Parenchyma - in Plants

In Plants

Parenchyma cells are thin-walled cells of the ground tissue that make up the bulk of most nonwoody structures, although sometimes they have lignified cell walls. Parenchyma cells in between the epidermis and pericycle in a root or shoot constitute the cortex, and tissue specialised for food storage commonly is parenchyma. They generally constitute "filler" tissue in the soft areas of the stems, leaves, root, flowers, fruits etc. Parenchyma cells within the center of the root or shoot constitute the pith. Parenchyma cells in the ovary constitutes the nucellus and are brick-like in formation. Parenchyma cells in the leaf constitute the mesophyll; they are responsible for photosynthesis and they allow for the interchange of gases.

Parenchyma is the most common and versatile ground tissue. It forms, for example, the cortex and pith of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma cells are living cells and may remain meristematic at maturity, meaning that they are capable of cell division. They have thin but flexible cellulose cell walls, and are generally polygonal when close-packed, but approximately spherical when isolated from their neighbours. They have large central vacuoles, which allows the cells to store and regulate ions, waste products and water.

Parenchyma cells have a variety of functions:

• In leaves, they form the mesophyll and are responsible for photosynthesis and the exchange of gases, parenchyma cells in the mesophyll of leaves are a specialized parenchymatous tissue known as chlorenchyma (parenchyma with chloroplasts).
• Storage of starch, protein, fats and oils and water in roots, tubers (e.g. potato), seed endosperm (e.g. cereals) and cotyledons (e.g. pulses and groundnut)
• Secretion (e.g. hydathodes, nectaries and cells lining the inside of resin ducts)
• Wound repair and the potential for renewed meristematic activity
• Other specialized functions such as aeration (aerenchyma) and support

The form of parenchyma cells varies with their function. In the epidermis of higher plants, only the guard cells have chloroplasts, but in the spongy mesophyll of a non-specialised leaf, most parenchyma cells exposed to light are full of chloroplasts. The mesophyll cells range from near-spherical and loosely arranged with large intercellular spaces to branched or stellate, mutually interconnected with their neighbours at the ends of the arms to form a three-dimensional network, as in the red kidney bean Phaseolus vulgaris and other mesophytes. These cells, with the epidermal guard cells of the stoma, form a system of air spaces and chambers that regulate the exchange of gases. They usually contain plastids with various functions, such as photosynthesis or food storage.