62 THE PRINCIPLES OF AGRICULTURE. 



The process of breaking up the molecules of carbonic 

 acid, and retaining the carbon, requires the aid of sun- 

 light, and is really a process of feeding, or upbuilding the 

 plant ; but the act of breathing, or taking oxygen from 

 the air, goes on continuously through the night, and is a 

 process of oxidation or slow burning and destruction of 

 the plant. 



Absorption of Water. But little water, if any, is ab- 

 sorbed by leaves from the atmosphere. A drooping plant 

 is quickly revived by watering, or by a shower of rain, 

 not so much by absorbing moisture through its leaves as 

 by the rapid passage of water into the stem and leaves 

 through its roots. 



Nitrogen from the Air. While free nitrogen constitutes 

 four fifths of all the air, it has generally been believed 

 that none of this is directly available for the use of 

 plants. Recent investigations, however, show that cer- 

 tain varieties of plants are probably able, in some way, 

 to make direct use of atmospheric nitrogen. This seems 

 to be especially true of the family of plants called legumi- 

 nous, or pod-bearing plants. This family includes peas, 

 beans, clover, lucern, etc. 



Ammonia, which exists in small quantities in the 

 atmosphere, is partly composed of nitrogen, and it is 

 thought the plant may obtain a little nitrogen by ab- 

 sorbing this gas through its foliage. 



Roots. The purpose of the root is to give the plant 

 support, holding it firmly in position ; to absorb nourish- 

 ment from the soil; and, in the case of biennial plants, 

 to store up a supply of food to support the plant the 

 second year. 



The radicle, or first root which descends into the soil 

 from the seed, soon subdivides into a number of small 



