11th Class Biology Mineral Nutrition In Plants Special Modes Of Nutrition

The method of taking in and synthesis of various types of foods by different plants and animals is called nutrition.

Generally plants are autotrophic in their mode of nutrition, but there are some examples which are heterotrophic in their mode of nutrition. These plants are unable to manufacture their own food due to lack of chlorophyll or some other reasons.

(1) Parasites : These plants obtain either their organic food prepared by other organisms or depend upon other plants only for water and minerals with the help of which they can synthesize their own food. The living organism from which the parasite obtains its organic food or water and minerals is called host. Any part of the body of parasite is modified into a special organ called haustorium which enters into the cells of host and absorbs food or water and minerals from the host.

Parasites can be classified into two categories :

(i) Total parasites. (ii) Semiparasites or partial parasites.

(i) Total parasites : These plants never possess chlorophyll, hence they always obtain their food from the host. They may be attached to branches, stem (stem parasites) or roots (root parasites) of the host plants.

Total stem parasite : Cuscuta is a rootless, yellow coloured, slender stem with small scale leaves, which twines around the host. The parasite develops haustoria (Small adventitious sucking roots) which enter the host plant forming contact with xylem and phloem of the host. It absorbs prepared food, water and minerals from the host plant.

Total root parasite : Total root parasites are common in the families like Orobanchaceae, Rafflesiaceae, Balanophoraceae, etc. Orobanche, Rafflesia and Balanophora are some of the common root parasites.

Orobanche is commonly known as broom rape. It has scale leaves and pinkish or bluish flowers. The tip of the root of parasite makes haustorial contact with the root of host and absorbs food from the host. Orobanche is usually parasitic upon brinjal, tobacco. In Rafflesia (stinking corpse lily) another root parasite, vegetative parts of the plant are highly reduced and represented by cellular filaments resembling fungal mycelium. These filaments get embedded in the soft tissue of the host while the flowers emerge out in the forms of buds.

Balanophora occurs as a total stem parasite in the roots of forest trees.

(ii) Semiparasite or partial parasite : Such parasitic plants have chlorophyll and, therefore, synthesize their organic food themselves. But they fulfill their mineral and water requirements from their host plants. These are two types :

Partial stem parasites : The well known example of partial stem parasite is Viscum album (mistletoe) which parasitizes a number of shrubs and trees. The mature plant of Viscum is dichotomously branched with green leaves born in pairs attached on each node of stem. The shoots are attached to the host by means of haustoria. The primary haustoria reaches upto cortex of the host which runs logitudinally. It sends secondary haustoria which make connection with the xylem of the host and absorb water and minerals Loranthus is another partial stem parasite.

Partial root parasites : The common example of partial (semi-parasite) root parasite is Santalum album (Sandal wood tree) which is an evergreen partial root parasite which grows in South India. It grows on the roots of Dalbergia sisso, Eucalyptus. Like other partial parasites, it also has green leaves and absorbs only minerals and water from the host plants.

Similarly, Striga on roots of sugarcane and Thesium on the roots of grasses are other partial root parasites.

(2) Saprophyte : These plants live upon dead organic matter and are responsible for conversion of complex organic substances into simple inorganic substances (minerals), e.g., some bacteria, some fungi (Yeast, Mucors, Penicillium, Agaricus), few algae (Polytoma), few bryophytes (Buxbaumia, Hypnum and Splanchnum), few pteridophytes (like Botrychium) and some angiosperms (Monotropa and Neottia) also.

Monotropa, commonly known as Indian pipe, lacks chlorophyll and is colourless or ivory white. It is found in Khasi hills and in the dense forests of Shimla. Monotropa, though usually referred to as a saprophyte, actually gets its nourishment from fungal mycelium which surround its roots. Such association between roots of higher plants and fungi is known as mycorrhiza. Neottia (Bird's nest orchid) grows in the humus rich soil of the forests. It has very few reduced leaves and thick pale yellow stem. The roots lack root hairs and the nutrients are absorbed by mycorrhiza.

(3) Symbiotic plant : Sometimes two different species of organisms spend much or all of their lives in close physical association, deriving mutual benefit. Such an association is known as symbiosis and each organism is known as symbiont. Symbiotic association is so close that symbionts appear to be different parts of the same plant.

Symbiotic association may be between two higher plants or between a higher plant and a lower plant. Some common examples of symbiosis are described below.

Lichens : Lichens is a special group of plants, when an alga and fungus live together and are mutually benefitted (alga provides food and fungus provides water minerals and protection of alga).

The fungus component of the lichens, called mycobiont, is generally a member of Ascomycetae or occasionally a Basidiomycetae. The algal component of the lichen is known as phycobiont and is generally a member of Chlorophyceae (e.g., Trebouxia) or Cyanophyceae (e.g., Nostoc, Gloeocapsa).

Mycorrhiza : It is a mutually beneficial association between a fungus and the root of higher plant. In such association the fungal mycelium forms a mantle over the root surface and some of the hyphae penetrate between cortical cells and metabolites are transferred in both directions (i.e., from fungus to the root cells and vice-versa).

Root nodules of leguminosae : Members of the sub-family Papilionaceae of the Leguminosae (e.g., pea, beans, trifolium) harbour species of Rhizobium, a nitrogen fixing bacteria. The bacteria form nodules in the roots. They fix elemental nitrogen of the atmosphere and make it available to the plant in forms that can be utilized. In turn they derive food and shelter from the leguminous plant.

Myrmecophily : It is the symbiotic relationship between ants and some higher plants. The ants obtain food and shelter from the plant. They protect the plant (e.g., Mango) from other animals. In Acacia sphaerocephala the stipules are hollowed to function as ant shelter. Leaflet tips (Belt's corpuscles) and rachis (extrafloral nectaries) possess feeding materials. A higher plant which is benefitted by association with ants is called myrmecophyte. The term myrmecophily is also used for pollination by ants.

(4) Insectivorous plants : These plants are autotrophic in their mode of nutrition but they grow in marshy or muddy soils, which are generally deficient in nitrogen and in order to fulfil their nitrogen requirement, these plants catch small insects. The organs and specially leaves of these plants are modified variously to catch the insects. These plants have glands secreting proteolytic enzymes which breakdown complex proteins into simple nitrogenous substances, which inturn are absorbed by these plants. Some of these plants are as follows :

Drosera (Sundew) : It is a herbaceous plant having spathulate or lunate leaves. The leaves are covered by glandular hair with a swollen tip. The glands secretes a sticky purple juice which shines like a dew drop in bright light sunshine, hence the name sundew. These long special hair are generally referred to as 'tentacles'. When an insect alights on the leaf, the tentacles curve due to thigmonasty. The insect is killed and its proteins are digested by pepsin hydrochloride. Similar tentacles are also found in Drosophyllum.

Utricularia (Bladderwort) : It is submerged floating aquatic herb which lacks roots. Some of the species of Utricularia also occur in moist soil. The leaves are dissected into fine segments and appear like roots. Some of the leaf segments are modified into pear-shaped sacs called bladders or utricles.

The bladders are triangular or semicircular structures having a single opening guarded by a valve. There are numerous bristles near the mouth and digestive glands inside. The bladders show special trap mechanism. The valve of the bladder opens on the inner side. When small aquatic animalcules enter the bladder along with water current, they get trapped inside. Their proteins are digested enzymatically. When a bladder is full of undigested matter, it degenerasis.

Nepenthes (Pitcher plant) : They are commonly found in tropical areas like Assam and Meghalaya (i.e., N. Khasiana). In this plant the leaf base is winged, petiole is tendrillar and the lamina is modified into pitcher. The pitcher has a distinct collar at the mouth and the apex is modified into the lid. The undersurface of the lid has alluring glands whereas the inner surface of pitcher is lined by numerous digestive glands and several downward directed hair. The lid attracts insects which slide down into the pitcher. The downward directed hair check their escape. The insect is killed and its proteins are digested by pepsin hydrochloride. Other insectivorous plants having leaf pitchers are Sarracenia, Cephalotus, Heliamphora, etc.

Dionaea (Venus fly trap) : It is a small herbaceous plant found mainly in America. The plant has a rosette of radiating leaves. The petiole is winged and photosynthetic. The lamina is bilobed and the midrib acts like a hinge between the two lobes of the lamina. Each lobe has 15-20 trigger hairs or bristles. These hairs are very sensitive to nitrogenous substances. When an insect alights on the leaf and touches the sensitive hairs, the two lobes of lamina fold along the midrib. Thus the insect is trapped in between the lobes. Pepsin hydrochloride secreted by the digestive glands, present in the upper part of the lobes digests the insect.

Sarracenia (Pitcher plant; Devil's boot) : This pitcher plant is found in the temperate regions. It has a very reduced stem which bears a rosette of leaves. The leaves are modified into pitchers. It can easily be distinguished from Nepenthes on the basis of its trumpet-shaped sessile pitchers. The pitchers of Sarracenia lack digestive enzymes and here the insects are decomposed by bacteria.

Pinguicula (Butterwort) : It is a herbaceous plant having a basal rosette of ovate leaves. The leaf margins are slightly curved in upward direction. The dorsal (upper) surface of leaf has two types of glands stalked and sessile. The stalked glands secrete mucilage while the sessile glands secrete digestive enzymes.

Aldrovanda (Water flea trap) : It is also a rootless, submerged aquatic plant (bog plant) recalling the habit of Utricularia. The leaves are bilobed with long petioles. There are five bristle like outgrowths associated with the lamina. The leaf surface is covered by visid stalked glands. The proteins of the insect are digested enzymatically.