List of nine heterotrophic plants:- 1. Parasites 2. Saprophytes 3. Symbionts 4. Insectivorous or Carnivorous Plants 5. Nepenthes or Pitcher Plant 6. Drosera or Sundew 7. Utricularia or Bladderwort 8. Dionaea or Venus’s Fly-Trap 9. Aldrovanda.
Heterotrophic Plant # 1. Parasites:
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.
Heterotrophic Plant # 2. Saprophytes:
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.
Heterotrophic Plant # 3. Symbionts:
Symbiosis is an association of two different kinds of organisms leading a life of mutual friendship. In connection with nitrogen-assimilation it has been mentioned that some bacteria can fix free nitrogen from the air. The bacteria attack the roots of leguminous plants and give rise to characteristic swellings or nodules by stimulating cell division in the cortical region.
They convert nitrogen into nitrogenous compounds within the cells which partly become available to the plants. In exchange, the bacteria draw carbohydrate food matters from the plant. The bacteria alone cannot fix nitrogen, nor can the host leguminous plant.
Only when there is an intimate association between the bacteria and the root-hair host cells nitrogen fixation can occur. Thus it is a case of symbiosis, what may also be termed reciprocal parasitism. These plants are called symbionts.
Some forest trees have roots encased by many soil fungi. This fungus-root association is called mycorrhiza. The fungus-infected roots lack in root-hairs and the fungal hyphae take up the function of the absorption of water and salts from the soil.
It has been observed that the root nodules of the legumes are sometimes pink or reddish in colour. Only recently the pink colour has been definitely identified to be haemoglobin with the characteristic absorption bands. The presence of haemoglobin in nature has been considered as general only in the vertebrate animals.
This view may have to be seriously modified by the discoveries that the pigment exists in a ciliate Parmoecium and in the root nodules of the legumes. Neither the plant cells (host) nor the bacteria grown separately synthesise haemoglobin.
There is thus no doubt that haemoglobin itself or the process connected with its synthesis may be directly linked with nitrogen fixation. The bacteria, after they have formed nodules by stimulating growth and multiplication of cells, supply the host plant with one or more factors essential for the synthesis of haemoglobin.
Heterotrophic Plant # 4. Insectivorous or Carnivorous Plants:
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 (Figs. 177 & 178).
Heterotrophic Plant # 5. 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.
Heterotrophic Plant # 6. 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.
Heterotrophic Plant # 7. 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.
Heterotrophic Plant # 8. 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.
Heterotrophic Plant # 9. Aldrovanda:
It is a root-less free-floating aquatic plant (Fig. 178) 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.