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The arrangement of leaves on a stem.



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.


References in periodicals archive ?
Nature is playing a geometric game with phyllotaxis," says Pan Atela, a mathematician at Smith College in Northampton, Mass.
Scientists have now come to believe that spiral phyllotaxis is a side effect of the biochemistry of growing plants.
Asymmetric flowers are lacking, except for flowers with spiral or irregular floral phyllotaxis (here not considered), and flowers with contort aestivation in each perianth whorl (Cabomba, Cabombaceae) (Endress, 2008a).
On corolla aestivation and phyllotaxis of floral phyllomes.
Phyllotaxis varies along the synflorescence (main shoot and branches); the disposition of the leaves in the vegetative zone is tristichous (phyllotaxis 1/3), whereas in the inflorescence branch zone the bracts, and their branches, have a spiral arrangement.
Regulation of phyllotaxis by polar auxin transport.
2006), spiral inflorescence phyllotaxis is thus very probably ancestral in the grasses, and the distichous arrangement of the inflorescence primary branches appears to be a derived trait.
Floral architecture and phyllotaxis in Calycanthaceae (Laurales).
On the origin of symmetry, branching and phyllotaxis in land plants.
Other changes accompanying size increase and changes in cell arrangement include shifts in phyllotaxis (appendage arrangement), an increased rate of appendage initiation, increased mitotic activity, and appendage initiation relatively higher on the apex (Gifford & Corson, 1971).
The "inflorescence" also has some characters that are more usual in flowers; it often bears lateral structures (the "flowers") directly without subtending foliar organs; the phyllotaxis is often paired or whorled; and there is frequently a residual terminal meristem.
In some of this work the relation between the leaf arrangement and the pattern of vascular organization in the shoot was clearly explained, calling attention to a rather neglected aspect of the theory of phyllotaxis (the pattern of leaf arrangement).