LEAVES 19 



This explains why leaves are adjusted in so many ways to 

 obtain light, as described in 12. It also gives name to 

 the process, photosynthesis, the name indicating that the 

 work is done in the presence of light. 



The process demands that carbohydrates shall be made 

 from raw materials common in nature and easily obtained 

 by plants, and in photosynthesis two such substances are 

 used. One of these is water, which in the plants com- 

 monly thought of is absorbed by the roots from the soil, 

 passes up through the stem, and reaches the green working 

 cells of the leaves through the veins. The other substance 

 is carbon dioxide, a gas present in small proportion in the air 

 (really in the form of carbonic acid gas), but one which is 

 being constantly renewed as it is used, so that it is always 

 available. Water is made up of one part of oxygen and two 

 parts of hydrogen; while carbon dioxide consists of two 

 parts of oxygen and one part of carbon. These are just the 

 elements that enter into the structure of a carbohydrate. 



In photosynthesis the elements of water and carbon 

 dioxide are separated and recombined to form a carbo- 

 hydrate, and when this has been accomplished it is found 

 that some oxygen has been left over. That is, in the 

 process oxygen is a waste product and is given off by the 

 working cells. Therefore, in the sunlight a leaf is absorbing 

 carbon dioxide and giving off oxygen; and this gas exchange 

 is the superficial indication that photosynthesis is going on. 



It is very easy to discover that oxygen is being given 

 off by a leaf exposed to light, and that the amount given 

 off (and hence the amount of work) depends upon the 

 intensity of the light. If an actively growing water-plant, 

 submerged in water in a glass vessel, be exposed to bright 

 light, bubbles may be seen coming from the plant and 

 rising through the water (Figs. 13 and 14). Shading the 

 vessel diminishes the number of bubbles. That the gas 

 being given off is mainly oxygen may be proved by invert- 



