Stems

Photo shows a stem. Leaves are attached to petioles, which are small branches that radiate out from the stem. The petioles join the branch at junctions called nodes. The nodes are separated by a length of stem called the internode. Above the petioles, small leaves bud out from the node.

Figure 1. Leaves are attached to the plant stem at areas called nodes. An internode is the stem region between two nodes. The petiole is the stalk connecting the leaf to the stem. The leaves just above the nodes arose from axillary buds.

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. (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. In Figure 2, we see the central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis); both are composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith.

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).

Part A shows a cross section of a flax stem. The pith is white tissue in the center of the stem. Outside the pith is a layer of xylem. The inner xylem cells are large, while ones further out are smaller. The smaller xylem cells radiate out from the center, like spokes on a wheel. Outside the xylem is a ring of phloem cells. The phloem is surrounded by a layer of sclerenchyma cells, then a layer of cortex cells. Outside the cortex is the epidermis. Part B is a painting of women working with linen cloth. One is smoothing the cloth on a table, and the other women are sitting with linen on their laps. Part C is a photo of flax plants, which have long, wide leaves that taper toward narrow tips.

Figure 4. The central pith and outer cortex of the (a) flax stem are made up of parenchyma cells. Inside the cortex is a layer of sclerenchyma cells, which make up the fibers in flax rope and clothing. Humans have grown and harvested flax for thousands of years. In (b) this drawing, fourteenth-century women prepare linen. The (c) flax plant is grown and harvested for its fibers, which are used to weave linen, and for its seeds, which are the source of linseed oil. (credit a: modification of work by Emmanuel Boutet based on original work by Ryan R. MacKenzie; credit c: modification of work by Brian Dearth; scale-bar data from Matt Russell)

PRactice Question

Which layers of the stem are made of parenchyma cells?

  1. cortex and pith
  2. phloem
  3. sclerenchyma
  4. xylem

Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment (Figure 5). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

Photos show six types modified stems: (a) Lumpy white ginger rhizomes are connected together. A green shoot projects from one end. (b) The carrion flower corm is conical-shaped, with white roots spreading from the bottom of the cone, just above the dirt. (c) Two grass plants are connected by a thick, brown stem. (d) Strawberry plants are connected together by a red runner. (e) The part of the potato plant that humans consume is a tuber. (f) The part of the onion plant that humans consume is a bulb.

Figure 5. Stem modifications enable plants to thrive in a variety of environments. Shown are (a) ginger (Zingiber officinale) rhizomes, (b) a carrion flower (Amorphophallus titanum) corm (c) Rhodes grass (Chloris gayana) stolons, (d) strawberry (Fragaria ananassa) runners, (e) potato (Solanum tuberosum) tubers, and (f) red onion (Allium) bulbs. (credit a: modification of work by Maja Dumat; credit c: modification of work by Harry Rose; credit d: modification of work by Rebecca Siegel; credit e: modification of work by Scott Bauer, USDA ARS; credit f: modification of work by Stephen Ausmus, USDA ARS)

Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants were cultivated for food hundreds of years ago in the Arizona desert in this video: Finding the Roots of an Ancient Crop.

Some aerial modifications of stems are tendrils and thorns (Figure 6). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant; common examples include roses, Osage orange and devil’s walking stick.

Photo shows (a) a plant clinging to a stick by wormlike tendrils and (b) two large, red thorns on a red stem.

Figure 6. Found in southeastern United States, (a) buckwheat vine (Brunnichia ovata) is a weedy plant that climbs with the aid of tendrils. This one is shown climbing up a wooden stake. (b) Thorns are modified branches. (credit a: modification of work by Christopher Meloche, USDA ARS; credit b: modification of work by “macrophile”/Flickr)