62 THE PKINCIPLES OF AGIUCULTURE. 



The process of breaking ii}) the inoU^ciiles of carl)onic 

 acid, and retaining the carbon, re(inires the aid of snn- 

 ligiit, and is really a process of feeding, or ui»building the 

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

 the ail", goes on continuously through the night, and is a 

 process of oxidation ov slow burning and destruction of 

 the ])laut. 



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

 sorbed by leaves from the atmosphere. A drooping plant 

 is (piickly revi^"ed by watering, or by a shower of rain, 

 not so much by absorl)ing moisture through its leaves as 

 by the raijid passage of water into the stem and leaA'cs 

 through its roots. 



Nitrogen from the Air. — While free nitrogen constitutes 

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

 that none of this is directly availal)le 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 ]>lant may obtain a little nitrogen by ab- 

 sorbing this gas through its foliage. 



Roots. — The purpose of the root is to give the jilant 

 support, holding it lii-mly in position ; to absorb nourish- 

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

 to store up a su])])ly of food to support the ])lant the 

 second year. 



The radicle, or first root wliirh descends into the soil 

 from the seed, soon subdivid"S into a number of small 



