Plant vascular system phloem

Vascular Anatomy. One aspect of the translocation system that is often overlooked is the influence of the structural features of the plant s vascular system on solute transport. For example, although much of what is known about phloem loading has been derived from a few dicotyledon leaves, all dicotyledon leaves are not similar. A notable example is the soybean leaf. Soybean leaves are specialized in that they have a unique cell type called the paraveinal mesophyll... 

Insecticides with systemic action are taken up relatively quickly by the plants and transported into the vascular system. According to the type of application, uptake occurs through the roots or the parts of the plant above ground. Distribution is chiefly by the xylem, but is also possible by the phloem and by diffusion from cell to cell. The persistence of activity is dependent on the type of substance, the intensity of breakdown in the plant or the soil, and environmental conditions. A much longer period of protection can be maintained if, by application of granulates at drilling or planting out, a depot of the substance is created in the soil from which the active substance is released slowly and taken up by the plants. 

An unusual and taxonomically useful trait found in the Myrtaceae involves the vascular system of the stem. In most dicotyledonous plants the food conducting cells of the vascular system, the sieve elements of the phloem, surround the water conducting cells, or xylem. In young stems there is usually another group of large cells that appear open in sections viewed under a light micro-... 

The chemical properties of cellulose contribute to the qualities that make linen such an attractive fabric (i.e., its smoothness, strength, and water absorbency). Bast fibers, found in the phloem (a component of the plant s vascular system) are used to make linen. They contain thicker cell walls than most of the other plant tissues. In a chemical process called retting, bast fibers are harvested after the rest of the plant is decomposed by bacteria. Because of the large amount of cellulose within their cell walls, bast fibers can withstand the numerous corrosive chemical reactions of decomposition. 

Distribution pathways are summarized in Figure 1.1. First of all, there can be movement within a compartment for example, any chemical introduced into an aquatic compartment can move to the extent that the water moves, whether or not the chemical is in solution or sorbed on a particle. This movement would be defined by the appropriate hydrological parameters. A chemical may find its way into the atmosphere where it may be transported in atmospheric currents In this situation the appropriate meteorological phenomena will determine the rate and direction of movement. Distribution in a plant or animal wifi be controlled by the transport mechanisms in that organism either the vascular system in an animal or the phloem in a plant. In a much broader context, the transport of a chemical in an ecosystem must have some relation to the overall mass flow in the system since the chemical moves with the food constituents of the various components in the ecosystem. 

The ability of transfer cells to move solutes in and out of the vascular system and their positioning at strategic locations (Gunning et al. 1968, Pate and Gunning 1972, Gunning and Pate 1974) suggests that transfer cells may play a role in controlling the flux and distribution of plant hormones via the xylem and phloem. 

Endogenous growth regulators may be translocated in the plant s vascular systems. This would seem to hold especially true for gibberellins and cytokinins, since activity has been detected in the xylem sap of many different herbaceous as well as woody species. Obviously these hormones are also exported from photosynthesizing leaves, as indicated by their presence in sieve tube sap. Although with less frequency, also auxin and ABA-like activity have been detected in both xylem and phloem sap (Table 3.1). 

For a full and detailed description of the vascular systems of plants, the reader is directed to a standard text such as Esau. The movement of solutes within plants takes place largely by two pathways. One route is via the extraprotoplasmic continuum (apoplast) of the plant and includes transport over short distances through the intercellular spaces and over long distances in the xylem vessels. Such transport is normally in an upward direction (acropetal). The second route is via the cytoplasmic continuum (symplast) of the plant and includes short-distance cell-to-cell transport through plasmodesmata and long-distance transport in the phloem sieve cells. Phloem translocation takes place in both upward and downward (basipetal) directions to the sites of new growth. 

Two features of bryophytes tend to restrict them to moist environments, such as bogs and woodlands. First, unlike vascular plants, bryophytes lack a system with xylem and phloem for efficient transport of water and food. Second, the male sperm cells of bryophytes must swim through water to reach the female egg cells. 

The existence of a closed circulatory system in higher animals provides the organism with an easy and efficient route for the transport of hormones from the site of synthesis to the target tissues. In plants some hormones appear to be transported directly in the vascular tissue for example, cytokinin, GA, and ABA move from the root to the shoot in the xylem GA moves out of young leaves in the phloem and ABA is transported out of wilting leaves in the phloem (Fig. 6.1). However, auxin is not transported directly in the vascular tissue, but instead appears to be transported in cells associated with the phloem (Fig. 6.1). Ethylene poses a special problem in that it is a diffusable gas. However, its precursor, 1-aminocyclopropane-l-carboxylic acid (ACC), is transported from the root to the shoot in the xylem. Therefore, using the traditional concept of a hormone as a translocated chemical messenger, ACC may be more aptly considered to be a hormone than ethylene. 

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