Fig. 19.—The bacillus of bubonic plague (× 1000).
Photo: F. Martin Duncan.
The typical plant lives by absorbing carbon dioxide gas, water, and mineral salts from the surrounding media. These substances, by means of energy which it gathers from the rays of the sun, the plant builds up into organic substances, to be used in the maintenance of life, and for growth and reproduction. This process of chemical construction occurs only in the green, exposed parts of the plant, and indeed can occur only in the presence of chlorophyll, the green colouring matter of the leaves.
Fig. 20.—The bacillus of typhoid (× 2500 diameters).
Photo: F. Martin Duncan.
The animal, on the other hand, lives by appropriating, either directly or indirectly, what the plant has produced. All flesh is indeed grass, in a different sense from that originally intended by the statement. It is this essential difference which lies at the root of all the plain and obvious distinctions between animals and plants. The plant has neither the necessity to go forth in search of its food materials, which nature brings to it, nor has it to spare of its painfully collected energy for the labour of locomotion. Hence it remains stationary. The animal must of necessity go to seek its more elaborate fare, therefore it moves. Moreover, to be successful in its search, the animal obviously requires a nervous system to direct and control its movements, which system, except in the simplest and crudest forms, is absent from the plant. In the main, then, the plant builds up and saves, the animal breaks down and spends. The plant is the producer, the animal the consumer.
Fig. 21.—Amœba.
K, Nucleus; V, contractile vacuole.