Certain angiosperm plants are parasites due to lack of chlorophyll. They depend on other Plants, insects etc. for their nutrition.
These Plants obtain their nourishment from the host by means of special type of root like structures called as haustoria.
The haustoria penetrate the tissue of the host to make connection with its vascular tissue and draw organic food from the phloem and also water and mineral salts from the xylem. These parasitic angiosperms may be stem parasites or root parasites according to the organ of the host, where the parasite makes its connection.
Further they maybe total or holoparasite and partial or semi parasite. Holoparasites obtain their total food, water and mineral supply from the host. Semi parasites possess chlorophyll and thus synthesize their own food. They depend on the host for water and mineral nutrients. Parasitic angiosperms are of the following types.
1. Total Stem Parasites
Cuscuta or dodder is one of the most common examples of total stem parasites. It usually attacks stems of different Plants. The Plant Cuscuta completely lacks chlorophyll and has no connection with the soil after seedling stage. The stem of Cuscuta Plant twines round the stem of host and sends out haustoria at some intervals into the host tissues to obtain nourishment.
II. Partial Stem Parasites
Plants like Loran thus, Viscum are partial stem parasites. They have green leaves and are able to manufacture carbohydrates. They absorb water and mineral salts from the stem of hosts through haustoria. Such plants are, therefore, called as partial stem parasites.
III. Total Root Parasites
Angiosperms like Orobanche, Balanophora, Raffles etc., are completely dependent on the roots of the host plant for obtaining food, water and mineral salts. Thus they are called as total root parasites. In Rafflesia, only the flower remains above the surface of the soil while the much branched vegetative body is underground and parasitises the host roots.
IV. Partial Root Parasites
Santalum is one of the most common examples of partial root parasites. The plant is an evergreen one. The roots of this plant send out numerous haustoria which penetrate the roots of nearby host plants from where they absorb organic food, water and mineral salts depending on the requirement of the parasite.
Saprophytes are heterotrophic plants which obtain their nutrition from dead and rotting organic substances. They secrete extracellular digestive enzymes like amylase, cellulose, chitinase, proteases etc., to accomplish the task of food absorption.
A symbiont is that organism which lives in close relationship with other organism for mutual benefit. The close relationship between two organisms is known as symbiosis or mutualism. Symbiotic relationship may be established in between two plants or a plant and an animal. Lichens, modulated roots of legumes, coralloid roots of Cycas and mycorrhiza etc., are common examples of symbiosis.
Mycorrhiza: The association of fungus with roots of vascular plants for absorption and translocation of water and nutrients constitute mycorrhiza.
4. Insectivorous Plants
These plants are green in color. They participate in photosynthetic process and manufacture carbohydrates. But they supplement their protein requirements by capturing small insects; they trap, digest and absorb the nitrogenous products from the insect’s body. Their leaves are specially modified for this purpose. According to the mode of catching the prey carnivorous plants may be classified into four groups.
i. Plants with sensitive glandular hairs on the leaf-surface, secreting a sweet sticky fluid, e.g. Drosera.
ii. Plants with special sensitive hairs-trigger hairs-on the leaf surface, e.g. Dionaea.
iii. Plants with leaf-segments modified into bladders, e.g. Utricularia.
iv. Plants with leaves modified into pitchers, e.g. Nepenthes. Some of the common insectivorous plants are described below.
1. Drosera (Sundew)
It is a small herbaceous plant which grows in moist soil or waterlogged condition. The leaves are 6-12 in number and from a rosette. Petiole is long with a disc shaped lamina, bearing about 200 club shaped hairs or tentacles on the upper surface.
Each tentacle has a glandular swollen reddish tip which secretes a thick sticky purple fluid. This fluid shines in the sun like drops of dew. Insects are attracted by the shining drops and odor and get entangled in the sticky fluid.
The tentacles start bending and curling and entrap the insect. Simultaneously the leaf margins also incurve. The entrapping tentacles secrete proteases which convert nitrogenous organic matter of the insect into amino acids which are subsequently absorbed. The tentacles remain in curled state till complete digestion of the insect. After digestion the tentacles unfold and the remains of the insect are blown off by air.
2. Utricularia (Bladderwort):
It is an aquatic free floating or slightly submerged rootless, small herbaceous plant. Some of the submerged leaves or leaf segments of this plant are modified into small shortly stalked bladders.
Each bladder has an opening guarded by on inward opening valve which is surrounded by a number of stiff pointed hairs. Inner wall of the bladder bear numerous digestive glands. Small aquatic insects enter the bladder along with the water current and get trapped.
Once the insects are inside the bladder, there is no escape for them. Digestive juices secreted by the glands inside the bladder contain proteolytic enzymes which digest the proteins of the body of the insects.
3. Nepenthes (Pitcher Plant):
These are small herbs or shrubs in which the leaves are modified to form pitchers. The leaf apex forms an attractive lid, while the petiole becomes tendril like. The leaf base is flat and looks like a leaf.
At the mouth of the pitcher, there are nectar glands. Inner wall of the pitcher bear glands at the upper side. The middle portion is slippery and curved downward. The pitcher contains acidic fluid at its base. The insects are attracted towards the pitcher due to the colored lid and enter into it in search of honey.
Insects slip and get drowned in the acidic fluid. Curved hairs on the lower part do not allow insects to come out. In the meantime the mouth of the pitcher gets closed by the lid. The glands of the pitcher secrete digestive juice containing proteolytic enzymes to digest the bodies of the insects. After digestion, the food is absorbed by the pitcher and the lid opens again.
4. Dionaca (Venus’ Fly-trap):
The plant is herbaceous. The leaves of the plant grow in the form of a rosette with the inflorescence axis in the centre. The leaf petiole is winged and the lamina has 12-20 teeth on its margin.
In the centre of the leaf, there occur three long pointed sensitive hairs also called trigger hairs, round which a number of rose-colored glands are present. When an insect touches these hairs, the two halves of the lamina suddenly close together at the midrib, imprisoning the insect. The digestive glands begin to secrete pepsin, hydro choric acid, which digest the proteins of the insect. After the leaf has absorbed, the digested food, the two halves of the leaves open out as before,
5. Pinguicula (Butterwort):
It is a small herbaceous plant with a rosette of large and fleshy leaves which bear a number of stalked and sessile glands on their upper surface. The stalked glands secrete sticky mucilage and the sessile glands secrete digestive enzymes.
When an insect sits on the leaf, it becomes entangled in the sticky substance. The margin of the leaf roll inward to enclose the insect. The proteolytic enzymes secreted by the sessile glands digest the protein content of the insect which is then assimilated by the leaf. The leaf becomes open again.
Among all the animate pathogens, fungi, bacteria and viruses cause major damage to the crop plants. Irish famine of 1845 was due to a fungal disease called late blight of potato. The causal organism was a fungus, Phytophthora infestans. Nearly 20-lacs of people died because of starvation as there was complete failure of potato production.
In the last three years of the Second World War (1943), Bengal had to face a serious famine. One of the reasons of the famine was attributed to the loss in the yield of rice due to leaf spot disease of the fungus, Helminthosporium.
Similarly, pathogenic bacteria cause serious damage to certain economically important plants. For example, citrus canker disease caused by Xanthomonas citri can clear the entire orchard and the production may reduce to zero.