Parenchyma Cells
Parenchyma cells (a) make up the major portion of the primary plant body. They are usually thin-walled and vary in shape from spherical with many flat surfaces, to elongated, lobed, or folded. As living cells that are unspecialized initially, they later differentiate to more specialized cells. Parenchyma cells are found in photosynthetic tissue of green leaves and green stems, in epidermis, below the epidermis in cortex (b), in pith, and in the vascular system.
As food storage cells, they occur in specialized organs such as bulbs and tubers, in seeds (as endosperm), and
in seed leaves (cotyledons). Specialized parenchyma tissue (also called aerenchyma) with intercellular air
spaces aids water plants in floating.
Parenchyma cells
may appear as secretory forms such as glandular and stinging hairs, nectaries, and salt glands.
Collenchyma Cells
Collenchyma cells (c) provide elastic support to stems and leaves due to variously thickened primary walls (d)
containing cellulose, hemicellulose, pectin, and water.
These closely arranged, living cells are short or elongated in shape. They are usually found near the surface in the cortex around vascular bundles (e) of leaf petioles and stems.
Sclerenchyma Cells
Sclerenchyma tissue cells function in mechanical support due to thick lignified secondary walls (f), which con�tain large amounts of cellulose and lignin. At maturity, some sclerenchyma cells no longer have living protoplasts. Fibers and sclerids are types of sclerenchyma
cells.
Fibers (g)
These are elongated cells with pitted cell walls (h).
They are found in water-conducting tissue, xylem (i) and food-conducting tissue, phloem (j), along leaf veins and margins, and surrounding vascular bundles in stems.
Examples of commercial fibers are hemp, flax, jute, rattan, and cotton, used in making rope, mats and baskets.
Sclereids
These are dense (lignified), short cells which may look like stones, rods, bones, stars, or branched structures. Familiar forms, with dense cell layer of sclereids
occur in nut shells, fruit pits, and the seed coats in legume tree pods (Fabaceae, see 100). Pear (k) and quince fruits contain “stone cells” (l), that is, nests of sclereids in the fleshy mesocarp (m) tissue.
Vascular Cells
Cells of the xylem tissue, tracheids (n), are elongated, have bordered wall pits (o) for water conduction, and are aligned side by side. Also, water and mineral conducting cells of the xylem, namely, the vessel
elements (p), have bordered pits (q) in their cell walls.
The vessel elements are aligned end-to-end to form long tubes. The xylem sap passes vertically through the vessel elements via end perforations that may be
parallel slits (r) or a single large opening (s). Conifers and primitive woody flowering have only tracheids for water conduction.
Sieve cells
These are enucleate (minus a nucleus), found in the phloem of conifers and primitive vascular plants such as ferns. The sieve cells are elongated and thin walled. Sieve-tube elements (t) are enucleate and found in more advanced flowering plants. Both sieve cells and sieve-tube elements form long end-to-end columns called sieve tubes. Sieve plates (u), consisting of primary pit fields, occur in the end walls of sieve tube elements. In sieve cells, the walls, over their surfaces, contain localized sieve areas containing many pores that allow for cell-to-cell solute transport. Cell-to cell connecting strands of cytoplasm pass through the
sieve plates.
Companion cells (v),
a specialized type of parenchyma, may be present in varying numbers in association with sieve tube elements.
