In this article we will discuss about the interdependence of plants and animals for nutrition.
The living things, plants and animals, exist in all conceivable types of situations in the world. They are present on the snow-clad tops of the mountains, in the fathomless depth of the ocean, in barren desert with scorching heat and extremely adverse climatic conditions, in water, on land and even in air.
The minutest bacteria, which cannot be seen without very high magnification of a microscope, are also plants; and they occur everywhere—in food we take, in water we drink, in air we breathe, in clothes we wear and even inside our own bodies. The vastness and variety of plants and animals have not only impressed inquisitive men but have also presented very interesting problems for scientific investigations.
An insight into the subject of organic evolution would reveal that living things were similar in the dim past. At one stage during that very long process bifurcation occurred and there had been progressive elaboration in the two lines culminating in modern plants and animals. These plants and animals are living in the world side by side deriving mutual benefit.
Plants are autotrophic or self-supporting bodies. Due to presence of the green pigment, chlorophyll, they can synthesise complex organic food matters like carbohydrates—sugar and starch, out of water and carbon dioxide gas utilizing the energy from sunlight. This unique process is known as photosynthesis.
Sugar forms the ground substance for the synthesis of other organic food matters. The whole animal kingdom including ourselves has to depend on plants directly or indirectly for the supply of food. The herbivorous animals feed purely on plant parts. The carnivores use animals as their food, which in their turn had their food from the plants.
During the process of photosynthesis plants take in injurious carbon dioxide gas from the air and give out almost equal volume of oxygen. All plants and animals respire day and night when oxygen is absorbed and carbon dioxide is given out. The amount of carbon dioxide given out by plants alone during respiration is not enough for photosynthesis; they need some more than what is supplied by animals.
At any rate if the plants had not absorbed carbon dioxide gas for photosynthesis, the air would have been vitiated, so much so that life would have been impossible on earth.
Thus besides providing us with the most essential thing, food, plants are purifying the air. For proper nutrition plants require a number of elements which are absorbed from the soil and air. Animal bodies and excreta are broken down to simpler elements by bacteria and other micro-organisms.
They are added to the soil increasing its fertility when plants can have healthy growth by utilizing those elements. Nitrogen is a very important element for the plants. Though enough of nitrogen is present in the air, plants can hardly utilise it. It mainly comes as nitrates and ammonia salts from the soil. The protein molecules of animal bodies and excreta are broken down making nitrogen available to the plants.
Some insects have the power of fixing free nitrogen, and some micro-organisms living in symbiotic association in the lumen of some herbivorous animals can do the same. The use of animal excreta, cow dung, etc., and bone dusts as manures is a very common practice with farmers.
The insectivorous plants like Nepenthes (pitcher plant), bladder wort, etc., get part of their nitrogen requirement from the bodies of the insects they catch. Many fungi grow at the expense of decomposed animal bodies or they lead parasitic existence on living animals. All these illustrations go to show that even for nutrition the plants and animals are interdependent.
Flowers are specialised organs of plants for securing reproduction or multiplication of race. For the formation of fruits and seeds union of two opposite gametes or reproductive cells present in the male and female part of the flower is essential.
The pollen grains containing the male gametes must be carried to the carpel, the female organ of the flower. This transference, called pollination, is brought about by animals in many cases. The brightly coloured or sweetly scented flowers have many insect visitors like bees, butterflies, wasps, etc. Even flowers with very, disagreeable odours have their own visitors.
These insects in course of their visits in search of honey or pollen earn the pollen grains from one flower to another. Slow-moving animals like snails are found to crawl on the inflorescence axis of aroids when they accomplish pollination. Birds are the pollinating agents in plants like silk cotton with large flowers.
Every seed has the potentiality of giving rise to a new plant. In fact, the baby plant or embryo remains in the seed in sleeping condition. If all the fruits would discharge their seeds in the shade of the mother plant the result would be disastrous.
The seeds crowded in a small area with limited resources would try to germinate and grow. A keen struggle for existence would ensue and only the fittest would survive and the rest perish. To prevent this danger of wastage plants disperse or scatter their seeds over far and wide areas. In many cases dispersal is secured through animals.
The grazing herbivorous animals very often carry the seeds and fruits during their movement in search of food. The ants and other insects carry small seeds and thus cause dispersal.
The seeds of many fleshy fruits remain uninjured even when they pass through the alimentary canals of birds, squirrels, bats, etc. Those animals discharge the seeds with excreta on far off places—trunks of big trees, roofs arid cornices of houses, where the seeds get very suitable medium—the excreta, for germination.
Dispersal through human agency is also a noticeable feature. Many plants of economic and ornamental value have been carried by human beings to different countries where the plants in course of time got naturalized. The water hyacinth, a veritable pest of Bengal today, was brought to this country for the beautiful lilac flowers.
Ants and other stinging insects often take shelter on big trees, particularly fruit trees like mango and litchi. These insects act as an army and protect the plants from enemies whenever necessary.
In some American acacias certain part of the leaf becomes hollow to be used as an abode of the insects and even some food materials are kept ready for them. The insects in return work as a troop of body guards and render defence service in time of necessity. This interesting device is known as myrmecophily, literally meaning love for insects.