AskDefine | Define phyllotaxis

User Contributed Dictionary

English

Etymology

From Greek φυλλον ‘leaf’ + ταξις ‘arrangement’ (from τασσειν ‘arrange’).

Pronunciation

IPA: /fɪlə'tæksɪs/

Noun

phyllotaxis
  1. the arrangement of leaves on a stem, or the mathematical principles governing such arrangement
    • 1985: Yet his Lordship would find this rate of proportion (…) likewise in all plants, in the disposition of their leaves, for which ordering he would make a name, that is, from the Greek, phyllotaxis. — John Fowles, A Maggot

Extensive Definition

In botany, phyllotaxis or phyllotaxy is the arrangement of the leaves on the stem of a plant. The basic patterns are alternate, opposite, whorled or spiral. With an alternate pattern, leaves switch from side to side. An alternate distichous phyllotaxis means that each leaf growing at a single node is disposed in a single rank along the branch (such as in grasses). In an opposite pattern, two leaves grow in opposite directions from the same node. In an opposite pattern, if successive leaf pairs are perpendicular, this is called decussate. A whorled pattern consists of three or more leaves at each node. An opposite leaf pair can be thought of as a whorl of two leaves. A whorl can occur as a basal structure where all the leaves are attached at the base of the shoot and the internodes are small or nonexistent. A basal whorl with a large number of leaves spread out in a circle is called a rosette. A multijugate pattern is a spriral composed of whorls. The pattern has also been observed to emerge in at least one animal cell (the red blood cell), during processes that perturb cellular fluid dynamics .
Insight into the mechanism had to wait until Wilhelm Hofmeister proprosed a model in 1868. The process begins with two primordia, nascent leaves, forming on opposite sides of the shoot. A third new leaf then erupts between them. Each old leaf pushes the upstart away. The golden angle is the blind result of this jostling. Since three golden arcs add up to slightly more than enough to wrap a circle, one of the old leaves overlaps and is pushed out in a radial line, like a rocket that escapes earth orbit. This vacates space in the inner generative spiral for a new leaf to form. The generative spiral should not be confused with the clockwise and counter-clockwise spirals that emerge in densely packed plant structures. Spirals are discerned by tracing the path from leaf to neighboring leaf and each leaf acquires new neighbors as it shoots out from the center and its sister primordia. New leaves move into the spaces opened as old leaves diverge. Phi is an irrational number and this guarantees that no two leaves ever follow the same radial line from center to edge. In modern times, researchers such as Snow and Snow have continued these lines of inquiry. Computer modeling and morphological studies have confirmed and refined Hoffmeister's ideas. Questions remain about the details. Botanists are divided on whether the control of leaf migration depends on chemical gradients among the primordia or purely mechanical forces. Lucas rather than Fibonacci numbers have been observed in a few plants and occasionally the leaf positioning appears to be random.

References

  • F.M.J. van der Linden
  • Frank M.J. van der Linden: Creating Phyllotaxis, The Stack-and-Drag model, in Mathematical Biosciences, NY 1996
  • Frank M.J. van der Linden: Creating Phyllotaxis from Seed to Flower, in Symmetry in Plants, Jean & Barabé eds., World Scientific, Singapore 1998

See also

phyllotaxis in German: Phyllotaxis
phyllotaxis in Spanish: Filotaxis
phyllotaxis in French: Phyllotaxie
phyllotaxis in Korean: 잎차례
phyllotaxis in Hungarian: Levélállás
phyllotaxis in Dutch: Phyllotaxis
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