General Physiology of the Plant. 



195 



plant will form a subject of study when we consider the 

 dynamical aspect of plant physiology. 



It is worth pointing out here that this analysis of plants 

 yields us most important results when we consider their life- 

 histories from an agricultural point of view. The whole 

 question of manuring is founded on a knowledge of the 

 chemical composition of plants. By the rotation of crops, 

 the farmer is able to grow a plant on a soil impoverished by 

 the preceding crop of many substances which its successor, 

 however, does not require. Meantime, by rainfall and the 

 natural disintegration of the soil, the land is becoming re- 

 plenished with exactly those substances in which it has be- 

 come deficient, enabling it, therefore, to supply food-stuffs 

 for another crop of the former kind in a succeeding year. 

 A comparison of the chemical composition of a few typical 

 crops, as, for example, those in the subjoined table, will 

 make this fact clearer. 



1,000 PARTS OF DRY SOLID MATTER CONTAIN (PrantI) : 



We have seen, then, that the food of plants is solid, liquid, 

 and gaseous, and that it is obtained (in the case of green 

 plants) from the soil and the atmosphere. We have already 

 considered the composition of the atmosphere (p. 41) ; we 

 have now to glance for a moment at the composition of the 

 soil. 



-O 2 



