The Structure and Types of Plant Stems

Investigating the Nature and Diversity of Stems

Aug 12, 2009 Dennis Holley

Stems come in two varieties: soft, green herbaceous stems or hard woody stems.

The most advanced plants on the planet are the vascular seed plants. These plants are taxonomically divided into two smaller groups: gymnosperms (mainly evergreen trees) and angiosperms (flowering plants).

Angiosperms are further divided into two taxonomic classes: monocotyledons and dicotyledons. Although the major physical differences between these two classes lie in their seeds, structural differences can also be noted in their stems, roots, and leaves.

The Structure of Monocot stems

A monocot stem is best understood by examining a cross section of a corn stem. The outermost tissue is a single layer of epidermis. Beneath the epidermis are two or three layers of fibrous sclerenchyma cells for support (Think reinforcing rods in concrete). Large balloon-like parenchyma cells (or pith) packed with stored food fill the interior of the stem.

Scattered throughout the parenchyma are the vascular bundles (conducting tubes). The vascular bundles are composed of both xylem and phloem tissue and are usually surrounded by a bundle sheath of fibers. The large tubes in the bundle are the xylem tissue while the smaller tubes are the phloem tissue.

Monocot stems are categorized as herbaceous. Herbaceous stems are usually green outside and soft inside. Those found in temperate zones are annual plants. That is, they die back and are replaced seasonally.

The Structure of Dicot Stems

Dicot stems can be either herbaceous or woody. In herbaceous dicots, the vascular tissues occur as a ring of separate vascular bundles lying just under the epidermis. As with monocots, each vascular bundle contains both xylem and phloem tissue, with the xylem toward the center of the stem and the phloem toward the outside.

This ring of vascular tissue surrounds the pith, a central area of food-storing parenchyma cells. Toward the outside of the stem is the cortex. Although the cortex consists of mainly of parenchyma cells, fibers often occur in this region. Covering the stem is the epidermis. In some herbaceous dicots, fibers that strengthen the stems can be found beneath the epidermis.

In woody dicots, the vascular tissue, especially the xylem, is more extensive and makes up the bulk of the stem. Like the herbaceous dicots, the pith occupies the center of the stem. Surrounding the pith are rings of secondary xylem. Each ring represents the xylem formed by the vascular cambium during one growing season and is thus called an annual ring.

The rings, which are easily visible to the unaided eye, are due to the different sizes of cells formed through the growing season. In the spring, when water is usually more abundant, the cells, known as springwood, are larger than those produced during the late summer or fall, known as summerwood. The portion of each ring with springwood appears lighter than the area with the smaller, densely packed cells of summerwood.

Since each ring typically represents one growing season, the age of a tree growing in temperate regions can be determined quite accurately by counting the annual rings. A type of gymnosperm, the bristlecone pine (Pinus longaeva) found in the White Mountains of eastern California, have members with more than 4,000 rings.

Surrounding the outermost ring of xylem is the vascular cambium, the meristematic (growth) region that produces both secondary xylem to the inside which becomes wood and secondary phloem to the outside which becomes bark. Large amounts of secondary xylem are produced each year but only a small amount of secondary phloem. Because they live and grow for many years, plants with woody stems are called perennials.

A small band of cortex can be found outside the phloem. In older trees, however, the cortex is completely replaced by cork (commonly called bark). The thickness and texture of the bark depends on the type of tree and varies from thin and papery in the cherry or paper birch to extremely thick in cork oaks.

The Structure of Gymnosperm Stems

On the North American continent, all gymnosperms species are in the form of evergreen trees. As such, their stem structure is similar to that of woody dicot angiosperms. However, gymnosperm stems often contain resin ducts within their cork layer. These ducts produce a thick, sticky material called resin that can often be seen oozing out of various spots on gymnosperms such as pine trees.

Woody dicots such as oaks, maples, and walnuts are called hardwoods while gymnosperms such as pine, spruce, and fir are called softwoods. The annual rings and radial structures within the wood called rays give sawn wood its characteristic grain.

The vascular seed plants number in the hundreds of thousands of species, yet all of them have stems of only two basic types: herbaceous and woody.

The copyright of the article The Structure and Types of Plant Stems in Botany is owned by Dennis Holley. Permission to republish The Structure and Types of Plant Stems in print or online must be granted by the author in writing.

Soft Herbaceous Stems are Annual Growths