THE INTER-RELATION OF PLANTS AND ANIMALS

THE lives of Plants and Animals which, perhaps, at first sight seem so entirely separate and individual are, actually, most intimately connected. The connection varies both as regards degree and endurance in time.

When two individuals live together in such intimate connection as to appear one, their union is known as symbiosis. Symbiosis. Apparently each member of the union benefits by the close partnership.

Among lowly plants a very perfect case of symbiosis is seen in Lichens. A Lichen is a community of Algae and Fungi living in such complete union that a Lichen expanse appears as one plant.

Such a union may occur between plants that differ widely in complexity. The invest- ment of Beech roots with a mycorrhiza-forming Fungus affords an example of such a partnership .

Just as there may be sym- biotic union between plant and plant, so may there also be symbiosis between two animals.

The Sea Anemone is a Coelenterate, closely related to Hydra. There are certain Sea Anemones that fix themselves firmly to the shells of Hermit Crabs . As the Crab moves the Anemone is carried from place to place. In all probability it is thus brought into regions of more plentiful food. Certainly the Crab benefits because it is difficult for its enemies to distinguish it, overshadowed as it is by the body and tentacles of the Anemone it carries.

Another Coelenterate, Hydra viridis , shelters numbers of unicellular Algae. Here is a case of symbiosis not between plant and plant, nor between animal and animal, but actually between animal and plant. The Algae are within the animal cells and undoubtedly help in the nutrition of Hydra by carrying on the work of photosynthesis.

There are, however, more fleeting relationships than this between animals and plants. A sort of temporary symbiosis may obtain.

In pollination the association of plant and animal is usually very fleeting, but it is certainly advantageous. The insect is fed, and through its agency the plant benefits, for pollination is followed by fertilisation.

In some few cases of seed-dispersal, too, there is a certain gain to the animal. The Ant is better off for harvesting those seeds whose caruncles are nutritious . The seeds benefit by germinating away from their place of origin. Generally, however, in cases of dispersal the animal is an unconscious, or even unwilling, agent – possibly suffering inconvenience because of the fruits that have attached themselves to some part of his body.

Plants and animals gain mutually as a result of the gaseous interchange between the individual and the environment. If plants lived in a world of their own they could maintain a satisfactory balance of gases for their physiological needs – carbon dioxide is, however, made more readily available for them because of animal respiration. The animal gains greatly by the volume of oxygen returned to the environment by plants in photo-synthesis.

The balance of advantage is decidedly on the side of the animals. No animal can live independently of plants, unless it be some primitive organism like Euglena, which contains chlorophyll, and therefore reaches a degree of independence.

Animals are unable to act upon the inorganic constituents of earth and air and build up organic compounds – but the plant can, in its green tissues, carry on such analytical and synthetic processes that, from these inorganic substances, complex foods result. An animal surrounded by such substances is helpless. It is entirely dependent upon the plant for food.

In our day, more than ever before, the dependence of animal-life upon plant-life is realised. Without vitamins our physiological processes would go entirely astray and our whole development would suffer. They are absolutely indispensable to normal animal life. For their formation they are dependent upon light. It is in plant-tissues, through the agency of light, that these all-important controlling forces are produced.

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