Stems

Stem
Stem

The stem is the part of a vascular plant that supports the leaves and reproductive structures, often at some distance above the ground.

Stems have two chief functions: to support various plant structures and to transport water and nutrients. Most stems support the leaves and reproductive organs at some distance above the ground.

Such elevation is important to both leaves, whose function is to trap light energy for photosynthesis, and flowers, because being positioned above the ground helps attract pollinators. Fruits also benefit from being held aloft because height improves the chances of long-distance seed dispersal.

While being off the ground has advantages for shoot-borne organs, it also has one big disadvantage. These organs are far from their source of water, the soil, and therefore run the risk of drying out unless adequatewater can be supplied.


There in lies the second function of stems: to conduct a supply of water from the roots to the organs. Stems also conduct nutrients in both directions between the roots and organs of the plant.

Structure

Unlike roots, which branch at irregular intervals and do not bear organs such as leaves or reproductive structures, stems have a definite pattern of organization that consists of alternating nodes and internodes. Each node is a point on a stem at which an organ is attached. Adjacent nodes are separated by an internode, a zone that lacks any attached organs.

The part of the stem closest to the ground is called the proximal end, while the part farthest away is called the distal end. In many plants, internodes at the proximal end are the longest. Toward the distal end, the internodes tend to be progressively shorter.

An actively growing stem has an apical meristem at its extreme distal end. In this region, new stem tissue is produced. In the apical meristem, cells actively divide and elongate, causing the stem to become longer.

That elongation process is called primary growth. Immature leaves are also produced at the apical meristem. Immediately after the new cells enlarge, they undergo differentiation to form primary tissue.

stems of Equisetum hyemale
stems of Equisetum hyemale

Newly formed stems tend to be soft, because the walls of the young cells are thin. Moreover, the stems are typically green because of the presence of photosynthesizing cells on the periphery. Such soft, green stems are termed herbaceous. These herbaceous stems often turn hard and woody through time.

At each node, the angle that is formed when a leaf attaches to the stem is called an axil. At that juncture is found an embryonic shoot, called an axillary bud. Normally, axillary buds remain dormant for some time. Eventually, however, many buds break dormancy and elongate to forma lateral shoot, called a branch.

Different species of plants have different patterns of leaf arrangement. Most plants display an alternate arrangement, in which only one leaf is inserted at a node. Plants with alternate leaves include oak trees, goldenrod, geraniums, and lilies.

Plants with two leaves at a node, in what is called an opposite arrangement, include maples, lilacs, phlox, and mints. Finally, many plants, such as trillium, catalpa, and bed straw, have three or more leaves at a node, in what is called a whorled arrangement.

Some plants exhibit very little internodal development. The result is that the plant consists of a cluster of leaves right at the soil surface. These are called rosette plants and are represented by dandelion, hawk weed, and plantain.

Woody Plants

Woody Plants stem
Woody Plants stem

Some plants produce an herbaceous stem that will develop a secondary growth of hard, woody tissue. These woody plants can be categorized as trees, which have only a single main stem emerging from the ground, or as shrubs, which have several stems coming from the ground.

The development of woody tissue from herbaceous tissue results from a process called lignification, in which the walls of cells inside the stem become thick and hard because of the presence of a material called lignin.

The lignified inner cells of the stem are colletively called wood. Lignification causes the stem to become more rigid, allowing it to support more weight.

In temperate climates with pronounced cold winters, an herbaceous stembecomes lignified during its first winter. In more moderate climates, stems become lignified gradually. In either case, the outside of the stem becomes covered with water-impermeable bark.

Woody plants that growing temperate areas vary in their rates of growth from one season to another. Stems grow most rapidly during the spring and summer. The rate of growth declines markedly at the end of summer and during fall. Little, if any, growth occurs during winter.

Woody plants, especially deciduous ones, have a few features that are lacking in herbaceous plants, which have unprotected stems. First, when a leaf drops off a woody plant at the end of the growing season, it leaves a scar on the stem.

The shapes of leaf scars vary from one species to another; some are circular, others are triangular, still others look like small lines. A second feature found on a woody plant is a terminal bud. That structure usually forms when the apical meristem stops actively growing at the end of the summer.

The terminal bud is typically dormant during the winter and is often enclosed by one or more hard, modified leaves called terminal bud scales. Those scales protect the bud against injury by cold and predators.

Stem growth in years
Stem growth in years

When warm weather or abundant rainfall returns, the bud breaks its dormancy, and a new stem is produced. As the young shoots begin to elongate, the bud scales fall off, resulting in scars that encircle the stem. Thus, the bud scales mark the juncture between the growth that occurs in two successive years.

Because a new set of bud scale scars is produced at the beginning of every growing season, one can tell the age of a stem by counting the number of sets of bud-scale scars from the base to the extreme distal end.

Finally, the bark that encircle swoody stems does not allow air to pass between the inside of the stem and the outside. That poses a problem for the cells immediately under the bark, which need oxygen to survive.

To help overcome this problem, many woody stems contain raised corky dots called lenticels scattered over the surface. The cells of the lenticels are spongy and allow air to diffuse to the living cells beneath.

Modified Stems

Many plants have stems that are horizontal instead of erect. Stolons, also called runners, are horizontal stems that lie above the soil surface. Stolons are found on strawberry plants, for example. Rhizomes are horizontal stems that lie below the soil surface.

Ferns, irises, milkweeds, and golden rods produce rhizomes. Both stolons and rhizomes often produce adventitious roots, which are roots that form along a stemor anywhere else roots typically do not grow.

Tubers are thickened rhizomes that store starch. The potato is probably the best-known tuber. Interestingly, plants such as kohlrabi have thickened tuberlike stems that are borne above ground.

Modified Stem
Modified Stem

A bulb is a bud like modified stem that has sets of thick leaves closely overlapping one another (as an onion does). A corm is similar to a bulb except that the leaves are thin and papery, and they surround a stem that is short and fleshy. Gladiolus and crocus both produce corms.

Some plants, especially those that grow in desert regions, have thick, succulent stems that serve to store and conserve water.

The stemis either pad like (such as that of a prickly-pear cactus) or barrel shaped (such as that of a saguaro cactus) and is typically covered by a thick layer of green photosynthesizing cells and sharp spines. Asparagus and a few other plants produce stems called cladophylls, which are flattened, highly branched, and capable of photosynthesis.

Some branch stems, such as those of hawthorn, are modified to form sharp structures called thorns. In contrast, the sharp structures on the surfaces of rose and blackberry stems, commonly referred to as thorns, are actually prickles, while sharpened leaves such as those of holly are called spines.

Finally, some woody plants, such as birches, produce stubby side branches called spur shoots that grow only a few millimeters each year.

Growth

Three types of primary tissue form the stem’s interior: dermal tissue, vascular tissue, and ground tissue. Dermal tissue forms a thin layer surrounding the herbaceous stem and protects it from drying out.

Vascular tissue is composed of xylem and phloem and serves to conduct water, minerals, and organic substances throughout the plant. In most plants, it forms a ring within the stem.

Woody plants have a ring of cells called the vascular cambium located within the vascular tissue. The vascular cambium actively divides during the growing season, producing wood to the inside and bark to the outside, and causes the stem to increase in width.

Finally, ground tissue has two main functions: photosynthesis and storage. It is found both at the extreme core of the stem and in a ring immediately inside the dermal tissue but outside the vascular tissue.

Many plants vary from these basic patterns. For example, in plants such as lilies and grasses, the cells of the vascular tissue occur in small clusters that are interspersed with the ground tissue. In woody plants, the vascular tissue makes up the bulk of the stem.

Many internal (hormonal) and external (environmental) factors affect stem growth. At least two classes of hormones appear to increase the growth of stems when applied to the plant: auxins and gibberellins.

Auxins cause individual cells to become longer, whereas gibberellins stimulate both cell division and elongation. Gibberellins appear to be important in the life cycles of biennials, which are plants that typically spend their first year as a rosette and then flower in their second year.

Before flowering, the second-year plant must produce an erect flowering stem, a process called bolting. When a biennial bolts, the new internodes are long rather than short, and gibberellins appear to cause this elongation.

Many environmental factors can influence stem growth, but light and physical damage to the apical meristem are perhaps the most dramatic.

Plants that growin light that comes from one side (instead of from the top) tend to bend toward the light, a process called phototropism. Plants that grow in complete darkness tend to have stems that are abnormally long and thin and are yellow in color. Such stems are said to be etiolated.

Finally, when the apical meristem is intact, lateral buds typically remain dormant. When, however, the meristemis removed by herbivory or clipping, at least one of the lateral buds is released from dormancy, producing a branch that then becomes the new meristem.

The process by which an intact apical meristem inhibits the growth of the laterals is termed apical dominance. Research has shown that phototropism, etiolation, and apical dominance are mediated through the hormone auxin.

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