PHYLLOTAXY

            The term phyllotaxy is derived from two Greek words, ‘Phylla’ means leaves and ‘taxis’ means arrangement, i.e., the manner of arrangement of leaves on the stem is called phyllotaxy. Plants arrange leaves on their stems and branches in such a way that the leaves get maximum amount of sunlight to perform their normal functions, more particularly manufacture of food.

            Three main types of phyllotaxy are found in plants –

            1. ALTERNATE:- When only a single leaf arises at each node, it is known as alternate phyllotaxy. Such type of phyllotaxy is noticed in China rose, Mustard, Sunflower, etc.

            The alternate phyllotaxy is further divided into three types as follows –

            a. Phyllotaxy 1/2 or 2-ranked or Distichous – If an imaginary spiral line is drawn starting from the base of a particular leaf and touching the base of all successive leaves then it will be seen that the 1^st, 3^rd, 5^th, 7^th, etc. leaves will fall in the same vertical line and similarly the 2^nd, 4^th, 6^th, etc. leaves will fall in another vertical line. The imaginary spiral is called the genetical spiral and the imaginary line joining the vertical row of leaves is called orthostichy.

            The positions of leaves are marked out on a circle; the leaves will be seen to be placed at half distance of the circle, each leaf being same distant from each other. This type of phyllotaxy is therefore referred to as phyllotaxy 2-ranked. Here the position of the leaves from angular divergence from 1^st to 2^nd leaf is 180⁰. Example – Grasses.

            b. Phyllotaxy 1/3 or 3-ranked or Tristichous – In this type of alternate phyllotaxy, the 4^th leaf stand vertically over the 1^st leaf. The genetic spiral spiral makes one complete turn to reach the 4^th leaf crossing three leaves in between. The 5^th leaf stand over the 2^nd leaf and thus leaves are arranged in 3 rows such as – (i) 1^st, 4^th, 7^th, 10^th. (ii) 2^nd, 5^th, 8^th, 11^th. (iii) 3^rd, 6^th, 9^th, 12^th. If a circle is drawn marking the position of leaves, it will be seen that, each leaf is placed at 1/3 distance and hence this type of phyllotaxy is called 1/3 or 3-ranked. Here the angular divergence is 1/3 of 360⁰ i.e., 120⁰. Example – Sedges.

            c. Phyllotaxy 2/5 or 5-ranked or Pentastichous – In this type of alternate phyllotaxy, the 6^th leaf stand over the 1^st. The genetic spiral makes two circles to reach the 6^th leaf. The arrangement is like – (i) 1^st, 6^th, 11^th (ii) 2^nd, 7^th, 12^th (iii) 3^rd, 8^th, 13^th (iv) 4^th, 9^th, 14^th (v) 5^th, 10^th, 15^th. In this type, the leaves are arranged in 5 rows and two turns of genetic spiral involved five leaves and hence this type of phyllotaxy is called phyllotaxy 2/5 or 5-ranked. Example – China rose.

            2. OPPOSITE:- When two leaves arise at each node, one opposite to the other, it is said to be opposite phyllotaxy.

            Opposite phyllotaxy are of two types –

            a. Decussate – If each pair of leaves stand at right angle to the other, it is called deccusately opposite, e.g., Basil (Ocimum spp.), Guava (Psidium guajava), etc.

            b. Superposed – If each pair of leaves stand directly over the lower pair in the same plane, it is called superposed phyllotaxy, e.g., Rangoon creeper (Quisqualis indicus).

            3. WHORLED:- When more than two leaves are present in a node and these are arranged in a circle or whorl, such type of phyllotaxy is known as whorled phyllotaxy. Example – Devil tree (Alstoma scholaris), Oleander (Nerium indicum), Allamanda spp., etc.

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