Meristematic Tissue

Meristematic tissue is made of young cells that are still actively dividing. Because they are perpetually immature, meristematic cells share a distinctive set of features quite different from any permanent tissue:

• Shape and packing — the cells are polyhedral or roughly isodiametric (about equal in all dimensions) and are packed together with no intercellular spaces at all, so there is no wasted space between dividing cells.
• Cell wall — thin, elastic and made almost entirely of cellulose, allowing rapid new wall formation after each division.
• Cytoplasm and nucleus — the cytoplasm is dense and fills most of the cell, with a large, distinct nucleus usually located centrally; this is typical of any cell that is metabolically very active.
• Vacuoles — either absent or reduced to a few very small ones, unlike the single large vacuole of a mature parenchyma cell.
• Metabolic rate — high, since the cells are constantly synthesising new cytoplasm and wall material to support division. …

Meristems can be classified in three different ways, and a single meristem is usually described using all three labels at once — one for its origin, one for its position, and one for its function.

By origin. The very first meristematic tissue of an embryo is the promeristem (or embryonic meristem), a tiny region right at the tip of the future root and shoot. As the plant grows, primary meristem develops directly from the promeristem and remains active at the root and shoot tips throughout the plant's life; it is what produces all the primary permanent tissues of the young plant body. Later in the life of many plants, cells of an already-differentiated permanent tissue can regain the ability to divide — a process called dedifferentiation — giving rise to secondary meristem. Because this reactivation always happens along the side of the axis rather than at a growing tip, secondary meristems are always lateral in position; the vascular cambium (both the intrafascicular and interfascicular strips) and the cork cambium are the two standing examples.

By position. An apical meristem sits at the very tip of a root, a shoot, or any of their lateral branches, and its activity is what makes the plant body grow in length — this is why it is also called the apical initial. In the shoot, this meristem occupies the true terminal tip; in the root it is protected and lies just behind the root cap, so it is described as sub-terminal rather than strictly terminal. Intercalary meristem is a pocket of still-dividing cells left behind, usually near the base or top of a node, after the apical meristem has moved on; it is short-lived and its activity is prominent mainly in monocots (it is what allows a grazed or cut grass blade to keep growing from its base). Lateral meristem runs along the sides of the root or stem's central axis and is responsible for increasing girth rather than length — the vascular (intrafascicular) cambium found in the vascular bundles of gymnosperms and dicot angiosperms is the standard example. …