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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.

Contents 1 Root Apical Meristems (RAM) 2 Stem Apical Meristems 3 Intercalary Meristems 4 Shoot Apical Meristems (SAM) 5 Floral Meristems (FM) 6 External link

The root apical meristem is covered by the root cap, which protects the apical meristem from the rocks, dirt and pathogens.

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.

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.

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.

The shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, sepals, petals, stamens and ovaries are initiated here at the rate of one every time interval, called a plastochron. It is where the first indications that flower development has been evoked are manifested. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as axillary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome.

Here are some scanning electron micrographs (SEMs) providing examples of shoot apical meristems. SEMs of apical meristems

The SAM consists of 4 distinct cell groups: -

• Stem Cells
• The immediate daughter cells of the stem cells
• A subjacent organising centre
• Founder cells for organ initiation in surronding regions

The four distinct zones mentioned above are maintain by a complex signalling pathway. The organisation centre expresses WUS proteins which maintains the stem cell identity of the overlying cells. The stem cells signal back with CLAVATA3 (CLE3) which is assumed to be a ligand for the CLE1 receptor kinase. When CLV1 interacts with CLV3 it initiates a signalling pathway that results in the repression of the expression of wus. This controls the size of the organising centre.

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).

• Research on the shoot apical meristem at meristemania.org