Category: 3. Plant Stem Tissue Patterns

These plants are usually grass or grass-like and do not grow to great size. Monocot stems do not have vascular cambiums or cork cambiums, as growth will not be lateral. The vascular bundles produced by the procambium are scattered throughout the stem, rather than organized in rings as in woody dicot stems. Every bundle is oriented with the xylem toward the center of the stem and the phloem toward the stem surface. The xylem in the vascular bundle generally consists of two large vessels with some small vessels in between them, while the phloem consists of sieve tubes and companion cells. The entire vascular bundle is wrapped in a sheath of sclerenchyma cells. The background tissue between vascular bundles is not divided into cortex and pith in monocots, but they do have similar function and appearance as the parenchyma cells in cortex and pith. The concentration of bundles and bands of sclerenchyma cells, give the stem the flexibility and strength to withstand the elements—such as a summer rainstorm. In grasses, there is an intercalary meristem at the base o each internode which contributes to growth in length, like apical meristems. During the growing season, the stems of the grasses elongate rapidly. Because there is no vascular cambium that would produce tissues to increase the girth of the plant, the growth is columnar with very little variation in diameter between the top and the bottom of the plant.

Palm trees are special because they grow to a considerable size, however, this is primarily due to the subsequent division and growth of their parenchyma cells. All this growth occurs without a true cambium developing. Other monocot stems have adaptations that allow for specialized growth. Monocot fibers, such as manila hemp and sisal, come from stems and leaves and are used for commercial products however, their fibers are not as strong as dicot fibers.

Wood is essentially secondary xylem growth. These stems look similar to herbaceous dicot stems up until the vascular cambium and the cork cambium start functioning. The differences are then quite obvious. While some tropical trees demonstrate year-round secondary growth, most trees in temperate climates grow in the spring and summer and cease through the winter. In the springtime, when water and resources are plentiful, the vascular cambium produces large xylem cells. During the summer months when resources and water may be lacking or reduced, the xylem cells are small. Pressed up against the large, light-colored xylem cells, the small xylem cells look like a thin dark ring. One year of xylem growth, called an annual ring, can be measured as the distance between the dark rings—or the distance between summer xylem growth. Summer growth is called summerwood while the large spring cells are called springwood. Much can be learned about the local environmental conditions through the years by looking at tree rings. If water is plentiful the rings will be wider than usual. Years with fires and blights will be evident, as well as insect infestations and fungal infections. All this by looking at a cross-section of a tree. In conifers, vessels and fibers are absent and thus the wood consists mainly of tracheids. It is important therefore to remember that environmental conditions affect xylem production and the dark rings may not be completely visible, one year’s growth is what constitutes an annual ring, not just dark circles.

The vascular cambium produces more xylem than phloem. In fact, the phloem will be difficult to locate as the cells are thinner than xylem and more likely to collapse under the pressure of the cambiums. Phloem grows to the outside of the vascular cambium and xylem grows to the inside. The oldest xylem is in the very center of the stem/trunk. The wood in the center is called heartwood. It is usually darker as the vessels and tracheids are filled with old resins, gums, and tannins. The younger wood where the xylem is still functioning is toward the outside of the stem nearest the cambium and is lighter in color. This younger wood is called sapwood. The main role of heartwood is structure and support since it is unable to conduct water and nutrients. The heartwood sometimes rots out of an otherwise living tree. Sapwood develops at roughly the same rate that heartwood is ‘retiring’ and thus vital conducting functions are not compromised. Recall that conifers (a group of gymnosperms) do not have vessels or fibers and are primarily tracheids. Conifers have resin canals scattered throughout the xylem tissue. Conifers are primarily considered to be softwoods while the wood of woody dicot trees is considered to be hardwood.

Bark is all of the tissues outside of the cambium, including the phloem. Some have gone so far as to distinguish between inner bark—primary and secondary phloem and outer bark—the periderm, which consists of cork tissue and cork cambium. The cells in these layers only function briefly as they usually become crushed and then slough off. New layers are annually produced by the cambiums. The youngest phloem cells are the ones nearest the vascular cambium and are most active in transporting nutrients, sugars, and water. Mature bark may be composed of alternating layers of crushed phloem and cork.

Herbaceous refers to non-woody plants. Plants that die after going from seed to maturity are called annuals. In general, most monocots are annuals, but there are annual dicot plants as well. Annual dicots are mostly composed of primary tissues, although there may be some minimal secondary growth. Remember, the plant only lives a year, so extensive secondary growth, or increases in width, really doesn’t make sense as far as using the plant resources. A cross-section of a herbaceous dicot stem will show discrete patches of xylem and phloem, vascular bundles, that are arranged in a proper ring separating the cortex and the pith. If secondary xylem and phloem are to emerge, they will arise from between the two primary tissues. Monocots will be discussed shortly.

In the study of plants, understanding the different tissue patterns is essential.

Steles

Steles are a central cylinder in most younger stems and roots, composed of primary xylem, phloem, and the pith, if present. Sometimes referred to as eusteles, which are vascular bundles in higher vascular plants.