Meristematic Tissues

Plant anatomy is the study of the internal structure of a plant's organs -- what tissues each organ is built from, how those tissues are arranged, and how that arrangement suits the organ's function. The starting point for this study is the meristem: a region of the plant body whose cells have not yet taken up a fixed identity and instead keep dividing, adding new cells that later mature into every other tissue in the plant.

A meristematic cell has a characteristic look that sets it apart from a mature cell: it is small and roughly isodiametric (about equal in every direction), its wall is thin and made only of cellulose (no secondary thickening), its cytoplasm is dense and fills almost the whole cell, its nucleus is large and prominent relative to the cell's size, and it has either no vacuole or only a few very small ones. Meristematic cells sit packed tightly against one another with no intercellular spaces, and it is exactly this combination of features -- a thin wall, a dense cytoplasm and an active nucleus, with nothing rigid or bulky to get in the way -- that keeps a meristematic cell free to divide again and again.

Meristems are classified, most usefully, by where in the plant body they occur:

Apical meristem. Found at the very tip of the main root and the very tip of the main shoot (and at the tip of every branch root and branch shoot too), the apical meristem is responsible for the plant's growth in length -- what is called primary growth. Cells cut off behind the apical meristem differentiate in turn into the primary permanent tissues of the root or shoot, so the apical meristem is, in effect, the source from which the whole primary plant body is built.

Lateral meristem. Found running as a cylinder parallel to the long axis of a root or stem, rather than at a tip, the lateral meristem is responsible for growth in girth -- what is called secondary growth. The vascular cambium (which adds new xylem and phloem) and the cork cambium or phellogen (which adds the protective outer bark) are the two lateral meristems of a dicot stem or root; a typical monocot stem lacks both and so, as a rule, does not thicken with age.

Intercalary meristem. Found not at a tip but somewhere along the length of an organ -- classically at the base of a leaf or at the base of an internode, as in grasses and many other monocots -- the intercalary meristem is a patch of tissue that is left behind by the apical meristem but has not yet lost its power to divide. Because it survives grazing or mowing (which removes only the tip), intercalary meristem is what allows a lawn or a cereal crop to regrow quickly from its base; with time, however, its cells too differentiate and it becomes ordinary permanent tissue.

All three positions can also be described by developmental origin -- a promeristem (the very first, embryonic dividing cells), which gives rise to primary meristem, which in turn produces the primary permanent tissues; a lateral meristem that appears later in a mature organ is called a secondary meristem. Whatever its position or origin, the essential job of every meristem is the same: to keep producing new cells that will, once they stop dividing, mature into the parenchyma, collenchyma, sclerenchyma, xylem and phloem that make up the rest of the plant body -- the permanent tissues studied next.