Everything about Vascular Tissue totally explained
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This is an article about vascular tissue in plants. For transport in animals, see Circulatory system.
Vascular tissue is a complex
tissue found in
vascular plants, meaning that it's composed of more than one cell type. The primary components of vascular tissue are the
xylem and
phloem. These two tissues transport fluid and nutrients internally. There are also two
meristems associated with vascular tissue: the
vascular cambium and the
cork cambium. All the vascular tissues within a particular plant together constitute the
vascular tissue system of that plant.
The cells in differentiated vascular tissue are typically long and slender. Since the xylem and phloem function in the conduction of water, minerals, and nutrients throughout the plant, it isn't surprising that their form should be similar to pipes. The individual cells of phloem are connected end-to-end, just as the sections of a pipe might be. As the plant grows, new vascular tissue
differentiates in the growing tips of the plant. The new tissue is aligned with existing vascular tissue, maintaining its connection throughout the plant.
The vascular tissue in plants is arranged in long, discrete strands called
vascular bundles. These bundles include both xylem and phloem, as well as supporting and protective cells. In
stems and
roots, the xylem typically lies closer to the interior of the stem with phloem towards the exterior of the stem. In the stems of some Asteriidae
dicots, there may be phloem located inwardly from the xylem as well.
Between the xylem and phloem is a
meristem called the
vascular cambium. This tissue divides off
cells that will be become additional xylem and phloem. This growth increases the girth of the plant, rather than its length. As long as the vascular cambium continues to produce new cells, the plant will continue to grow more stout. In
trees and other plants that develop
wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the
epidermis of the stem, woody plants also have a
cork cambium that develops among the phloem. The cork cambium gives rise to thickened
cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of
secondary growth.
In
leaves, the vascular bundles are located among the spongy mesophyll. The xylem is oriented toward the
adaxial surface of the leaf (usually the upper side), and phloem is oriented toward the
abaxial surface of the leaf. This is why
aphids are typically found on the underside of the leaves rather than on the top, since the phloem transports sugars manufactured by the plant and they're closer to the lower surface.
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