608 The Outline of Science 



carbon dioxide and water, and gives up its store of energy. This 

 continued production of carbon dioxide would, if there were no 

 counter-process, lead to a cumulative vitiation of the atmosphere. 

 It was the attempt to find the counter-process that led to the 

 discovery of the plant's mode of nutrition. The discovery was 

 made by the English chemist and philosopher, Joseph Priestley, 

 one of the founders of modern chemistry. 



What Food is Used for 



The sugars and the like that green plants make in their leaf- 

 laboratory may be used in the leaf itself, or first transported to 

 some other part. The plant moves but little, and then slowly; 

 its temperature is scarcely raised above that of the surrounding 

 air; and so it requires only a fraction of the energy used up by 

 the animal, in moving and heat production. What energy it does 

 expend is chiefly employed in carrying out chemical transforma- 

 tions and in the processes of growth. This energy is obtained, as 

 in the animal, by oxidation or combustion processes, and an ap- 

 preciable quantity of carbohydrate is used up in this way. Here 

 we may note that in plants the using up of oxygen and the giving 

 off of carbon dioxide, which may be briefly called respiration, is 

 demonstrable only in the dark. It undoubtedly goes on all the 

 time, but as long as the plant is illuminated the respiratory process 

 is completely masked by the much more active counter-process of 

 photosynthesis which we have discussed. Carbohydrates are also 

 transported from the leaves to other parts of the plant to be re- 

 stored as reserves, often after conversion into other forms. The 

 commonest food reserve is starch. This storage of food is a 

 characteristic feature of plant life, and it is wrapped up with 

 the habit of evading adverse conditions, such as cold in tem- 

 perate climates or drought in arid regions, by passing into an 

 inactive resting state. The resting parts, such as bulbs or tubers 

 or seeds, usually contain a good store of food, and at the expense 

 of this the plant can make a flying start when conditions are once 



