A meristem is the tissue in most plants containing undifferentiated cells called meristematic cells, found in zones of the plant where growth can take place. Meristematic cells give rise to various organs of a plant and are responsible for the growth.
Differentiated plant cells generally cannot divide or produce cells of a different type. Meristematic cells are incompletely or not at all differentiated, and are capable of continued cellular division. Therefore, cell division in the meristem is required to provide new cells for expansion and differentiation of tissues and initiation of new organs, providing the basic structure of the plant body. Furthermore, the cells are small and protoplasm fills the cell completely. The vacuoles are extremely small. The cytoplasm does not contain differentiated plastids (chloroplasts or chromoplasts), although they are present in rudimentary form (proplastids). Meristematic cells are packed closely together without intercellular cavities. The cell wall is a very thin primary cell wall. Maintenance of the cells requires a balance between two antagonistic processes: organ initiation and stem cell population renewal.
There are three types of meristematic tissues: apical (at the tips), intercalary (in the middle) and lateral (at the sides). At the meristem summit, there is a small group of slowly dividing cells, which is commonly called the central zone. Cells of this zone have a stem cell function and are essential for meristem maintenance. The proliferation and growth rates at the meristem summit usually differ considerably from those at the periphery.
Apical meristems are the completely undifferentiated (indeterminate) meristems in a plant. These differentiate into three kinds of primary meristems. The primary meristems in turn produce the two secondary meristem types. These secondary meristems are also known as lateral meristems because they are involved in lateral growth.
There are two types of apical meristem tissue: shoot apical meristem (SAM), which gives rise to organs like the leaves and flowers, and root apical meristem (RAM), which provides the meristematic cells for future root growth. SAM and RAM cells divide rapidly and are considered indeterminate, in that they do not possess any defined end status. In that sense, the meristematic cells are frequently compared to the stem cells in animals, which have an analogous behavior and function.
The number of layers in meristem varies according to plant type. In general the outermost layer is called the tunica while the innermost layers are the corpus. In monocots, the tunica determine the physical characteristics of the leaf edge and margin. In dicots, layer two of the corpus determine the characteristics of the edge of the leaf. The corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the root and the stem.
Apical dominance is the phenomenon where one meristem prevents or inhibits the growth of other meristems. As a result, the plant will have one clearly defined main trunk.
In some plants, the lateral bud located in the axil of each leaf does not grow to form branches, especially at first. The apex of the stem has an actively growing apical bud. Apical dominance is thought to be caused by the apical bud producing plant hormone IAA (auxin) in abundance. This auxin is transported basipetally from the apical bud. The auxin causes the lateral buds to remain dormant.
The difference in response between the two kinds of buds is explained in their sensitivity to the auxin concentration. Clearly the lateral buds are more sensitive to auxin than the apical bud. There is a concentration of auxin at which the apical bud is stimulated to grow while the lateral buds are inhibited. That concentration would be near the letter “C” of “Concentration” in the graph.
When the apical bud is removed, the source of IAA is removed. Since the auxin concentration is much lower, the lateral buds can now grow. In fact their growth will be stimulated by a relativley small drop in auxin concentration. Thus, decapitating (pruning) a shoot will cause it to branch.
Recent investigations into apical dominance and the control of branching have revealed a new plant hormone family termed strigolactones. These compounds were previously known to be involved in seed germination and communication with mycorrhizal fungi and are now shown to be involved in inhibition of branching.