196 Ehmcntary Biology. 



Composition of the soil, The composition of the soil 

 is as varied as that of the rocks by the disintegration of 

 which it is in the main produced. The animals and plants 

 living on it, however, add a considerable quantity of matter 

 to the soil by their excrement while living, as well as by the 

 products arising from the decomposition of their dead 

 bodies. The character of the soil is dependent in the first 

 instance on the nature of the subsoil and rock beneath, i.e. 

 on the nature of the geological formation of the district in 

 question. The character of the vegetation, as a rule, alters 

 very considerably according as the soil is clayey, loamy, 

 sandy, gravelly, chalky, or peaty. Not only SD, but the 

 presence or absence of various constituents determines 

 whether the soil will be wet or dry, cold or warm, rich or 

 poor. Particular crops are adapted to some soils rather 

 than to others a fact very familiar to all agriculturists. In- 

 deed, the occurrence and distribution of "many wild plants 

 within their area of habitat depends entirely on the presence 

 or absence of the suitable soil and the requisite amount of 

 moisture. 



General physiology of the plant It has already been 

 pointed out in the early chapters of this book how necessary 

 it is that the great law of the conservation of energy should 

 be continually borne in mind when the physiology of pro- 

 toplasm is being discussed, and it will be found most advis- 

 able if we adopt this law for our guide in discussing the 

 physiology of the plant in general. Protoplasm is a highly 

 complex store of potential energy, in virtue of the possession 

 of which the plant is able to perform certain duties, related 

 to the maintenance, partly of tribal, partly of individual life. 

 Without protoplasm, so far as we know, no protoplasm is 

 formed. Since, then, this store must be built up ere it can 

 be broken down and used, it follows that we must discuss 

 the physiology of the plants first from the anabolic, and 

 secondly from the katabolic point of view. (Compare sec. ii. 

 Chap. L, sees. ii. and vi. Chap. II., and sees. i. andii. Chap. 



