![]() Some early scientists-notably Leonardo da Vinci-made observations of the spiral arrangements of plants. This gives rise to a self-propagating system that is ultimately controlled by the ebb and flow of auxin in different regions of the meristematic topography. When a leaf is initiated and begins development, auxin begins to flow towards it, thus depleting auxin from area on the meristem close to where the leaf was initiated. Leaves become initiated in localized areas where auxin concentration is higher. The pattern of leaves on a plant is ultimately controlled by the accumulation of the plant hormone auxin in certain areas of the meristem. In some cases, the numbers appear to be multiples of Fibonacci numbers because the spirals consist of whorls. These also turn out to be Fibonacci numbers. In the botanical literature, these designs are described by the number of counter-clockwise spirals and the number of clockwise spirals. This phyllotactic pattern creates an optical effect of criss-crossing spirals. The most famous example is the sunflower head. Examples can be found in composite flowers and seed heads. This tends to occur with a basal configuration. With larger Fibonacci pairs, the pattern becomes complex and non-repeating. The number of leaves is sometimes called rank, in the case of simple Fibonacci ratios, because the leaves line up in vertical rows. The numerator and denominator normally consist of a Fibonacci number and its second successor. In beech and hazel the angle is 1/3, in oak and apricot it is 2/5, in sunflowers, poplar, and pear, it is 3/8, and in willow and almond the angle is 5/13. The rotational angle from leaf to leaf in a repeating spiral can be represented by a fraction of a full rotation around the stem.Īlternate distichous leaves will have an angle of 1/2 of a full rotation. A basal whorl with a large number of leaves spread out in a circle is called a rosette. Ī 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. Examples of trees with whorled phyllotaxis are Brabejum stellatifolium and the related genus Macadamia. The whorled arrangement is fairly unusual on plants except for those with particularly short internodes. If the arrangement is both distichous and decussate, it is called secondarily distichous.Ī decussate leaf pattern Decussate phyllotaxis of Crassula rupestris In genera of the Aizoaceae, such as Lithops and Conophytum, many species have just two fully developed leaves at a time, the older pair folding back and dying off to make room for the decussately oriented new pair as the plant grows. It is common in members of the family Crassulaceae Decussate phyllotaxis also occurs in the Aizoaceae. In an opposite pattern, if successive leaf pairs are 90 degrees apart, this habit is called decussate. It also occurs in other plant habits such as those of Gasteria or Aloe seedlings, and also in mature plants of related species such as Kumara plicatilis.Ī Lithops species, showing its decussate growth in which a single pair of leaves is replaced at a time, leaving just one live active pair of leaves as the old pair withers ![]() Examples include various bulbous plants such as Boophone. With an alternate (spiral) pattern, each leaf arises at a different point (node) on the stem.ĭistichous phyllotaxis, also called "two-ranked leaf arrangement" is a special case of either opposite or alternate leaf arrangement where the leaves on a stem are arranged in two vertical columns on opposite sides of the stem. An opposite leaf pair can be thought of as a whorl of two leaves. With an opposite leaf arrangement, two leaves arise from the stem at the same level (at the same node), on opposite sides of the stem. ![]() Veronicastrum virginicum has whorls of leaves separated by long internodes. Leaves may also be whorled if several leaves arise, or appear to arise, from the same level (at the same node) on a stem. The basic arrangements of leaves on a stem are opposite and alternate (also known as spiral). Two different examples of the alternate (spiral) leaf pattern
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