2. Origin and Development of Stems

Origin and Development of Stems

Recall that the apical meristem is responsible for vertical growth, or increase in the length of a stem. Prior to the start of the growing season, the cells in the apical meristem are dormant. The apical meristem is protected at the tip of the twig, by the covering bud scales and by the leaf primordial. The leaf primordia are tiny embryonic leaves that will develop into mature leaves after bud scales drop off and growth commences. When a seed germinates or a bud begins to grow, the cells in the apical meristem undergo mitosis. From these cells three primary meristems will develop:

  1. The outermost meristem is the protoderm, which gives rise to the epidermis. This layer is usually one cell thick and becomes coated with a waxy cuticle.
  2. The second layer is the procambium, which is a cylinder of strands. This layer gives rise to the primary xylem and primary phloem cells.
  3. The innermost meristem is the ground meristem from which arises two tissues composed of parenchyma cells. The tissue in the center of the stem is the pith. These cells are large and may break down shortly after being formed which leaves a cylindrical hollow area. If they do not break down, they will be compressed by new additions to the plant girth by other meristems. The second parenchyma tissue that arises is called the cortex. Cortex may be quite extensive and also crushed or replaced in woody stems. The function of both tissues is food storage. If chloroplasts are present the tissues may function in producing food.

It is important to note that all five of the above-mentioned tissues—epidermis, primary xylem, primary phloem, pith, and cortex—are produced by the apical meristem and are, thus, primary tissues as the plant is increasing in length. Xylem and phloem tissue branch off from the main vascular cylinder and enter into the leaf or bud. Each branching of vascular tissue is called a trace. Each trace branch leaves a small thumbnail shaped gap in the cylinder of tissue and are called leaf gaps and bud gaps.

In between the primary xylem and primary phloem, a thin band of cells retains its meristematic nature. This band becomes the vascular cambium of one of the two lateral meristems.

In woody plants and some others, a second cambium arises from the cortex or sometimes the epidermis or phloem. The second cambium is called the cork cambium or phellogen and is responsible for producing cork cells. Recall that the cork cells become filled with suberin which waterproofs the cells. The resulting cork tissue constitutes the out bark of woody plants and functions to reduce water loss and to protect the stem against mechanical injury. We will revisit the role of cork later on in discussing biotechnology and propagation. For now, though, understand that cork tissue cuts off food and water supplies to the epidermis which results in sloughing off. Also, understand that cork tissues do not form a solid cylinder around the exterior of a woody stem. This is to allow vital gas exchange with the environment.

Before we go on, it is important to remember the difference between monocots and dicots, the two main divisions of flowering plants. Most of the distinguishing revolves around the seed leaves, which are called cotyledons. Cotyledons function in storing food needed by the young seedling until true leaves grow and are able to take over the food supplying function.

  1. Monocotyledon (monocot) plants—These plants form from seeds that have one embryonic seed leaf (hence the ‘mono’ in monocot).
  2. Dicotyledon (dicot) plants—These plants form from seeds that have two (hence the ‘di’ in dicot) embryonic seed leaf.

Cone-bearing trees, conifers, such as pines, have multiple cotyledons, usually eight, in their seed structure.