CHAPTER III 



THE NUTRITION OF THE ROOT 



§ 4. What substances must be present in the soil for the 

 continuous and effective nutrition of plants ? 



In order to find out what substances are necessary for the 

 growth of cultivated plants, a number of chemical analyses 

 have for years been made of the most widely differing plants, 

 and it has been found that only a very small number of 

 chemical elements are present in all of them. From this 

 small number, too, a few must be excepted, which occur in 

 some individuals grown in special localities, and which are 

 therefore, as we might say, accidentally taken up by some 

 plants, while equally robust specimens of the same species 

 are devoid of them. The results of these analyses caused 

 physiologists to try to cultivate plants in media the composi- 

 tion of which was known, but which in themselves had no 

 influence on the plant. The choice lay between distilled water 

 or quartz sand which had been sterilised by heat. To these 

 media were added the substances which had always been 

 found in the ash of plants. These substances were added in 

 various forms, and one or other was occasionally omitted, so 

 as to find out which substances must always be present in 

 order that the plant may grow successfully. These water and 

 sand cultures resulted in showing that even some of the sub- 

 stances which were present in the ash of all cultivated plants 

 might be absent without lessening the growth of plants ; and 

 hence we may assume that they are only present in all our 

 cultivated plants because they are present in every soil, and 

 are therefore taken up by all roots. 



The knowledge thus obtained enables us to divide the 

 substances found in the ash of plants into three categories : 



