Modes of Nutrition in Plants
The chemical substances that provide nourishment to living organisms are called nutrients. Depending on the mode of nutrition the organisms are classified as autotrophs and heterotrophs.
Organisms which utilise carbon dioxide as their sole source of carbon for the formation of organic food by the process of photosynthesis are called autotrophs (self nourishing). In addition to carbon dioxide, autotrophs require water and several inorganic ions.
If the autotrophs prepare their own food by utilising chemical energy they are called chemoautotrophs.
Organisms which are incapable of photosynthesising, obtain certain organic compounds from other autotrophs and they are called heterotrophs and this type of nutrition is referred to as heterotrophic nutrition.
Different types of heterotrophic nutrition are as follows:
1) Parasites, obtain their nutrition from hosts. These parasites maintain physical contacts with the host plant through haustoria (parasitic roots) These haustoria penetrate into host tissue and make connections with the conducting elements of host and draw nourishment.
Example: Total stem parasite like cuscuta and root like orobanche are never green and consequently they have no power to prepare their own food. They get all their food supply from the host plants.
Cuscuta (Dodder) - Portion of Parasite Attached to the Stem of Host
Orobanche ramoseThe young seedling of santalum grows independently upto one year of age. After this, few roots develop haustoria, which make contact with roots of neighbouring trees. Viscum, a partial stem parasite has green leaves and thus is capable of manufacturing food, but is dependent on host plant for water supply.
Viscum : Plant Attached to Host Stem Cut Open to Show Haustoria
2) Saprophytes get their nutrition from dead and decaying organic matter.
Example: Fungi like Rhizopus, Agaricus
Neottia (Bird's nest orchid)
Monotropa (Indian pipe)
Neottia (Bird's nest plant) and Monotropa (Indian pipe) are flowering plants whose roots constitute a mycorrhizal association with fungal hyphae, which help in absorption.
3) Insectivorous plantsBack to Top
They are said to be partly autotrophic and partly heterotrophic. These autotrophs supplement their nutritional requirements by trapping and digesting insects and other small animals. The trapped insects are killed and their proteins are digested by proteolytic enzymes secreted by the epidermis of the leaf. They can live without insects but their growth is stimulated when they digest the insects. The leaves of these plants are modified in several ways for trapping insects.
Example: Pitcher plants, Drosera, Utricularia and Dionea
4) SymbiontsIn a symbiotic association, two organisms live in close physical contact with each other and are of mutual benefit to one one another.
Examples: Lichen and Micorrhiza