The Plantlet. 45 



■whicli the clover belongs * are able to appropriate nitro- 

 gen from the air (260). Phosphorus and sulfur assist 

 in the formation of albuminous substances; potassium 

 assists in assimilation (59); calcium f and magnesium,, 

 while uniformly present, seem to be only incidentally 

 useful. Iron is essential to the formation of chlorophyll 

 (58). 



Of all the materials obtained by plants from the soil, 

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

 and potassium (254) are needed in such quantities that 

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

 water is not deficient. 



63. Water is Necessary to Growth. An adequate sup- 

 ply of water is the most important condition for the well- 

 being of plants, since it not only ser\e.s in nutrition, but 

 is the vehicle by which all other food constituents are 

 distributed throughout the plant. Comparatively few 

 soils are so poor as to be incapable of producing good 

 ciops when sufficiently supplied with water, while the 

 richest soils are unproducti\'e when inadequately supplied 

 with it. Much of the benefit of manuring undoubtedly 

 comes from the increased capacity it gives the soil for 

 holding and transmitting water (93). 



The supplying of food material is not the only office 

 performed by water in the plant. The unfolding and 

 expansion of the plantlet is largely due to a strong ab- 

 sorptive power for water possessed by the protoplasm 

 within the cells. This force causes all living parts of 



* Leguminosse. 



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. 



