I02 THE LIFE OF THE PLANT 



experiments we have made. The most sterile soils, 

 when watered with such a solution, become fertile in 

 the sense of becoming quite fit for the purpose of feeding 

 the plant. 



Such are the results, so brilliant in their simplicity, 

 to which the study of the physiology of the root has 

 brought us. Let us, however, remember that this 

 simplicity is the result of many years' stubborn labour 

 of scores of scientific investigators. 



A question naturally arises here : would it be right 

 to conclude that all the other substances which form 

 the main bulk of the soil are quite useless to the plant ? 

 Evidently not. Some of the substances, while they do 

 not serve as food at any given moment, may be of use 

 as food at some future time ; others, without taking 

 any direct part in nutrition, may indirectly contribute 

 towards it. For instance, besides saltpetre and 

 ammonia, the soil contains considerably larger quantities 

 of nitrogen in the form of organic matter. Yet this 

 nitrogen is of no immediate use for the purposes of 

 nutrition ; soil, which contains nitrogen only in that 

 form, is almost sterile ; but this nitrogen may gradually 

 change into ammonia and nitric acid, and then servfe 

 as food. This is an example of a substance useless at 

 any given moment, yet serving as a future supply of 

 food. Substances in the soil may be useful to the 

 plant in still other ways. They are of value to it by 

 reason of their capacity for retaining moisture, and 

 for absorbing heat, and they help to hold the nutrient 

 substances and distribute them uniformly. 



With regard to this last property the capacity of the 

 soil for absorption is remarkable. If we fill a funnel 

 with soil and water it with some nutrient solution, and 

 then collect the water after it has passed through the 

 soil, we find that it contains very little of the nutrient 

 substances. Ammonia, phosphoric acid, and potassium 

 are especially absorbed ; all these, as we have seen, being 



