USE OF WATER IN PLANTS 77 



greater area of soil being thus moistened. 

 This may be observed in the sugar-cane and 

 tobacco. In many large trees, amongst other 

 plants, practically all the water is thrown 

 away from the trunk so that there is a dry 

 space beneath the leaves and branches ; water 

 is not wanted there, for there are no young 

 roots to absorb it near the trunks of such 

 plants. Close observation has revealed a rela- 

 tionship between the direction and spread of 

 the rootlets and the drainage system of the 

 leaves of a plant. In those plants with widely 

 spreading roots the water is conducted towards 

 the margin of the plant system (cacao, mango). 

 In those with bulbous roots, or with closely 

 tufted rootlets, or with deep, penetrating 

 taproots, the water is commonly conducted 

 towards the centre (violets, canna, beet, lilies). 



7. A plant breathes, just as animals do, 

 and also obtains a large proportion of its food 

 from the air through the agency of its leaves. 

 In order, however, to understand the various 

 processes which go on in the leaf it is neces- 

 sary to know something concerning the com- 

 position of the atmosphere. 



