Meristem

Meristem is a type of embryonic tissue in plants consisting of unspecialized, youthful cells called meristematic cells and found in areas of the plant where growth is or will take place - the roots and shoots.

Differentiated plant cells have shapes and structures that would make it problematic for them to produce cells of a different type. Thus, cellular divisions of the basic cells of the meristem provide new cells for expansion of tissues, providing the basic structure of the plant body. Though each plant grows according to a certain set of rules, each new root and shoot can go on growing for as long as it is alive; This growth is indeterminate, making the overall shape of the plant not determinate in advance. This is the primary growth. Each meristem can therefore develop into a complete plant. As a consequence, cloning is rather easy. New plants can grow from cuttings or broken plants. This is called asexual reproduction or vegetative reproduction.

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, protoplast fills the cell completely. The vacuoles ( = cavities) are extremely small. The cytoplasm does not contain plastids, chloroplasts, even if they are present in rudimentary form. Meristematic cells fit together closely. There are no intercellular cavities. The cell wall is very thin.

The top of the meristem consists of the root cap, the quiescent center and the merismatic zone, followed by the elongation zone.

There are two kinds of meristems :

• apical meristems (terminal meristems).
• lateral meristems (axillary meristems).

The most general form of meristem is the apical meristem (also called terminal meristem), found in buds at the tips of shoots and roots, producing root and shoot tissue. At the tip of the root is the apical meristem protected by a root cap. Buds can be naked (with the growing leaves visible), protected by not-overlapping scales (valvate buds) or by overlapping scales (imbricated buds).

Apical meristems are completely undifferentiated (indeterminate).

They differentiate into three different 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, 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.

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.

The position of a lateral meristem on a branch or shoot, is known as a node. Tissue between nodes is known as the internode.

References

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