Plant: Tissue Systems

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There are many variants of the generalized plant cell and its parts. Similar kinds of cells are organized into structural and functional units, or tissues, which make up the plant as a whole, and new cells (and tissues) are formed at growing points of actively dividing cells. These growing points, called meristems, are located either at the stem and root tips (apical meristems), where they are responsible for the primary growth of plants, or laterally in stems and roots (lateral meristems), where they are responsible for secondary plant growth. Three tissue systems are recognized in vascular plants: dermal, vascular, and ground (or fundamental).

Dermal System
The dermal system consists of the epidermis, or outermost layer, of the plant body. It forms the skin of the plant, covering the leaves, flowers, roots, fruits, and seeds. Epidermal cells vary greatly in function and structure.

The epidermis may contain stomata, openings through which gases are exchanged with the atmosphere. These openings are surrounded by specialized cells called guard cells, which, through changes in their size and shape, alter the size of the stomatal openings and thus regulate the gas exchange. The epidermis is covered with a waxy coating called the cuticle, which functions as a waterproofing layer and thus reduces water loss from the plant surface through evaporation. If the plant undergoes secondary growth—growth that increases the diameter of roots and stems through the activity of lateral meristems—the epidermis is replaced by a peridermis made up of heavily waterproofed cells (mainly cork tissue) that are dead at maturity.

Vascular System













The vascular tissue system consists of two kinds of conducting tissues:
  1. the xylem, responsible for conduction of water and dissolved mineral nutrients. The xylem consists of two types of conducting cells: tracheids and vessels. Elongated cells, with tapered ends and secondary walls, both types lack cytoplasm and are dead at maturity. The walls have pits—areas in which secondary thickening does not occur—through which water moves from cell to cell. Vessels usually are shorter and broader than tracheids, and in addition to pits they have perforation—areas of the cell wall that lack both primary and secondary thickenings and through which water and dissolved nutrients may freely pass.
  2. the phloem, responsible for conduction of food. The xylem also stores food and helps support the plant. The phloem, or food-conducting tissue, consists of cells that are living at maturity. The principal cells of phloem, the sieve elements, are so called because of the clusters of pores in their walls through which the protoplasts of adjoining cells are connected. Two types of sieve elements occur: sieve cells, with narrow pores in rather uniform clusters on the cell walls, and sieve-tube members, with larger pores on some walls of the cell than on others. Although the sieve elements contain cytoplasm at maturity, the nucleus and other organelles are lacking. Associated with the sieve elements are companion cells that do contain nuclei and that are responsible for manufacturing and secreting substances into the sieve elements and removing waste products from them.
Ground System
The ground, or fundamental, tissue systems of plants consist of three types of tissue. The first, called parenchyma, is found throughout the plant and is living and capable of cell division at maturity. Usually only primary walls are present, and these are uniformly thickened. The cells of parenchyma tissue carry out many specialized physiological functions—for example, photosynthesis, storage, secretion, and wound healing. They also occur in the xylem and phloem tissues.

Collenchyma, the second type of ground tissue, is also living at maturity and is made up of cells with unevenly thickened primary cell walls. Collenchyma tissue is pliable and functions as support tissue in young, growing portions of plants.

Sclerenchyma tissue, the third type, consists of cells that lack protoplasts at maturity and that have thick secondary walls usually containing lignin. Sclerenchyma tissue is important in supporting and strengthening those portions of plants that have finished growing.

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