Table of Contents
In this primary and secondary meristematic tissue post we have briefly explained about primary cells of meristematic tissue, secondary cells of meristematic tissue, and other cells of meristematic tissue.
A meristem is a site in the plant body where new cells of meristematic tissue form and the complex processes of growth and differentiation are initiated. Growth means the irreversible increase in size that comes from both cell division and cell enlargement. Cell differentiation refers to the changes that a cell undergoes structurally and biochemically so that it can perform a specialized function. Since cells of meristematic tissue and tissues are derived from meristems, we do not consider meristems themselves to be tissues.
There are different categories of meristems, each with a specific function. Shoot and root apical meristems are at the tips of branches and roots; they are the ultimate sources of all cells of meristematic tissue in the plant. Primary meristems, the next level of meristems, originate in apical meristems and produce, or more correctly, differentiate into, the primary tissues. The secondary meristems produce the secondary tissues. These categories of meristems allow vascular plants to grow very large and to great age.
Diagram of a tomato plant showing the relative positiions of the root apical meristem (RAM) and shoot apical meristem (SAM), the primary meristems (protoderm, ground meristem, and procambium), and the secondary meristems (vascular cambium and cork cambium) in both the shoot and root systems.
Primary and Secondary Meristematic Tissue
The vascular plant body is polar, meaning that it has a shoot end and a root end. At the tip of each branch is a shoot apical meristem (SAM), and at the tip of each root is a root apical meristem (RAM).
These two apical meristems are the sites of the formation of new cells of meristematic tissue by cell division. Theoretically, apical meristems could operate forever. This does not occur, however, because some factor will always limit the size of a plant. Whether it’s because of a scarcity of nutrients, or structural limitations, or heredity, eventually a plant ceases to grow. A branch, for example, can carry only a certain weight before it breaks. Also, each plant and plant organ (leaves, stems, and roots) has a system for genetic regulation of growth; every species seems to have an optimum size.
If you made a very thin longitudinal section through them you would see that the cells of meristematic tissue of the shoot apical meristem (SAM) and root apical meristem (RAM) and those just basal to them are small, with relatively dense protoplasts.Cells of meristematic tissue with these characteristics are usually capable of dividing and so are referred to as meristematic cells.
Cells of meristematic tissue immediately basal to the shoot apical meristem are ordered into distinct files of cells. These newly ordered cells are still meristematic (they can divide); they are in a sense the embryonic stages of the tissues. These groups of cells of meristematic tissue are called the primary meristems, and they have two roles: to form the primary tissues and to elongate the root and shoot.
There are three primary meristems: protoderm, procambium, and ground meristem. The cells of meristematic tissue of the protoderm differentiate into the epidermis. The procambium cells of meristematic tissue differentiate into the cells of the primary xylem and primary phloem. The ground meristem differentiates into the cells of the pith and cortex of stems and roots and the mesophyll of leaves.
The primary meristems near the tips of the roots and shoots are the site of most elongation. They produce new cells of meristematic tissue, which then enlarge mostly by elongation. However, in many plants the branch or root continues to increase in girth as well. This increase in girth requires lateral growth, which involves the formation and activity of the next category of meristems, called secondary meristems.
The secondary meristems are responsible for cell division, initiation of cell differentiation, and growth in a lateral direction, thereby increasing the thickness and circumference of stems and roots. The wood in trees, for example, is really secondary growth resulting from the activity of secondary meristems. Not all plants have secondary meristems. There are thousands of species that grow only one season and usually lack secondary growth.
Leaves also usually lack secondary growth. The bodies of many plants have two secondary meristems: the vascular cambium and the cork cambium. Vascular cambium differentiates into secondary xylem and secondary phloem, and the cork cambium into the periderm. These will be discussed at length in the chapters on stems and roots, which follow.
Summary of meristems and the tissues they generate.
There are several other meristems. In stems, as an example, an intercalary meristem occurs within the stem to regulate its elongation. As leaves develop, there are leaf-specific meristems that regulate leaf shape. The intercalary meristem at the base of grass leaves allows the leaf to continue to grow after being grazed or mowed. Other meristems are involved in forming buds and roots in unusual places, such as at the base of trees, and also in the repair of wounds.