In stems, the xylem and the phloem form a structure called a vascular bundle in roots, this is termed the vascular stele or vascular cylinder.Īll animals are made of four types of tissue: epidermal, muscle, nerve, and connective tissues. The xylem and phloem always lie adjacent to each other (Figure 1). Unlike xylem conducting cells, phloem conducting cells are alive at maturity. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel elements and tracheids (both of which conduct water), and xylem parenchyma. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Secondary tissues are either simple (composed of similar cell types) or complex (composed of different cell types). (credit: modification of work by “(biophotos)”/Flickr scale-bar data from Matt Russell) The vascular bundles are encased in ground tissue and surrounded by dermal tissue. Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. This light micrograph shows a cross section of a squash ( Curcurbita maxima) stem. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. They differentiate into three main types: dermal, vascular, and ground tissue. Such cells take on specific roles and lose their ability to divide further. Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. This tissue enables the monocot leaf blade to increase in length from the leaf base for example, it allows lawn grass leaves to elongate even after repeated mowing. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). Lateral meristems facilitate growth in thickness or girth in a maturing plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Meristematic tissues consist of three types, based on their location in the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Plant tissue systems fall into one of two general types: meristematic tissue and permanent (or non-meristematic) tissue. Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions.
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