Phyllotaxy

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Phyllotaxy may be defined as the pattern of attachment or arrangement of leaves on the stem or branch is such that they get appropriate amount of sunlight for photosynthesis. It is of three types;

Alternate: here a single leaf arises at each node in alternate manner e.g. mango, China rose.

Opposite: a pair of leaves at each node and lie opposite to each other, it is two types;

  • Opposite decussate: In pairs at right angles to one another, e.g. Calotropis, Tulsi.

  • Opposite superposed: a pair of leaves that stands directly over the lower pair in the same plane, e.g. guava.

Whorled: when more than two leaves at each node are arranged in a whorl, e.g. Alstonia, Nerium.

Image showing Phyllotaxy types; alternate, opposite and whorled.

Image Showing Types of Phyllotaxy.

Image showing Phyllotaxy types; alternate, opposite and whorled.

Modifications of leaves

Although the function of leaves is to synthesize food, in some cases they get modified into distinct structures to perform special functions like support and protection to plant, storage of food and water or to catch insects as in case of insectivorous plants.

Table showing modification of leaves.
Table showing modification of leaves.

Types

Characters

Examples

Tendril

Thin wiry, entire leaf or leaflet becomes tendrillar for the purpose of climbing

Pea and glory lily

Spines

Leaves become spiny and serve the purpose of defence and help in reducing transpiration

Opuntia and Aloe

Phyllode

When the petiole becomes leaf like and help in photosynthesis

Australian acacia

Leaves of Insectivorous plants

The lamina assumes the form of a pitcher with a lid to trap the insects. The inner walls of the pitchers possess a number of digestive glands that secrete a fluid while in bladderwort some segmented leaves get modified into bladders, they help in trapping insects

Pitcher plant (Nepenthes), Bladderwort (Utricularia)

Image showing modification of leaves; tendril, spines, phyllode and pitcher plant.

Image Showing Modification of Leaves.

Image showing modification of leaves; tendril, spines, phyllode and pitcher plant.

Heterophylly (heteros = different)

Some plants show more than one type of leaves in the same plant or occurrence of dissimilar foliage leaves on a plant is called Heterophylly. Heterophylly is commonly seen in rooted emerged type of hydrophytes e.g. Limnophila heterophylla, Trapa bispinosa (water chestnut).

Anisophylly

Occurrence of dissimilar leaves on a node is called Anisophylly e.g. Goldfussia glomerata, Boerhaavia diffusa.

Functions of Leaf

  1. Manufacture of carbohydrates: main function of the leaf is to manufacture food particularly carbohydrates. Chloroplasts found in the leaf cells, trap the solar energy which is then utilized in the synthesis of carbohydrates from carbon dioxide and water by the process of photosynthesis.

  2. Exchange of gases: To facilitate the exchange of gases between the atmosphere and the plant body numerous minute openings called stomata, develop usually on the undersurface of the leaf. The stomata remain open during day light. In the process of respiration of all the living cells the oxygen is taken in and carbon dioxide is given out while in photosynthesis the green cells absorb carbon dioxide and give out oxygen.

  3. Transpiration: the excess of water is lost from the aerial parts of plants in the form of water vapours is called transpiration. Occurs in mostly through stomata but sometimes also takes place through cuticle and lenticels.

  4. Storage of food: fleshy leaves of succulents, such as Indian aloe, purslane and fleshy scale leaves of onion store up water and food material for the future use of the plants. Fleshy leaves of many desert plants store a large quantity of water, mucilage and food material.

  5. Vegetative propagation: The leaves of Bryophyllum, Begonia and Kalanchoe produce buds on their margins. Each such bud develops into a new plant.

  6. Protection: The leaves also give necessary protection to the axillary bud. The leaves modified into thorns and spines give protection to the plants from animals.

  7. Guttation: Exudation of excess of water containing salts takes place in liquid form from leaf margins in plants growing in humid climate.

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