Plant cells have a rigid cellulose-based cell wall and a large central vacuole. They have three types of tissue: surface, basic, and vascular. Epidermal cells can be specialized as guard cells or for water absorption. Basic tissue types include parenchyma, collenchyma, and sclerenchyma. Xylem and phloem are the two types of vascular tissue.
Plant cells differ structurally from the cells of most other organisms in a few key ways. Specifically, they are usually larger than animal cells and are surrounded by a rigid cellulose-based cell wall. They also often have a large central vacuole that takes up most of the cell, and if they photosynthesise, the cells will have chloroplasts. This does not mean that all of these cells are the same and, in fact, several different types of cells are found in most plants.
Plants basically have three types of tissue, which are made up of different types of cells. The surface tissue forms the outer protective layer that covers the plant. Basic, or simple, tissues are usually composed of only one cell type and are usually grouped according to the level of cell wall thickness. Vascular tissues are complex tissues made up of more than one cell type. There are only two types of vascular tissue: xylem and phloem.
The surface tissue, or epidermis, of a plant is often only one cell thick, although it can be much thicker if the plant is living in a very dry environment and protection from water loss is paramount. It consists of epidermal cells, which often have a very large vacuole. The cell wall facing the outside of the plant is often thicker than the cell wall facing into the plant.
Epidermal cells in leaves can be specialized as guard cells. These cells control the opening and closing of tiny holes in the leaves, called stomata. In this way they regulate the movement of gases into and out of the plant. The function of the epidermal cells that cover the roots is the absorption of water from the soil. To increase surface area, many epidermal cells grow long hairs, or filaments, from their surfaces.
There are several basic tissue types, including parenchyma, collenchyma, and sclerenchyma. Parenchyma consists of parenchyma cells and occurs in the roots, leaves and stems of plants. These plant cells are relatively unspecialized and contain large vacuoles and a thin cell wall. Within the leaves and stems, most of the chloroplasts are found in the parenchymal cells. They give cells their green color and enable photosynthesis.
Collenchyma cells are longer than parenchyma cells and their cell walls are much thicker. Their function is to provide support in young plants and in the stems and leaves of older non-woody plants. Sclerenchyma cells also provide plant support and are much more specialized than collenchyma cells. They have a thick secondary wall which is hardened to strengthen the plant and these cells are usually dead by maturity.
Xylem and phloem are the two types of vascular tissue found in a plant. Xylem consists of parenchyma cells and two specialized cells called tracheids and vascular elements. Both the tracheids and the vessel elements are dead and their function is to provide support and transport of water from the roots to the rest of the plant.
Phloem tissue is alive and consists of parenchyma and sclerenchyma cells. Furthermore, it contains specialized plant cells called sieve tube cells and companion cells. The function of phloem is to transport material through the plant in all directions and they contain no nucleus and very little cytoplasm. Companion cells are closely associated with sieve tube cells and are thought to perform cellular functions for both cell types.
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