Parenchyma:
Parenchyma refers to a tissue composed of living cells variable in their morphology and physiology. The word parenchyma is derived from the Greek para, beside and en-chain, to pour, meaning a semi-liquid substance poured beside other tissues. Parenchyma is often considered as the fundamental or ground tissue in which other tissues, notably the vascular are embedded.
Ontogenetically parenchyma is the precursor of other tissues. Phylogenetically also it is more ancient than other tissues as seen by the fact that the body of primitive organisms is always parenchymatous.
Parenchyma tissue is the principle seat of such essential activities of the plant like photosynthesis, assimilation, respiration, storage, secretion etc. Developmentally, parenchyma cells are relatively undifferentiated and are unspecialised morphologically and physiologically when compared with other complex tissues.
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Distribution:
Parenchyma cells are present in almost all parts of the plant, like the pith and cortex of roots and stems, mesophyll of leaves, the flesh of succulent roots, the endosperm of seeds etc. As components of heterogene ours tissues parenchyma cells form vascular rays.
Cell shape:
Parenchyma cells are spherical or polygonal in shape. Sometimes they may be elongated and columnar as in the leaves. Studies based on geometric models have indicated that closely packed parenchyma cells have the shape of a polyhedron with approximately fourteen faces. I Parenchyma cells are closely packed or arranged loosely with large number of intercellular spaces.
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Cell structure:
The cells have thin walls and have dense cytoplasm with a prominent nucleus. Parenchyma cells in storage organs have abundant reserve food materials like proteins, fats and carbohyderates. Parenchyma cells found in leaves has number of choloroplasts. These help in photosynthesis.
Cell types:
Based on their structure and function parenchyma cells are classified into the following types.
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Simple parenchyma which forms the ground tissue, Chlorenchyma which is the chief photosynthetic tissue in the plants, and Aerenchyma which has parenchyma cells with large intercellular spaces storing air. Aerenchyma is present in hydrophytes and helps in respiration and buoyancy.
Chlorenchyma cells found in dorsiventral leaves are of two types viz. Palisade parenchyma and spongy parenchyma. Cells of palisade parenchyma are columnar and are filled with chloroplastss. Spongy parenchyma cells also have chloroplastss, but they are rich in starch grains. Parenchyma cells found in Xerophytes (Aloe, Agave etc.) act as water storage tissues.