Here is your short essay on Phyllotaxy

The mode of arrangement of leaves on the stem is known as phyllotaxy. There is a definite law according to which the leaves are arranged on the stem of a particular species, there are three categories of phyllotaxy which fall under two principal types known as cyclic and acyclic.

In acyclic phyllotaxy a single leaf is present at each node. Acyclic phyllotaxy is also known as spiral or alternate phyllotaxy.

In cyclic type, at each node there are’ more than one leaf. In cyclic type, the leaves are arranged in a cycle in which the angle¹ between adjacent leaves known as angular divergence is the same. If there are two’ leaves at the node, two will be placed opposite (i.e. at an angular distance of two right angles).

If there are three leaves, the angle between leaves in the same whorl is 120° (i.e. one third of a circle). If four, it is 90° and so on. Cyclic type shows two distinct categories of phyllotaxy: opposite and whorled depending on presence of two or more than two leaves at each node respectively.

(i) Alternate:

Here, a single leaf arises at each node. These alternate to each other. It is the commonest type of phyllotaxy, e.g., mustard, sunflower, China rose, etc. If an imaginary line is drawn from the base of one particular leaf through the bases of the successive leaves, it forms a spiral. This imaginary spiral line is known as genetic spiral.

When looked from top, one finds in each cycle of spiral one leaf comes to lie directly on another leaf which is placed on a vertical line. So, on the stem one finds definite number of rows of leaves, known as orthostichy. Alternate phyllotaxy is classified further on the basis of number of leaves in relation to spirals. These are as follows.

l. Distichous or two ranked or 1/2 phyllotaxy:

In this case, leaves are arranged in two vertical rows. It is found that the third leaf remains over the first while completing one spiral. Fourth leaf also stands over the second. So, there are two leaves in each spiral. The fifth leaf stands over the first and the third and so on, e.g., wheat, rice. In this case, the angular divergence is 180° (360° x 1/2).

2. Tristichous or three ranked or 1/3 phyllotaxy:

In tristichous phyllotaxy, there are three leaves in one spiral. The fourth leaf stands vertically over the first one, and one cycle of the genetic spiral involves three leaves the fifth leaf stands over the second, the sixth over the third, and the seventh remains over the fourth and the first.

There are three orthostichies, i.e., leaves are arranged three rows or ranks. The mode of arrartgement in the first, fourth, seventh and tenth leaf remains in one row while the third, sixth, ninth and twelveth in the second r and second, fifth, eighth and eleventh in the third row, e.g., Cyperus. Here the angular divergence is 120°. (36o°x3/8).

3. Pentastichous or five-ranked or 2/5 phyllotaxy:

In this case, the leaves are arranged in five vertical rows. There are five leaves two spirals. The mode of arrangement is first and sixth leaf on the first orthostichy, second arid seventh on the second, three and eighth on the third, fourth and ninth on fourth orthostichy. Hence, It is called 2/5 phyllotaxy, making an angular divergence of 144⁰ (3600×2/5). A common example is China rose.

4. Octastichous or eight-ranked or 3/8 phyllotaxy:

The arrangement of leaves is found along eight vertical rows. The mode of arrangement is first and ninth leaf on first row, second and tenth on second row, third and eleventh on fourth row and so on. In this case, there are eight leaves in three spirals. Here, the angular divergenre between two consecutive leaves is 135⁰ (36o°x3/8). A common example is papaya.

(ii) Opposite:

When, two leaves arise at each node, and are opposite to each other, it is opposite phyllotaxy. When one pair of leaves stand at right angle to the next upper or lower pair, the phyllotaxy is called opposite decussate, e.g., Ocimum, Ixora, Calotropis.

In some other plants, the successive pairs are placed exactly on top of each other so that all the leaves lie in one plane. A pair of leaves is seen to stand directly over the lower pair in the same plane. Such an arrangement is said to be opposite superposed, e.g., Quisqualis.

(iii) Whorled or Verticillate:

Here three are more leaves, which form a whorl at each node, are arranged in a circle or whorl, e.g., Alstonia, Allamanda, Nerium.

Leaf mosaic:

Phyllotaxy is the arrangement of leaves on the stem in such a manner that each leaf gets adequate amount of sunlight for manufacture of food material.

The arrangement allows the leaves to secure maximum amount of sunlight with minimum amount of overlapping. Such a condition is also obtained by a differential growth of the stalk of different leaves on the twig so that all the leaves are brought to the same level.

It resembles with a flat mosaic surface. The leaf stalks are unequal and leaves originating lower down have longer stalks. Leaves twist and turn to fill up all the gaps where light is available, -e.g., Acalypha.

FUNCTIONS OF LEAF

The leaves perform three main functions such as manufacture of food, exchange of gases between the atmosphere and the plant body, and evaporation of water. Some of the functions are described below. The functions of modified leaves are not dealt here as they carry on specialized functions.

A. Manufacture of food:

It is the primary function of green leaves. The process is called photosynthesis by which leaf produces food material in the presence of sunlight and green pigment-chlorophyll.

B. Interchange of gases:

It takes place both for respiration and photosynthesis. There are numerous, minute openings present on the leaves, called stomata, through which gaseous exchange takes place between the atmosphere and the plant body.

C. Evaporation of water:

Water absorbed by the root is evaporated from the leaf surface during the day time. It generally takes place through the stomata openings. The phenomenon is known as transpiration. This process allows the plant to absorb water and minerals from the soil and conduct the same to the top of the plant.