Explain briefly why few plants develop bushy appearance after few years of growth
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Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers and buds; in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.
Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (Figure 1). Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud.
Stem Anatomy
Micrograph shows a stem about 1.2 millimeters across. The central pith layer is about 800 microns across. Pith cells stain greenish-blue and are about 50 to 100 microns in diameter in the middle, and smaller toward the outside. Surrounding the pith is a ring of xylem cells about 75 microns across and four cells deep. Xylem cells, which are about 15 microns across, radiate out from the center in rows. Rows of green-staining phloem cells radiate out from the xylem cells. Phloem cells are about half the size of xylem cells. Outside the phloem is a ring of cells that make up the peripheral cortex. Cells in the peripheral cortex are rounded rectangles that lie perpendicular to the phloem. The outermost epidermis is made up of cells similar in shape to the peripheral cortex cells but a bit larger. On opposite faces of the stem the peripheral cortex bulges outward, forming buds about 150 microns across.
Figure 2. The stem of common St John’s Wort (Hypericum perforatum) is shown in cross section in this light micrograph. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis) are composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith. (credit: Rolf-Dieter Mueller)
The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.
Parenchyma cells are the most common plant cells (Figure 2). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.
Collenchyma cells are elongated cells with unevenly thickened walls (Figure 3). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.
Micrograph shows collenchyma cells, which are irregularly shaped and 25 to 50 microns across. The collenchyma cells are adjacent to a layer of rectangular cells that form the epidermis.
Figure 3. Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to plant structures. (credit: modification of work by Carl Szczerski; scale-bar data from Matt Russell)
Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope (Figure 4).
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