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vascular tissue[′vas·kyə·lər ′tish·ü]
in plants, a tissue that conducts water and mineral substances absorbed from the soil, as well as the products of photosynthesis and other metabolites. Vascular tissues consist of variously shaped elongate cells (prosenchyma cells). They are distributed in masses or bundles in a complex with mechanical and parenchyma tissues.
Arising from procambium and cambium, vascular tissues form an integral system that connects all organs of the plant. The tissues include xylem (primary and secondary) and phloem (primary and secondary). The principal vascular elements of xylem are tracheids (elongate cells distributed in strands and connected through bordered pits) and vessels (long tubules consisting of a large number of separate cells, the transverse walls between which disappear, thus forming perforations). The principal vascular elements of the phloem—sieve cells (elongate living cells placed one over the other) and sieve tubes—are united by strands of cytoplasm that pass through numerous perforations in the transverse walls of their membranes. The principal substances that move through the xylem are water and minerals; predominantly organic matter moves through the phloem. However, in the spring before the buds open, for example, organic matter stored in the previous year may move through the wood.
Highly differentiated vascular tissues are present only in vascular plants. Mosses and lower plants have no such tissues. Primary specialized vascular cells in the form of tracheids with annular and spiral thickenings of the membranes have been discovered in the earliest terrestrial flora, that is, the psilo-phytes. In the process of evolution, plants developed tracheids with scalene thickenings, uniformly thickened membranes, and numerous bordered pits. The most perfect vascular elements— the vessels—arose in the process of evolution in some ferns and gymnosperms, but they attained the greatest complexity in an-giospermous plants.
I. S. MIKHAILOVSKAIA