THE FIIEE AVATKIi OP THE SOIL. 217 



111 the first iilaoe it is an unfailing and sufficient source of 

 its elements, — hydrogen and oxygen, — and undoubtedly 

 enters directly or indirectly into chemical combination 

 with the carbon taken up from cnrbonic acid, to form sug- 

 ar, starch, cellulose, and other carhohydi-ates. In the 

 second place it performs important physical offices ; is the 

 vehicle or medium of all tlie circulation of matters in the 

 plant ; is directly concerned, it would appear, in imbibing 

 gaseous food in the foliage and solid nutriment through 

 the roots ; and by the force with which it is absorbed, di- 

 rectly influences the enlargement of the cells, and, per- 

 haps, also the direction of their expansion, — an effect shown 

 by the facts just adduced relative to the clover crops ex- 

 amined by Ritthauseii. 



Indirectly, also, Avater performs the most important ser- 

 vice of continually solving an<l making accessible to crops 

 the solid matters in the vicinity of their roots, as has 

 been indicated in tlie chapter on the Origin of Soils. 



Combined Water of the Soil. — As already stated, there 

 may exist in tlie soil compounds of which water is a chemi' 

 cal component. True clay (kaolinite) and the zeolites, as 

 well as the oxides of iron that result from weathering, con- 

 tain chemically combined water. Hence a soil which has 

 been totally deprived of its hygroscopic Avater by drying 

 at 212°, may, and, unless consisting of pure sand, does, 

 yield a further small amount of water by exposure to a 

 higher heat. This combined water has no direct influence 

 on the life of the plant or on the character of the soil, ex^ 

 cept so far as it is related to the properties of the com 

 pounds of which it is an ingredient. 



§2. 



THE AIR OF THE SOIL. 



As to the free Oxygen and Nitrogen which exist in the 

 interstices or adhere to the particles of the soil, there ia 

 10 



