92 THE LIFE OF THE PLANT 



not important for the nutrition of the plant ; but if 

 under exactly the same conditions we get a weaker 

 plant in the second case, we shall be justified in attribut- 

 ing the difference to the difference in the conditions, 

 i.e. to the absence of the eliminated substance. This 

 chapter in the physiology of the plant affords us a 

 series of good, simple illustrations of a strict and 

 repeated application of the second canon of inductive 

 reasoning. ' If an instance in which the phenomenon 

 under investigation occurs, and an instance in which it 

 does not occur, have every circumstance in common 

 save one, that one occurring only in the former, the 

 circumstance in which alone the two instances differ 

 is the cause, or an indispensable part of the cause, of 

 the phenomenon.' * 



Thus by eliminating one after another all the sub- 

 stances discovered in the plant and in the soil, we 

 discover which of them is absolutely indispensable as 

 food for the plant. Let us study the principal results of 

 these experiments. 



First of all our attention centres upon the organic 

 substances, the products of decay. Everyday experi- 

 ence proves that dark soils are more fertile than light 

 ones. It would seem, then, that the black humus 

 must form the main food of the plant. Yet exact 

 experiment tells quite a different tale. We may 

 thoroughly calcine black mould, burning away the whole 

 of the organic matter in it, and yet it remains a soil in 

 which a plant will normally develop. We can grow on 

 such a white soil a plant that could not be distinguished 

 from any grown on the best of moulds. Therefore, it is 

 not from the humus that the plant obtains its food. 

 It can grow without it. We have already seen that the 

 largest part of the mineral matter in the soil, the part 

 we have called the insoluble substratum, is useless as 

 food ; so the circle of substances which can be looked 



1 J. S. Mill, A System of Inductive Logic, etc., p. 256. 



