissue first evolved a special type of leaf, referred to as a microphyll. A microphyll typically has a single midvein, and arises from a stem which does not have leaf gaps, in regions of parenchyma (i.e, unspecialized) tissue where the vascular strand leads into the leaf base. The microphyll may have originated as an outgrowth of a vascularized stem, or by evolutionary simplification of a complex branch system. The leaves of certain modern plants in... 

At the stem apex, alkaloids are present in all the young undifferentiated cells. According to Molle (6), the most recently formed cells have comparatively little, the precipitations increasing to a maximum density at a short distance behind the actual apex. The zone of tissue differentiation is also abundantly supplied, but as differentiation proceeds, alkaloids disappear from the vascular strands, and then from the central tissues of the pith. When differentiation is complete, the alkaloids are located principally in three concentric layers, in the epidermis and outer cortical layers just below it, in parenchyma within and adjacent to the phloem, and in the periphery of the pith just inside the xylem strands. The xylem parenchyma and medullary rays also possess alkaloids after they have disappeared from the conducting elements. 

The epidermis of Cinchona leaves is said to contain no alkaloids, but they accumulate in large cells to form a special hypodermal layer (23). c. Tissues In and Near the Vascular Strands. Examples occur in bella-... 

Fig. 5. Vertical and horizontal sections of a pea nodule approximately 4 weeks old and 3 mm in length attached to a root which is also sectioned. Root vascular strand, RV root peiicycle, RP root endodermis, RE root cortex, RC nodule cortex, NC nodule vascular strand, NV nodule pericycle, NP nodule vascular strand endodermis, NVE nodule endodermis, NE nodule meristem, M infection zone, I early symbiotic zone, ES late symbiotic zcme, LS senescent zone, Sn nodule surface, NS. (Drawn after Bond, 1948, and from sections from Newcomb, 1976, and Syono et al., 1976.)...

Clasen et al. (1974), found developmental changes in endothelial cells (causing vascular strands ), and reported the presence of PAS-positive globules in perivascular astrocytes of the cerebellum and basal ganglia. These droplets, originally described by Blackman in 1936, were described as small accumulations of phagocytosed oedema fluid, possibly containing lead. The oedema fluid in the white matter was found to contain albumen and sodium. These vascular strands were noted in both rat and human material, and were believed to be collapsed capillaries due to arrested development,... 

Flavonoids have been reported at high levels in epidermal tissues, particularly in leaves [11]. A recent immunohistochemical study of parsley seedlings provided evidence for chalcone synthase (the entry point to flavonoid biosynthesis) in epidermal tissue, and also in vascular strand tissue of parsley leaves [40]. Polar lAA transport is known to be associated with non-conducting cells of vascular strands [ 1, 12, 27, 49]. 

For convenience we will discuss regeneration first, then normal differentiation. This distinction is somewhat artificial, because most experiments in which hormones have been added to plants have involved the wounding the plants (the hormone being substituted for excised organs). However, I shall restrict the term regeneration to studies in which the continuity of some tissue, such as a vascular strand, is severed and subsequently reforms from cells that would not normally differentiate into such tissue. 

More precise determinations were made by Thompson and Jacobs (1966), who made the wound in one of the smaller vascular strands in the flat sides of Coleus stems (rather than in the large strand in the corners), so that the regeneration would be almost all in one plane and would consist of fewer cells. 

In his elegant study of hormonal control of the normal differentiation of phloem fibers (described below), Aloni (1976) pointed out that the fibers did not differentiate from the parenchymatous cells of the wound callus, even though sieve tubes and tracheary cells did. The differentiation of phloem fibers was always limited to the longitudinal vascular strands, never being found in the intervening parenchyma. In that sense, one could say that they do not regenerate however, their regeneration has been seldom studied. 

SO the completed vascular system is ready at an early stage for translocation of metabolites from the scutellum to the embryo, and/or (if it occurs) for the transport of gibberellins synthesized at the node to the scutellum, whence they can be released. There is evidence, coming from the use of radioactive gibberellic acid, that this hormone can indeed be transported through the vascular strand of the scutellum [112]. 

Patterns of reserve mobilization 1. Pattern of mobilization not associated with vascular strands (commonest type) 2. Mobilization begins around vascular strands 3. Mobilization around vascular strands delayed... 

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