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The apical meristem, or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of root in plants. Its main function is to begin growth of new cells in young seedlings at the tips of roots and shoots (forming buds, among other things). Specifically, an active apical meristem lays down a growing root or shoot behind itself, pushing itself forward. Apical meristems are very small, compared to the cylinder-shaped lateral meristems.
Apical meristems are composed of several layers. The number of layers varies according to plant type. In general the outermost layer is called the tunica while the innermost layers are the corpus. In monocots, the tunica determine the physical characteristics of the leaf edge and margin. In dicots, layer two of the corpus determine the characteristics of the edge of the leaf. The corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the root and the stem.
Root Apical Meristems
The root apical meristem is covered by the root cap, which protects the apical meristem from the rocks, dirt and other pathogens.
Stem Apical Meristems
The stem apical meristem is multicellular and, unlike the root apical meristem, has no cover. Rudimentary leaves may develop as scales, hardening at the end of the growing season to protect the stem apical meristem.
Intercalary Meristems
The intercalary meristems occur only in monocot stems between mature tissues. They are cylindrical meristems located around the nodes and are an adaptation to grazing herbivores and landmowers.
Besides growing additional leaves, an apical meristem may also develop into flowers. However, once differentiated into a flower, the meristem loses its meristematic ability and thus terminate the growth of that shoot. The lateral meristems behind the flowered apical meristem will then take on the functions as the apical meristem(s). This may be observed on a fruit tree at the beginning of growing season.
Shoot Apical Meristems (SAM)
The source of all above-ground organs. Cells at the SAM summit serve as stem cells to the surrounding peripheral region, where they proliferate rapidly and are incorporated into differentiating leaf or flower primordia.
Floral Meristems (FM)
As flowers derive originally from shoots, the apical part of the flower primordium is composed of meristematic tissue, referred to as the floral meristem (FM).
"Apical meristem"