The Plantlei. 45 



gen and oxygen. These enter into the construQtion of 

 nearly all tissues. Nitrogen is one of the constituents 

 of protoplasm (13). Most plants depend upon soluble 

 nitrates in the soil for their nitrogen supply, but those 

 of the natural order to which the clover belongs* are 

 able to appropriate nitrogen from the air (260). Phos- 

 phorus and sulfur assist in the formation of albumin- 

 ous substances; potassium assists in assimilation (58); 

 calcium"!" and magnesium, while uniformly present, seem 

 to be only incidentally useful. Iron is essential to the 

 formation of chlorophyll (57). 



Of all the materials obtained by plants from the soil, 

 but three, aside from water, viz., nitrogen, phosphorus 

 and potassium (253) are needed in such quantities that 

 the plants are likely to exhaust the supply, so long as 

 water is not deficient. 



62. Water is Necessary to Growth. An adequate 

 supply of water is the most important condition for the 

 well-being of plants, since it not only serves in nutri- 

 tion, but is the vehicle by which all other food con- 

 stituents are distributed throughout ■ the plant. Com- 

 paratively few soils are so poor as to be incapable of 

 producing good crops when sufficiently supplied "with 

 water, while the richest soils are unproductive when 

 inadequately supplied with it. Much of the benefit of 

 manuring undoubtedly comes from the increased ca- 

 pacity it gives the soil for holding and transmitting 

 water (92). 



* Leguminosae. 



t Lime, which is a compound of calcium, appears to be essential to 

 the fruiting of some plants, as the peanut, while detrimental to the 

 fruiting of others, as the cranberry and huckleberry. 



