LEAVES AND LIGHT 



its leaves and turns its stems so as to get as much light as 

 possible. 



This light, as we have seen, is taken in by the plant. It 

 is used to make the gas, carbonic acid,^ unite with water : 

 when these are made to join together, they form sugar; if 

 the sugar is burnt the heat and light appear again. 



By changing the amount and arrangement of the mole- 

 cules in sugar, starch or vegetable fats, and many other sub- 

 stances can be formed. But it is the sunlight that makes 

 all this possible. 



Thus the sun not merely supplies the motive power for all 

 animal and vegetable activity but, by its influence, flowers, 

 leaves, and stems move and turn in such ways that they are 

 in the most convenient position to intercept its light. 



The sunlight, though all-important in the life of most 

 plants, kills many kinds of bacteria and bacilli which love the 

 darkness. The well-known radium rays are also destructive 

 to bacteria, and hinder the growth of certain fungi (Bec- 

 querel's rays have a similar effect). The X-rays are not so 

 well understood, but one can close the leaflets of the Sensitive 

 Plant by means of them. 



Carbonic acid gas forms but a small proportion of the 

 atmosphere which surrounds a growing plant. Yet there 

 is no lack of it, for when the leaf is at work forming 

 sugar the particles of gas are rushing into the leaf, and 

 other particles come from elsewhere to take their place. 

 Every fire and every Iw'eath given off" by an animal 



^ The gas Carbonic acid consists of one part of Carbon and two of 

 Oxygen. It is invisible, just as are the gaseous states of many liquids 

 and solids. Water-vapour is not visible, though water (liquid) and ice 

 can of course be seen. Starch, sugar, cellwall substance, etc. , all con- 

 tain Carbon, Oxygen, and Hydrogen. Vegetable fat is not well under- 

 stood, but starch helps to form it. 



