THE ROOT 91 



elements found in the ash of the plant, together with 

 two other compounds containing nitrogen, namely 

 saltpetre, a salt of nitric acid, and ammonia, a com- 

 pound of nitrogen and hydrogen. Hence, the four 

 elements of organic matter, the elements of ash and the 

 two compounds of nitrogen, nitric acid and ammonia, 

 are the substances to which the analysis of the soil 

 points as the possible sources of food for the roots of the 

 plant. Let us see which of these possible sources will 

 prove to be the actual indispensable sources. In order 

 to find this out we must interrogate the plant itself, 

 by placing it under such circumstances as to oblige it 

 to give us an answer to the question. 



How otherwise, indeed, can we determine what 

 substances are really necessary for the nutrition of the 

 plant ? At first sight it would seem quite sufficient to 

 analyse the plant and determine which substances 

 enter into its composition, and to declare these the 

 necessary substances. But doubts at once arise as to 

 the soundness of such a conclusion. Many of the 

 substances found in the plant might clearly be, as it 

 were, luxuries ; their presence might even be accidental 

 and useless, following merely as the result of their 

 existence in the environment. We can only recognise as 

 necessary, substances in the absence of which the very 

 existence and development of the plant are impossible. 

 This can be determined only by means of an exact 

 experiment like the one which proved the sterility of the 

 insoluble mineral substratum of the soil. The essential 

 conditions for such an experiment are as follows : 

 we provide one plant with all the substances discovered 

 by analysis in the plant itself, or in the soil on which it 

 successfully grows ; another exactly similar plant beside 

 it we supply with these same substances, omitting one, 

 and observe the consequences. If we do not notice any 

 apparent difference in the development of the two 

 plants, we may conclude that the eliminated substance is 



