The procambium, the third of the primary developmental tissues, differentiates to form primary xylem and primary phloem as well as the vascular cambium. The vascular cambium produces cells that differentiate into secondary xylem and secondary phloem. It also regenerates the supply of cells in the vascular cambium.
An example of xylem is the woody tissue at the center of most trees. (Palm trees are a notable exception.) Smaller bundles of xylem are found in the roots, stems, and leaves of most plants, even when they are not woody. Xylem tissues are made of four cell types: fibers and parenchyma cells (which also occur in sclerenchyma and parenchyma) and xylem vessel elements and tracheids (which are found only in the xylem). These cells work in concert to move water upward through the plant. The xylem vessel elements and tracheids provide the actual channels for the movement, and the fibers serve largely as physical supporting structures.
The parenchyma cells are responsible for some lateral movement in the xylem tissues. These parenchyma cells also have the ability to revert to a meristematic condition, providing a mechanism for the xylem to replace damaged cells. Tracheids and vessels have unusual, patterned secondary cell walls that resist the physical stresses involved in moving xylem sap. The cell organelles are lost before the vessels and tracheids are functional. The sap moves through a channel where the body of the cell had been before it was lost. Xylem is another tissue that contains cell types that function when they are dead.
An example of phloem is the tissue on the inside of the bark of most trees (again, palm trees are an exception). Smaller bundles of phloem are found in the roots, stems, and leaves of most plants, even when they are not woody. Phloem tissues are made of fibers, parenchyma cells, sieve tube elements, and companion cells. These four cell types work in concert to move sugars, other organic molecules, and some ions throughout the body of the plant.
The sieve rube elements (or sieve cells, in some plants) provide the actual channels for the movement. The fibers serve largely as physical supporting structures. The parenchyma cells are responsible for some lateral movement and also provide a mechanism to replace damaged cells. Sieve tube elements and sieve cells have unusual, perforated cell walls whose appearance indeed resembles a sieve. Many of the cell organelles are lost before the sieve rube elements and sieve cells are functional. The phloem sap moves through living cells, but they resemble no other cells in the plant.
The companion cells (which in some plants are called albuminous cells, to indicate a different developmental origin) are similar to parenchyma cells, but they provide substantial metabolic support to the sieve tube elements and sieve cells. These cells function together. The companion cells could live independently, while the sieve tube element could not, but the important function is carried out by the sieve rube element.
Most species that grow in girth are woody, and the wood of woody plants is composed almost entirely of secondary xylem. The bark of woody plants is made of phloem and corky layers. There are two principal cambia, the vascular cambium and the cork cambium. Both contribute to the increase in girth. The vascular cambial tissues produce the cells that will differentiate to form the sec ondary xylem and phloem of woody species. The cork cambium produces the corky cells on the outside of the bark.
See also: Ground Tissues
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