Xylem plant vascular system

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-... 

Just few weeks after the formation of xylem, it contains mostly dead cells, but continues to play a role of utmost importance for the plant the transport of sap. Hence the vascular system so produced is called sapwood. This active zone may vary in thickness and number of growth rings, commonly up to about 15 years, which represents several centimeters in radial thickness. As a rule, the more... 

The fungus attacks the plant through the root system, where it penetrates 1-2 mm from the root tip and grows toward the vascular system (3). Hyphae in the xylem vessels produce spores that are transported upward in the xylem fluids and eventually infect the foliar plant parts (4). To prevent this systemic infection, the plant must seal the infected vessels and kill the fungus. 

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. 

XRS X-ray spectrometry see XRF xylem woody vascular conductive system in plants (for the transport of water and the predominantly inorganic substances dissolved in it)... 

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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