34.6: Plant Cells and Tissues
Plant tissues are collections of similar cells performing related functions. Different plant tissues will have their own specialized roles and can be combined with other tissues to form organs such as flowers, fruit, stem, and leaves. Two major types of plant tissue include meristematic and permanent tissue.
Meristematic tissue, the primary growth tissue in plants, is capable of self-renewal and indefinite cell division. Every cell in the plant originates from a meristem. Meristematic tissue is classified into one of three types depending on its location inside the plant - apical, lateral, and intercalary. Apical meristems are meristematic tissue located at the tip of root and stem, which enable elongation of plant length. Lateral meristems are present in the radial portion of the stem and root and increase the thickness or girth of the maturing plant. Intercalary meristems occur only in monocots at the base of the internode and leaf blade. The intercalary meristems increase the length of the leaf blade.
Permanent plant tissues are either simple (composed of similar types of cells) or complex (consisting of different kinds of cells). For example, dermal tissue is a simple permanent tissue that forms the outer protective covering. It protects the plant from physical damage and enables gas exchange. In non-woody plants, the dermal tissue is a layer of tightly packed cells called the epidermis. The cuticle, a waxy epidermal coating, is present on leaves and stems that prevent water loss. The epidermis has unique functions in different plant organs. For example, roots, water, and minerals absorbed from the soil enter through the epidermis.
Vascular tissue, in contrast, is an example of complex tissue that enables the transport of water and minerals through the plant. The vascular system is composed of two specialized conducting vessels: xylem and phloem. Xylem conducts water and minerals from the roots to different parts of the plant, and itself consists of three types of cells: xylem vessel, tracheids (both of which hold water), and xylem parenchyma. Phloem conducts organic compounds from the site of photosynthesis to different parts of the plant. It includes four different types of cells: sieve cells (which conduct photosynthesis), phloem parenchyma, companion cells, and phloem fibers. In the stem, the xylem and phloem together form a structure called a vascular bundle. In roots, this is called the vascular cylinder or vascular stele.
Parenchyma, Collenchyma and Sclerenchyma
Plant anatomy divides the organism into four primary organs - root, stem, leaf, and flower. These can subsequently be divided into three tissue types. For example, leaves consist of three different tissues -dermal, vascular, and ground tissues. Further, these tissues are each composed of up to three cell types-parenchyma, sclerenchyma, or collenchyma.
Parenchyma cells are living, metabolically active, and usually bounded by a thin and flexible primary cell wall. In general, parenchyma cells account for 90 percent of the cells found in herbaceous seed plants. These often occur in the cortex or pith of stems or roots, and the fleshy tissue of many fruits. Most parenchyma cells retain the ability to divide, making them essential in wound healing and tissue regeneration. Moreover, parenchyma cells perform specialized functions in plants such as photosynthesis, storage, or transport, and aid the vascular tissue by forming a route to exchange nutrients within or between xylem and phloem.
Collenchyma cells are also living, and are elongated in structure, consisting of an irregular thick cell wall that provides support and structure to the plant. These are the least common cell type and have cell walls composed of cellulose and pectin. The epidermal tissue of young stem and leaf veins consists of collenchyma cells. There are three general classifications of collenchyma cells, depending on location and pattern of cell wall thickenings - angular, annular, lamellar, and lacunar.
Sclerenchyma cells form protective or supportive tissue in higher plants. At maturity, these cells have limited physiological activity and are usually dead. Sclerenchyma cells have a cell wall with a thickened secondary layer made up of cellulose, hemicellulose, and lignin. The orientation of the cellulose provides a diverse combination of strength, flexibility, and stiffness in plant organs subjected to different compressive and tensile forces. Sclerenchyma occurs in three different forms - fibers, sclereids, and water-conducting sclerenchyma.