Families of heterotrophic nature (parasitic, saprophytic and insectivorous)

Families of heterotrophic nature (parasitic, saprophytic and insectivorous):-

Parasitic Angiosperms:-

> Plants, which grow on other living organisms and draw nourishment therefrom, are called parasites, and the living organisms attacked are referred to as the hosts. Many fungi lead parasitic existence on plant hosts and often cause enormous damage to the crop plants.

> They send out slender thread-like filaments into the body of the host for absorbing food matters. Phanero­gamic parasites may be devoid of chlorophyll and are thus entirely dependent on the host.

> They are known as total parasites. Cuscuta or dodder (B. Swarnalata), Rafflesia, etc., are examples of total parasites. Cuscuta has weak stems which twine round the host. It produces special sucking roots or haustoria, which penetrate into the tissues of the host. These roots absorb food matters, water and salts from vascular bundles of the host plant.

> Orobanche is another total parasite growing on the roots of the plants of the potato family (Solanaceae), particularly tobacco. Rafflesia, yielding the largest flowers in the world, is a remarkable root-parasite. It is a native of Java and Sumatra.

> Partial parasites have normal green leaves for manufacturing food, but having no connection with the soil they draw the entire supply of water and dissolved salts from the host. Loranthus, commonly growing on the stems of mango and other trees, is a partial parasite. Sandal wood plants attach themselves to the roots of other trees. So they are also partial parasites.

Saprophytic Angiosperms:-

> Saprophytes are the non-green plants which derive food directly from decaying organic matters. Many fungi like moulds and mushrooms growing on rich humus or decaying organic matters are saprophytes.

> They usually send out small slender filaments, called hyphae, into the substratum and make use of the dead organic matters as food by enzyme action. A few flowering plants like Monotropa entirely lack in chlorophyll and draw nourish­ment from the partially decayed organic matters of the soil, like the saprophytic fungi.

Insectivorous Angiosperms:- These are green plants capable of manufacturing carbohy­drates. As the name suggests, the plants have peculiar devices for catching insects which supply them with a part of nitrogenous food requirements. Insect bodies are digested by secretion of enzymes and ultimately absorbed. Some of the common insectivorous plants will be described here.

i. Nepenthes or Pitcher Plant:-

> These are herbaceous plants. The leaf blade is modified into the hollow pitcher having a coloured hood at the top for attracting the insects. Many hairs remain pointed downwards inside the body of the pitcher.

> Small insects slip down in the cavity and get drowned in water which partly fill the pitcher. A large number of glands are present on the inner wall which secrete enzymes, and the insect bodies are digested and absorbed.

ii. Drosera or Sundew:-

> These are also small plants. The spathula like leaves of Drosera bear many glandular hairs called tentacles. Due to the secretion of viscous fluid the tips of the tentacles glisten in sunlight and look like so many dew- drops. Small insects are naturally attracted.

> As soon as one alights on the leaf some move­ment of the tentacles is initiated. Quite a good number of them bend downwards and impri­son the poor insect. Then the dead body is digested by enzyme action and nitrogenous food matters are absorb­ed. Having finished that the tentacles re­sume their original posi­tion and get ready for next prey.

iii. Utricularia or Bladderwort:- These are aquatic plants having much segmented compound leaves. Some of the leaflets are modi­fied into swollen bladders for catching insects. Each bladder has a valve-like door which opens inwards, so that when the insects push in, they are caught. Glands present on the inner surface of the bladder secrete enzymes to digest the insect bodies.

iv. Dionaea or Venus’s Fly-Trap:-

It is a small insectivorous plant native of America. Here the leaves have three very sensitive hairs on each half, and a good number of teeth-like projections occur on the margin. When an insect alights on the blade, the two halves close quickly, as if they are hinged to the mid-rib and the teeth- like projections get interlocked. Thus the insect is caught and the body is digested by usual enzyme action.

v. Aldrovanda:- It is a root-less free-floating aquatic plant commonly found in the saline marshes near Calcutta and in the jheels of East Bengal. Here also the leaves have many sensi­tive hairs on the upper surface and teeth-like projections on the margin. The two halves of the leaves fold along the mid-rib and entrap the insects, the mechanism being same as in Venus’s fly-trap.