Leaves become initiated in localized areas where auxin is absent. The pattern of leaves on a plant is ultimately controlled by the local depletion 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. Decussate phyllotaxis of Crassula rupestris
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