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Modes of Plant Nutrition

Depending on the mode by which organisms obtain their food or raw materials for food (nutrients) they are classified into two main groups, Autotrophs (self nourishing) and Heterotrophs (dependent on others for nourishment). Autotrophic organisms are the green plants containing chlorophyll as they synthesize their own food through the process of photosynthesis. Heterotrophs are organisms which obtain their food from other organisms, either from autotrophs or from other heterotrophs. The main mode of animal nutrition is heterotrophic. In this method of feeding, complex food materials that the heterotroph takes in are broken down by digestion, into simpler substances, which are then absorbed and incorporated into body structures. The term holozoic is also used for such type of nutrition. In contrast, plants absorb simple compounds namely carbon dioxide, water and salts and build up complex substances, which can be oxidized for energy or synthesised into living protoplasm and cell walls. The term holophytic (feeding like plants) is applied to such modes of nutrition.

Autotrophic Nutrition

The process by which green plants build up carbohydrates from carbon dioxide and water is called photosynthesis. The energy for this process is obtained from sunlight which is absorbed by chlorophyll. Oxygen is given off as a byproduct. In land plants the water is absorbed from the soil by the root system and the carbon dioxide from the air through the stomata on the leaves. Photosynthesis goes on principally in the leaves, though any green part of the plant can photosynthesise (see next Chapter for a detailed account of photosynthesis).

Photosynthesis is a process unique to green plants and algae, a few protists and several groups of bacteria. The process is of a very high magnitude on a worldwide scale. According to one estimate, there are at least 1.6 X 1010 tons of carbon fixed per year on land and between 2 and 14 X 1010 tons per year fixed in the oceans. An amount of oxygen equivalent to that present in Earth's entire atmosphere is produced by photosynthesis every two years.

Heterotrophic Nutrition

Besides the autotrophic mode of nutrition many groups of plants adopt heterotrophic ways of nourishing themselves. A few plants get their part nourishment from symbiotic relationship with bacteria. A few are parasites obtaining their food directly from the living bodies of other plants. Some other plants show saprophytic way of nutrition by obtaining their nourishment from dead organic matter and still another group of plants supplement their photosynthetic diet by capturing and digesting flies and other insects (carnivorous plants). Let us examine some of the heterotrophic mode of plant nutrition in detail.

Symbiotic Nutrition

The nodules of the leguminous plants represent an excellent example of symbiosis, a relationship in which two different organisms live in close association, in intimate physical contact, with mutual benefit. 

Parasitic Nutrition

All higher plants are not photosynthetic autotrophs. During the course of evolution a group of plants has lost the ability to feed itself by photosynthesis. Some of these plants obtain their food directly from the living bodies of other plants. These are called parasitic plants and the most familiar examples are mistletoes and dodders. Mistletoes are green and actually carry on some photosynthesis. But the photosynthetic activity of these plants is not enough to support the normal growth of the plant. Another parasitic plant, the Indian pipe, (Monotropa uniflora), is now known to derive all of its nutrition from the nearby photosynthestic plants by way of a 'fungal bridge' connecting the roots of the two plants.

Many fungi are examples of parasites. The parasitic fungi may be of two types: facultative and obligate parasites. Facultative parasites are those that can grow parasitically but can also grow by themselves on definite artificial media. Obligate parasites, on the other hand, are those that cannot be grown on any available defined medium, as they may have some unknown nutritional requirements. Examples of obligate parasites include various mildews and rusts.

Saprophytic Nutrition

This is the method of feeding in which fairly complex chemical compounds are absorbed into the plant. Some of these compounds are absorbed directly, and others are first digested outside the organism by enzymes secreted or in the material. Decaying organic matter like rotting wood and humus are good materials for saprophytes to feed on. Many fungi are saprophytes obtaining their energy, carbon and nitrogen directly from decaying organic matter. The saprophytic fungi along with bacteria, are the major decomposers of the biosphere.

Predatory Nutrition

Some plants adopt a carnivorous mode of nutrition to supplement their photosynthetic diet. These plants capture and digest flies and other insects. The best known examples of carnivorous plants are Dionea (Venus flytrap) and Drosera (sundew). These plants are normally found in boggy regions where the pH is more acidic. Relatively little nitrogen is recycled into these soils because most decay causing organisms require a more neutral pH. Hence, these carnivorous plants have developed various adaptations for the capture and digestion of animals. This activity provides the plants with a supplementary source of nitrogen.

Sarracenia produces pitcher shaped leaves which collect small amounts of rain water. Insects are attracted to the pitchers either by the bright colour or nectar. Once the insect is inside the pitcher they are trapped by downward pointing hairs. A combination of digestive enzymes and bacteria digest these animals and the digested material is absorbed.

Drosera (sundew), has leaves covered with hairs that secrete a clear, sticky liquid which contains a lot of sugar. An insect touching the leaf gets stuck. Digestion is accomplished by the secretion of appropriate enzymes and digested materials containing carbon and nitrogen are taken up.

Pitcher Plant (Sarracenia)

Venus Fly Trap (Dionea)

Bladderwort (Utricularia)

         Sundew (Drosera)                     Active fly paper (Pinguicula)

Insectivorous Plants

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