ON VITAL PEODUCTION 81 



of animals, resolving those substances into water, carbonic acid, 

 and nitrate. The plants then utilise the latter substances as 

 crude foodstuffs. 



Plant Metabolism. — -We have now to consider the further 

 history of the working substance of life after it has undergone 

 the chemical and energetic degradations that are the results of 

 animal and bacterial metabolism. Returning to our inanimate 

 engine, it may be recalled that the working substance, or steam, 

 expands and does mechanical work on the pistons, and actuates 

 the mechanism. Then it passes through the condenser, having 

 lost its available energy. It is returned to the boiler and is 

 heated, and so takes up fresh available energy, and the cycle of 

 operations recommences. We take the same general view of the 

 animate engine. The working substance, which is a mixture of 

 fats, proteids, and carbohydrates, passes through the animal body, 

 undergoing chemical transformations, doing mechanical work, 

 and (it may be) heating the body. Then it passes out from the 

 body as the excretions, having lost most of its available energy, 

 and it is further acted upon by bacteria, when it loses the re- 

 mainder. It must now be transformed so as to reacquire avail- 

 able energy, just as the cold water entering the steam boiler again 

 takes up energy in the form of heat. 



This absorption of energy by the life working substance is 

 effected in the tissues of green plants, and we must now refer 

 briefly to the metabolism of the latter. 



A green plant is an organism — ^that is, something that trans- 

 forms energy of itself, grows, and reproduces its individual form 

 and mode of behaviour. Its growth is an obvious thing, and so 

 it is clear that it absorbs material from the medium in which it 

 lives. Now, with rare exceptions* the plant organism does not 

 take in visible foodstuff — that is, we can see no obvious process 

 of feeding, mastication, digestion, etc. We know that it can 

 only grow under certain conditions — a plentiful supply of water 

 to its roots and air to its leaves — and it must also have free 

 access to sunlight. In the failure of such conditions the plant 

 does not grow. 



Its foodstuff is necessarily contained in the air surrounding its 

 green leaves and in the water of the soil that contains its roots. 

 Examining the air contained in an enclosed space, we find that 



* Insectivorous plants. 



