90 THE LIFE OF THE PLANT 



divided into three groups according to their degree of 

 solubility. One part of them, the smallest, dissolves 

 in water ; the second, larger part does not dissolve in 

 water, but dissolves in acids ; and, lastly, the third and 

 by far the largest part does not dissolve either in water 

 or in acids. These three degrees of solubility correspond 

 roughly to the three degrees of accessibility of those 

 substances to the plant. The substances of the first 

 group which dissolve in the water of the soil are appar- 

 ently easily accessible to the plant ; the substances of 

 the second group are less accessible ; while the sub- 

 stances of the last group are totally inaccessible to the 

 plant, unless in the course of many years they are partly 

 transformed into substances belonging to one of the 

 first two groups. 



Thus at any given moment we find among the mineral 

 constituents of the soil, first, a basis, useless at present, 

 but containing stores of food which will be available in 

 the remote future ; next, stores of food within com- 

 paratively easy reach of the plant ; and, lastly, a very 

 small quantity of matter which serves the plant for 

 immediate use. The truth of this statement can be 

 easily proved. We have only to take the most fertile 

 soil, to calcine it, and to treat it with an acid, in order 

 to get an almost white residue which will prove totally 

 sterile. 



It follows that we may look upon the greater part of 

 the soil at any given time as a substratum which merely 

 serves to hold plants firmly, without taking any im- 

 mediate part in their nutrition. The food of the plant 

 must be sought in the remaining component parts of the 

 soil, i.e. in the humus, and in those parts which dissolve 

 in water and acids. Let us see how the twelve elements 

 discovered in the plant are distributed in these parts. 

 The organic humus contains four elements : carbon, 

 hydrogen, nitrogen, and oxygen. Substances soluble 

 in water and acids consist of salts containing all the 



