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The following points highlight the top four types of heterotrophic plants. The types are: 1. Parasites 2. Saprophytes 3. Symbionts 4. Insectivorous Plants.
Heterotrophic Plants: Type # 1. Parasites:
The total parasites, like dodder (Cuscuta) and broomrape (Orobanche) are never green and cannot synthesize their own food. In such cases, they develop haustoria or sucking roots which go into the vascular bundles of the host plant and absorb from them the prepared food and water.
Heterotrophic Plants: Type # 2. Saprophytes:
For example, Monotropa, bacteria and fungi grow on decaying vegetable or animal matter and absorb organic food material from their substrata.
Heterotrophic Plants: Type # 3. Symbionts:
Two organisms that live in close physical association and are of mutual benefit to each other are called symbionts and the condition is known as symbiosis. Lichens and mycorrhiza are examples of symbionts.
Lichens:
A lichen is a plant consisting of two separate plants a fungus and an alga, so closely associated with each other as to appear a single plant.
Mycorrhiza:
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Some fungi live in close association of other higher plants where they are mutually beneficial to each other. Here fungi develop in the roots of higher plants.
Heterotrophic Plants: Type # 4. Insectivorous Plants:
On the other hand the insectivorous plants are capable of manufacturing carbohydrates due to the presence of chlorophyll, but since they grow in swampy soil which are deficient in nitrogen they cannot synthesize enough proteins. They overcome this deficiency by catching small insects and digesting them. For this purpose, their leaves are specially modified in various ways.
The best known examples of insectivorous plants are-the pitcher plants (Nepenthes), the sundew (Drosera), venus fly trap (Dionaea), butterwort (Pinguicula) and the bladderwort (Utricularia).
Pitcher Plant:
In India, only one species, i.e., Nepenthes khasiana has been found. The pitcher plants are herbs or climbing under shrubs which often climb by means of tendrils.
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The pitcher itself is the modification of the leaf-blade. The insects are attracted to these pitchers either by their bright colours or by their nectar. Below the mouth the inside of the pitcher is covered with numerous smooth and sharp hairs, all pointing downwards.
The insects that slip into water are prevented from coming out by the hairs present in the pitcher. The trapped insects eventually die and are decomposed by microorganisms. The proteins are converted into amines and are absorbed by the inner surface of the pitcher.
The Sundew Plant:
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Three species of Drosera are found in India. The sundew plant is a small herb. Each leaf of Drosera remains covered on the upper surface with numerous glandular hairs known as tentacles. Each gland secretes a viscous fluid which glitters in the sun like dew-drops and hence the name sundew. An insect landing on the leaf gets stuck and the glandular hairs bend down and prevent the insect from escaping.
The enzymes secreted by the hairs digest the insect and the products are absorbed by the leaf surface. The enzyme called pepsin hydrochloric acid, acts on the insect and changes the proteins of its body into soluble and simple forms.
