Differences between animals and plants
At the outset of evolution two main lines diverged, resulting in the two great groups of the living world: the Plant Kingdom and the Animal Kingdom. Different methods of nutrition caused these kingdoms to evolve along completely separate paths, resulting in such radically contrasting types as a tree and the monkey which climbs in it.
Since animals require proteins, and since plants alone have the power of forming them from inorganic substances, it necessarily follows that all animals are dependent upon plants for their food supply.
The other important difference between animals and plants is the result of their different modes of nutrition. Plants are able to obtain the necessaries of life from the air and the soil, which they can find almost anywhere, and as a consequence they have no need of locomotion.
Animals, on the other hand, in their search for ready-made proteins, must move about in order to find their food. Hence, as a general rule, we find that animals have the characteristics of irritability and movement developed to a high degree, whereas plants are stationary and show only slight or sluggish movements in response to stimuli. There are certain exceptions to this general rule. Microscopic plants are often as highly mobile as microscopic animals, while, on the other hand, some animals, such as sponges, are sedentary like plants.
The plant-like type of nutrition is called holophytic and the animal-like type is called holozoic.
The holophytic mode of nutrition produces large quantities of the carbohydrate substance cellulose. This contributes largely to the rigidity of the plant framework, which is not inconsistent with a stationary mode of life, though it would be a serious hindrance to an animal. Cellulose is not formed by any arumal and is therefore a distinctive mark of vegetable nature.
Primitive animals are very much like primitive plants: indeed, there are some organisms which can be put into either category, since they are able to feed either holozoically or holophytically. For example, Euglena is a minute, motile organism, which, under suitable conditions, is able to build up its protein from simple substances in the water around it, like a plant; but should conditions be unfavourable, it can live as an animal, using protein for its food.