94 GENEKAL CONSIDEKATIONS 



It is supplied as the result of combustion, of fermen- 

 tation and putrefaction, and from the respiration of 

 plants and animals. The process by which it is 

 taken up and used as food by green plants is known 

 as photosynthesis. This is accomplished by the 

 agency of chlorophyll, the green coloring matter of 

 the leaves, when this is acted on by strong light. 

 During this process carbonic acid is absorbed and 

 a considerable quantity of oxygen is liberated, thus 

 masking and reversing the ordinary process of respi- 

 ration. At night or when in artificial darkness res- 

 piration continues and photosynthesis ceases, so that 

 then green leaves behave like other living tissues and 

 absorb oxygen and give off carbonic acid. Since 

 light then plays so important a part in the nutrition 

 of plants, we can readily see that the leaves are 

 special organs for exposing green chlorophyll-bearing 

 tissue to the action of light. This fact has an im- 

 portant practical bearing on methods of cultivation. 

 Plants that are crowded too closely together can 

 never grow as large or develop as rapidly as those 

 that have sufficient space in which to unfold their 

 leaves. Different kinds of plants vary widely in the 

 amount of light which they require in order to attain 

 their best development. Some are adapted to the 

 dim half light of deep forests and cannot endure 

 the full strong sunlight. Others are adapted to the 

 open, and thrive best where the light and heat are 

 strongest. The agriculturist in the tropics in par- 

 ticular must know the requirements of his different 

 plants as to light, and must regulate his practices 

 accordingly. 



