ROOT AND SHOOT APICAL MERISTEMS
The vascular plant body is polar, meaning that it has a shoot
end and a root end.The
procambium cells differentiate into the cells of the primary
xylem and primary phloem.This increase in girth requires lateral growth, which
involves the formation and activity of the next category of
meristems, called secondary meristems.The ground meristem
differentiates into the cells of the pith and cortex of stems
and roots and the mesophyll of leaves.Whether it's because of a scarcity of
nutrients, or structural limitations, or heredity, eventually a
plant ceases to grow.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.PRIMARY MERISTEMS
If you made a very thin longitudinal section through them
(SAM and RAM) you would see that the cells of the SAM and
RAM and those just basal to them (toward the more mature
cells) 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 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.Theoretically, apical meristems could operate forever.