130 The Study of Animal Life PART n 



less ; at last it slowly dies and withers away ; the cells are 

 left empty ; and that is why the stake cut from the old hard 

 part in the middle of the tree could not grow, it was quite 

 dead. 



5. The Machinery of Life. We have found that, in 

 some way, the protoplasm within the cells is the machinery 

 of life. For simplicity, we shall speak of protoplasm as 

 living matter. This living matter in plants is such that 

 it can transform the energy of sunlight into potential energy 

 of complicated substances such as wood. This trans- 

 formation of energy is one of the chief labours of plants in 

 the world. A great deal of the energy that reaches their 

 living matter is used for their own upward growth ; so that, 

 as we said before, thousands of tons of matter are every 

 year, over every acre of forest, raised high into the air. 



In animals the living machinery is in certain ways of a 

 different nature. An animal eats the substances made by 

 the plant ; the potential energy stored in these is used by it 

 in moving about, and so transformed into energy of motion. 

 The life of plants is chiefly shown in the storage of energy, 

 the life of animals in the use of that store. Chiefly we say, 

 for plants also move to a slight extent ; as a whole, when 

 they twine around a tree or bend towards the sun ; and in 

 their parts when the sap rises and falls. Animals also, to a 

 slight extent, build up substances of high potential energy. 



So far all is certain, but when we inquire by what arrange- 

 ment of parts the living matter is able to be a machine for 

 the transformation of energy, we are unable to form any con- 

 ception. Soon after the discovery of the cells and their living 

 contents, certain philosophers, who must have very faintly 

 realised the necessary physical conditions, arguing from the 

 analogy of machines as men construct them, supposed that 

 the activities of the living machinery could be deduced 

 from the structural arrangements of the cells ; they sup- 

 posed that the living matter, a part within it called the 

 nucleus, and the cell-wall, were in themselves the parts of 

 a machine, and that the various activities of the cells were 

 due to varying shapes of wall, and disposition of its visible 

 parts. It was soon shown, however, that the wall was not 



