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the arrangement of leaves on the stem, reflecting the symmetrical structure of a shoot. Leaf arrangement, which depends primarily on where the leaf primordia arise on the stem apex, is usually an element in the taxonomy of plants. There are three principal types of phyllotaxy: alternate, if one leaf occurs at each node of the stem (oak, birch, grasses, the Umbelliferae); opposite, if two leaves appear at a node on opposite sides (maple, lilac, the Labiatae); and whorled (verticillate), if more than two leaves appear (oleander, water thyme, water milfoil).
The common feature of all three types of leaf arrangement is equal angular distance between the leaves that are at the same node or on successive nodes of a spiral, which is called the basic genetic spiral. Opposite and whorled leaf arrangement is characterized by the alternation of the leaves of neighboring pairs or whorls; in such cases the number of leaves is double that at one node. Alternate leaf arrangement may be varied in the number of orthostichies (vertical files of leaves) and the magnitude of the angles of divergence between successive leaves; this is expressed by a formula of phyllotaxy that represents the fraction of the angle of divergence in segments of the circumference. Most often found are 1/2 (double-row leaf arrangement), 1/3 (three-row leaf arrangement), and 2/5 phyllotaxy; phyllotaxies of 3/8, 5/13, 8/21, etc., are less common. The denominator is the number of orthostichies; the larger it is, the less shading of leaves by one another occurs.
The regularity of leaf arrangement is due to the dimensions of the growing point and the leaf primordia and their reciprocal influence. According to one hypothesis, each leaf primordium forms a physiological field around itself that inhibits the development of new primordia in its immediate vicinity. According to another theory, the development of each succeeding leaf primordium is not inhibited but stimulated by the preceding one.
T. I. SEREBRIAKOVA