The following points highlight the three special modes of nutrition in angiosperms. The modes are: A. Parasitic Angiosperms B. Insectivorous Plants and C. Saprophytic Angiosperms.
A. Parasitic Angiosperms:
These plants obtain their nourishment from other living plants called as the hosts (which are also angiosperms) by means of special roots called as haustoria. The haustoria penetrate the tissue of the host to make connection with its vascular tissue to draw readymade food from the phloem and water and mineral salts from xylem.
Parasitic angiosperms may be of 4 types:
(1) Total Stem Parasites:
One of the very common examples of such parasites is dodder or Cuscuta. It completely lacks chlorophyll and has no connection with the soil after the seedling stage. Therefore, it is totally dependent on the host for organic food materials and supply of water and mineral salts.
The thin, pale yellow and leafless stem of Cuscuta (Fig. 12.1) twines round the stem and sometimes leaves of the host (which may be herb, shrub, or tree) and sends out haustoria at some intervals into the host tissue to obtain nourishment.
(2) Partial Stem Parasites:
There are certain angiosperms like Loranthus and Viscum or Misletoe (Fig. 12.2) which parasitise the stem of host plant (which are some trees and shrubs) to draw only water and mineral salts from it. They can manufacture their own food due to the presence of green leaves. Such plants are therefore, called as partial stem parasites. The haustoria penetrate the host tissue and make connection only with its xylem.
(3) Total Root Parasites:
Angiosperms like Orobanche, Striga, Balanophora, Rafflesia etc., are completely dependent on the roots of the host plant for food and supply of the water and mineral salts. They are called as total root parasites. In Rafflesia, only the flower (which is largest in diameter among the Plant Kingdom) remain above the surface of the soil while the much branched vegetative body is underground and parasitizes the host roots.
(4) Partial Root Parasites:
A very common example of partial root parasites is Santalum album (Sandalwood tree). The roots of this ever green plant send out numerous haustoria which penetrate the roots of nearby host plants like Albizzia, Dalbergia, and Eucalyptus etc.
B. Insectivorous Plants:
These plants are capable of manufacturing carbohydrates due to the presence of chlorophyll, but because they grow in swampy soils which are deficient in nitrogen they cannot synthesize enough proteins. They overcome this deficiency by catching small insects and digesting them. Their leaves are specially modified in various ways for this purpose.
Some of the common insectivorous plants are as follows:
(1) The Pitcher Plants:
These are usually small herbs or shrubs in which the leaves are modified to form pitchers. One of the very common examples of these plants is Nepenthes. In this plant the lamina of the leaf is modified to form pitcher, leaf apex forms an attractive lid, while the petiole becomes tendril like. The leaf base is flat and looks like a leaf (Fig. 12.4).
At the mouth of the pitcher there are nectar glands. Inner walls of the pitcher bear glands at the upper side; the middle portion is slippery while the lower part bears hairs which are curved downward. The pitcher contains acidic fluid at its base.
The insects are attracted towards the pitcher due to the coloured lid and enter into it in search of honey. Insects slip on the inner walls of the pitcher and are 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 is closed by the lid. The glands of the pitcher secrete digestive juices containing proteolytic enzymes to digest the bodies of the insects. After digestion, the food is absorbed by the pitcher and the lid becomes open again.
(2) Drosera (or Sundew):
Drosera is a small herbaceous plant growing in marshy places (Fig. 12.5 A). The leaves of this plant bear numerous hair-like structures called as tentacles. Each tentacle has got a gland at the tip (Fig. 12.5 B) which secretes a sticky fluid. This fluid shines in sunlight and appears as dew, hence the plant is called sundew.
When an insect is attracted by the shining sticky fluid and tries to sit on the leaf, it becomes entangled in the sticky fluid. Very soon the sensitive tentacles surround the insect and curve inward on it. Digestive juices secreted by the glands contain proteolytic enzymes which digest proteins of the insect body. Digested food is finally assimilated by the leaves and the tentacles again come in their original straight position.
(3) Utricularia (or Bladderwort):
Utricularia (Fig. 12.6 A) is an aquatic, free-floating, rootless, small herbaceous plant. Its leaves are very much dissected some of the segments of which are modified to form small bladder like structures. Each bladder has got an opening guarded by an inwardly opening valve which is surrounded by a number of stiff, pointed hairs. Inner walls of the bladder bear numerous digestive glands.
Small water insects enter the bladder along with the water current and are trapped there. Digestive juices secreted by the glands inside the bladders contain proteolytic enzymes which digest the proteins of the insect.
(4) Dionaea (or Venus Fly Trap):
Dionaea is a herbaceous insectivorous plant with a rosette of more or less prostrate leaves. Each leaf has got a winged petiole which is separated from the lamina by a constriction. The lamina is divided into two rounded halves which can move upward along the mid-rib and have teeth-like margins. Upper surface of the two halves of the lamina bears numerous digestive glands. Each half bears three pointed hairs on its upper side (Fig. 12.7).
As soon as an insect sits on the leaf-blade, the two halves curve upward. The teeth like margins of the sides interlock with each other and the insect is enclosed. Digestive juices secreted by the glands contain proteolytic enzymes which digest the proteins of the body of the insect. The digested food is assimilated by the leaf and again the two halves of the leaf blade become open.
(5) Pinguicula (or Butterwort):
Pinguicula (Fig. 12.8) is a small herbaceous plant with a rosette of leaves which bear a number of stalked and sessile glands on their upper surfaces. The stalked glands secrete a sticky mucilaginous substance while the sessile glands secrete digestive juices.
When an insect sits on the leaf, it becomes entangled in the sticky substance. The margins of the leaf roll inward to enclose the insect. Digestive juices containing proteolytic enzymes digest the proteins of the body of the insect which is then assimilated by the leaf. The leaf becomes open again.
C. Saprophytic Angiosperms:
Certain angiosperms like Monotropa (Fig. 12.9) and Neottia (Bird’s Eye Nest) which lack chlorophyll grow on soil rich in humus or dead organic matter in the forests. These plants (which are orchids) obtain their nourishment from the humus through mycorrhiza i.e., the fungal filaments associated with their roots.
