BREATHERS IN WATER AND BREATHERS IN AIR 379 



constantly be given off from the sea, which we know is not the 

 case. The requisite oxygen is, in fact, dissolved in the water, 

 which has absorbed it from the atmosphere. Anyone who has 

 tried to keep animals in an aquarium knows that a shallow vessel 

 is better for the purpose than a very deep one, and this is because 

 there is a relatively large surface by which oxygen can be absorbed. 

 In a very large and deep aquarium either the water must be con- 

 stantly renewed, or air must be frequently pumped in. 



Relation between Plants and the Breathing of Animals. — The 

 statement made in the last sentence requires a certain amount of 

 qualification, for not only do animals depend in the long run upon 

 plants, as regards food, but there is an intimate connection be- 

 tween them in the matter of breathing. It ought to be clearly 

 understood that plants breathe exactly in the same way as animals, 

 so far as essentials are concerned, taking in oxygen gas and giving 

 out carbonic acid gas. Yet, as regards ordinary green plants, the 

 statement is often made that "animals breathe in oxygen and 

 breathe out carbonic acid gas, while plants breathe in carbonic 

 acid gas and breathe out oxygen ". This is absolutely incorrect, 

 and why it is so will become apparent if we consider for a moment 

 how, and upon what, green plants feed. Such plants act, so to 

 speak, as intermediaries between the mineral and animal kingdoms, 

 for they convert very simple substances into the materials of 

 which their own bodies are composed, and thus prepare food for 

 animals, as these depend upon a diet of very complex chemical 

 nature (see p. 270). The food of a green plant consists, in fact, 

 of water with mineral substances dissolved in it, and also of car- 

 bonic acid gas, this too being dissolved in water in the case of 

 aquatic plants. It is the last kind of food with which we are here 

 concerned. The green colouring matter (chlorophyll) which is 

 characteristic of ordinary plants enables the living substance (pro- 

 toplasm) with which it is associated to use sunlight in such a way 

 as to bring about chemical action between water and carbonic acid 

 gas. The result of this is twofold, for in the first place a sub- 

 stance is formed which is a step upward in complexity, and in the 

 second place oxygen gas is liberated as a sort of by-product. It is 

 this oxygen gas which is given out by the plant and erroneously 

 supposed to be a result of breathing, whereas it really has to do 

 with the feeding. It is so large in amount as to greatly exceed, 

 during the daytime, the carbonic acid gas which is breathed out by 



