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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.

ROOT AND SHOOT APICAL MERISTEMS
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 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.

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. The cells of the
protoderm differentiate into the epidermis. The
procambium cells 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,
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.