Meristem

A meristem is a tissue in all plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place.

Differentiated plant cells generally cannot divide or produce cells of a different type. 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.

Meristematic cells are analogous in function to stem cells in animals, are incompletely or not at all differentiated, and are capable of continued cellular division (youthful). 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.

Meristematic zones
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.

Meristems located at a bud on a branch or shoot are known as a node. Tissue between nodes is known as the internode.

Apical meristems
The apical meristem, or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of roots in plants. Its main function is to begin growth of new cells in young seedlings at the tips of roots and shoots (forming buds, among other things). Specifically, an active apical meristem lays down a growing root or shoot behind itself, pushing itself forward. Apical meristems are very small, compared to the cylinder-shaped lateral meristems.

Apical meristems are composed of several layers. The number of layers 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.

Shoot apical meristems
The source of all above-ground organs. Cells at the SAM summit serve as stem cells to the surrounding peripheral region, where they proliferate rapidly and are incorporated into differentiating leaf or flower primordia.

The shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, sepals, petals, stamens and ovaries are initiated here at the rate of one every time interval, called a plastochron. It is where the first indications that flower development has been evoked are manifested. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as axillary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome. The SAM consists of 4 distinct cell groups: -


 * Stem Cells
 * The immediate daughter cells of the stem cells
 * A subjacent organising centre
 * Founder cells for organ initiation in surronding regions

The four distinct zones mentioned above are maintained by a complex signalling pathway. The organisation centre expresses WUS proteins which maintains the stem cell identity of the overlying cells. The stem cells signal back with CLAVATA3 (CLV3) which is assumed to be a ligand for the CLV1 receptor kinase. When CLV1 interacts with CLV3 it initiates a signalling pathway that results in the repression of the expression of WUS. This controls the size of the organising centre.

Root apical meristems
The root apical meristem (RAM) is covered by the root cap, which protects the apical meristem from the rocks, dirt and pathogens.

Stem apical meristem
The stem apical meristem is multicellular and, unlike the root apical meristem, has no cover. Rudimentary leaves may develop as scales, hardening at the end of the growing season to protect the stem apical meristem.

Intercalary meristem
The intercalary meristems occur only in monocot stems between mature tissues. They are cylindrical meristems located around the nodes and are an adaptation to grazing herbivores and landmowers.

Besides growing additional leaves, an apical meristem may also develop into flowers. However, once differentiated into a flower, the meristem loses its meristematic ability and thus terminate the growth of that shoot. The lateral meristems behind the flowered apical meristem will then take on the functions as the apical meristem(s). This may be observed on a fruit tree at the beginning of growing season.

Floral meristem
As flowers derive originally from shoots, the apical part of the flower primordium is composed of meristematic tissue, referred to as the floral meristem (FM).

Apical dominance
Apical dominance is phenomenon where one meristem prevents or inhibits the growth of other meristems. As a result the plant will have one clearly defined main trunk. For example, in trees the tip of the main trunk bears the dominant meristem. Therefore the tip of the trunk grows fast and is not shadowed by branches. If the dominant meristem is cut of, one or more branch tips will assume dominance. The branch will start growing faster and the new growth will be vertical. Over the years the branch may begin to look more and more like an extension of the main trunk. Often several branches will exhibit this behaviour after the removal of apical meristem, leading to a bushy growth.

The mechanism of apical dominance is based on the plant hormone auxin. It is produced in the apical meristem and transported towards the roots in the cambium. If apical dominance is complete, it prevents any branches from forming as long as apical meristem is active. If the dominance is incomplete, side branches will develop.

Primary meristems
Apical meristems may differentiate into three kinds of primary meristem:
 * Protoderm - lies around the outside of the stem and develops into the epidermis.
 * Procambium - lies just inside of the protoderm and develops into primary xylem and primary phloem. It also produces the vascular cambium, a secondary meristem.
 * Ground meristem develops into the pith. It produces the cork cambium, another secondary meristem.

These meristems are responsible for primary growth, or an increase in length or height.

Secondary meristems
There are two types of secondary meristems, these are also called the lateral meristems because they surround the established stem of a plant and cause it to grow laterally (i.e. larger in diameter).
 * Vascular cambium - produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arborescent. This does not occur in plants which do not go through secondary growth (known as herbaceous plants).
 * Cork cambium - gives rise to the bark of a tree.

Indeterminate growth of meristems
Though each plant grows according to a certain set of rules, each new root and shoot meristem can go on growing for as long as it is alive; In many plants meristematic growth is potentially indeterminate, making the overall shape of the plant not determinate in advance. This is the primary growth.

Cloning
Under appropriate conditions, each shoot meristem can develop into a complete new plant or clone. Such new plants can be grown from shoot cuttings that contain an apical meristem. Root apical meristems are not readily cloned, however. This cloning is called asexual reproduction or vegetative reproduction and is widely practiced in horticulture to mass-produce plants of a desirable genotype. This process is also known as mericloning.