Parenchyma cells are the most common plant cells (Figure 99). The stem, the root, the
interior of the leaf, and the fruit pulp all contain them. Photosynthesis and other metabolic processes
are carried out by parenchyma cells, which also aid in wound healing. Some parenchyma cells also
store starch.
155
Figure 99. In this light micrograph, the stem of common St. John's wort (Hypericum perforatum) is
displayed in cross section. Parenchyma cells make up the central pith (greenish-blue in the center) and
peripheral cortex (narrow zone, 3-5 cells thick, just beneath the epidermis). The pith is surrounded by
vascular tissue made up of xylem (red) and phloem tissue (green, between the xylem and cortex).
73
.
Collenchyma cells are elongated cells with unevenly thickened walls (Figure 100). They
primarily support the stem and leaves structurally. These mature cells, which are alive, are typically
located beneath the epidermis. A celery stalk's "strings" are an illustration of collenchyma cells.
Figure 100. This light micrograph demonstrates the uneven wall thickness of collenchyma cells. They
support the structures of plants
74
.
Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of
them are dead at maturity. Sclerenchyma cells come in two different varieties: fibers and sclereids.
Both types have secondary cell walls that have deposits of lignin, an organic substance that is an
essential part of wood, thickening them. Sclereids are smaller-sized cells while fibers are long, slender
73
(credit: Rolf-Dieter Mueller)
74
(credit: modification of work by Carl Szczerski; scale-bar data from Matt Russell)
156
cells. Pears get their grainy texture from sclerids. Sclerenchyma fibers are used by humans to make
rope and linen
75
(Figure 101).
