Delphinidin

There is an apical meristem (tissue in which cells actively divide) at the tip of each stem, and it is this meristem that contributes to an increase in the length of the stem. The apical meristem is dormant before the growing season begins.

It is protected by bud scales of the bud in which it is located and also to a certain extent by leaf primordia (singular: primordium), the tiny embryonic leaves that will develop into mature leaves after the bud scales drop off and growth begins. The apical meristem in the embryonic stem of a seed is also dormant until the seed begins to germinate.

When a bud begins to expand or a seed germinates, the cells of the apical meristem undergo mitosis, and soon three primary meristems develop from it. The outermost of these primary meristems, the protoderm, gives rise to the epidermis. Although there are exceptions, the epidermis is typically one cell thick and usually becomes coated with a thin, waxy, protective layer, the cuticle. A cylinder of strands constituting the procambium appears to the interior of the protoderm. (The procambium produces waterconducting primary xylem cells and primary phloem cells that have several functions, including the conduction of food.)

The remainder of the meristematic tissue, called ground meristem, produces two tissues composed of parenchyma cells. The parenchyma tissue in the center of the stem is the pith. Pith cells tend to be very large and may break down shortly after they are formed, leaving a cylindrical, hollow area. Even if they do not break down early, they may eventually be crushed as new tissues produced by other meristems add to the girth of the stem, particularly in woody plants. The other tissue produced by the ground meristem is the cortex.

The cortex may become more extensive than the pith, but in woody plants, it, too, eventually will be crushed and replaced by new tissues produced from within. The parenchyma of both the pith and the cortex function in storing food or sometimes, if chloroplasts are present, in manufacturing it.

All five of the tissues produced by this apical meristem complex (epidermis, primary xylem, primary phloem, pith, and cortex) arise while the stem is increasing in length and are called primary tissues. As these primary tissues are produced, the leaf primordia and the bud primordia (embryonic buds in the axils of the leaf primordia) develop into mature leaves and buds. As each leaf and bud develops, a strand of xylem and phloem, called a trace, branches off from the cylinder of xylem and phloem extending up and down the stem and enters the leaf or the bud. As the traces branch from the main cylinder of xylem and phloem, each trace leaves a little thumbnail-shaped gap in the cylinder of vascular tissue. These gaps, called leaf gaps and bud gaps, are filled with parenchyma tissue.

A narrow band of cells between the primary xylem and the primary phloem may retain its meristematic nature and become the vascular cambium, one of the two lateral meristems. The vascular cambium is often referred to simply as the cambium. The cells of the cambium continue to divide indefinitely, with the divisions taking place mostly in a plane parallel to the surface of the plant. The secondary tissues produced by the vascular cambium add to the girth of the stem instead of to its length.

Cells produced by the vascular cambium become tracheids, vessel elements, fibers, or other components of secondary xylem (inside of the meristem, toward the center), or they become sieve tube members, companion cells, or other components of secondary phloem (outside of the meristem, toward the surface). The functions of these secondary tissues are the same as those of their primary counterparts— secondary xylem conducts water and soluble nutrients, while secondary phloem conducts, in soluble form, food manufactured by photosynthesis throughout the plant.

In many plants, especially woody species, a second cambium arises within the cortex or, in some instances, develops from the epidermis or phloem. This is called the cork cambium, or phellogen. The cork cambium produces boxlike cork cells, which become impregnated with suberin, a waxy substance that makes the cells impervious to moisture. The cork cells, which are produced annually in cylindrical layers, die shortly after they are formed. The cork cambium may also produce parenchyma-like phelloderm cells to the inside. Cork tissue makes up the outer bark of woody plants; it functions in reducing water loss and in protecting the stem against mechanical injury.

Cork tissue cuts off water and food supplies to the epidermis, which soon dies and is sloughed off. In fact, if the cork were to be formed as a solid cylinder covering the entire stem, vital gas exchange with the interior of the stem would not be possible. In young stems, such gas exchange takes place through the stomata, located in the epidermis. As woody stems age, lenticels develop beneath the stomata. As cork is produced, the parenchyma cells of the lenticels remain so that exchange of gases (e.g., oxygen, carbon dioxide) can continue through spaces between the cells. Lenticels occur in the fissures of the bark of older trees and often appear as small bumps on younger bark. In birch and cherry trees, the lenticels form conspicuous horizontal lines.

Differences between the activities of the apical meristem and those of the cambium and cork cambium become apparent if one drives a nail into the side of a tree and observes it over a period of years. The nail may eventually become embedded as the stem increases in girth, but it will always remain at the same height above the ground, as the cells that increase the length of a stem are produced only at the tips.