606 The Outline of Science 



the co-operation of sunlight. All plants, as we have said, absorb 

 air, water, and salts; all of them split up carbon dioxide (carbonic 

 acid gas), liberating the oxygen, which goes off free once more 

 into the air, and building up the carbon compounds which make 

 all higher life possible. Thus we see that we owe to the plant the 

 oxygen of the air necessary for life. In an aquarium which has no 

 artificial aeration, the animals will die if there are not enough 

 green plants to keep up the supply of oxygen in the water, and 

 the same is true over the earth as a whole. It is plants alone that 

 can make organic matter (energy-yielding material) direct from 

 dead or inorganic material. Every living animal in breathing 

 gives out carbonic acid gas, and it is this gas which makes badly 

 ventilated rooms stuffy when there are large fires and many 

 people breathing. Yet it is food for the plant! The laboratory 

 of the green leaf, then, manufactures an energy -yielding product. 

 In the leaf -cell the chlorophyll acts as an energy transformer and 

 absorber. As energy cannot be created any more than it can be 

 destroyed, what is the source of this energy which the plant makes 

 available? 



If we burn a piece of cotton wool, which consists almost 

 wholly of the carbohydrate called cellulose, its carbon combines 

 with oxygen from the air, and the combustion results in carbon 

 dioxide, water vapour, and a fluff of mineral ash. But besides 

 these materials there are also liberated two forms of energy, 

 namely heat and light. This energy was locked up in the cellulose, 

 and it follows that if the plant has built cellulose from the raw 

 materials carbon dioxide and water, it must have got possession 

 of some energy corresponding to that which escapes as heat and 

 light during burning. The source of this energy is sunlight. 



The Capture of Sunlight 



A green leaf is a few cell-layers thick, and is traversed by 

 veins, the transport system by which water enters the leaf and 

 elaborated food is carried away. Their branching network pro- 



