What is Phyllotaxy
The mode of arrangement of leaves on stem and its branches is called phyllotaxy or Phyllotaxis. The object of phyllotaxy is to avoid shading one another and to provide sufficient light to the leaves. There are three principal types of phyllotaxy.
Alternate or spiral: A single leaf arises at each node in alternate manner, e.g., mango, China rose, mustard, tobacco, sunflower, etc.
The simplest type of such a phyllotaxy is alternate or spiral distichous in which the leaves of a branch form two alternate rows (e.g., Grass). In others leaves form 3, 4, 5 or several rows called orthostichies. The condition is called tristichous, tetrastichous, pentastichous, etc. In such cases phyllotaxy is determined by passing a thread along the bases of successively higher leaves till a leaf comes to lie exactly above the first one (which is counted zero). The spiral made by the thread is called genetic spiral. Phyllotaxy is written by taking the number of circles as numerator and the number of leaves denominator, e.g., 1/3, 2/5, 3/8, 5/13, 8/21. These phyllotaxic series are also called Schimper-Brown series in which each member of the series is the sum total of numerators and denominators of the two previous ones. Angle between two successive leaves or angular divergence is calculated by multiplying the phyllotaxy with 360, e.g., 1/3 × 360 = 120.
Opposite: Two leaves arise at each node in opposite directions. It is of two types:
Decussate: Pairs of leaves at nodes are at right angle, e.g., Ixora, Ocimum, Calotropis, etc.
Superposed: Pairs of leaves at nodes are parallel, e.g., Eugenia, Syzygium, Quisqualis, etc.
Whorled: More than two leaves at each node are arranged in a circle or whorl, e.g., devil tree (Alstonia) oleander (Nerium), moyna (Vangueria), etc.