Meristem

From Encyclopedia Jr, free information reference for Kids

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

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.

[edit] Meristematic Zones

Meristems may be either apical or lateral.

The most general form of meristem is the apical meristem (also called terminal meristem), found in buds at the tips of shoots, and at the root tip, and responsible for shoot and root growth, respectively. At the tip (apex) of the root the apical meristem is covered and protected by a root cap of differentiated cells. Buds can be naked (with the growing leaves visible), protected by non-overlapping scales (valvate buds) or by overlapping scales (imbricated buds).

Apical meristems are completely undifferentiated (indeterminate). They 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 the protoderm. It develops into the primary xylem, primary phloem. It also produces the vascular cambium, a secondary meristem which may continue to produce secondary xylem and phloem throughout the life of the plant.
• Ground meristem develops into the pith and the cork cambium, another secondary meristem.

The vascular cambium and cork cambium are called lateral meristems because they surround the established stem and make it grow larger in diameter. This is called the secondary growth, giving rise to wood. These plants are called arborescent or fruiticose. Secondary growth does not occur in all plants; those without it are called herbaceous.

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

[edit] 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.

[edit] 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.

[edit] See also

• Apical meristem
• Cork cambium
• Vascular cambium

[edit] References

• Neil A. Campbell and Jane B. Reece Biology, 6th edition. Benjamin Cummings.