IRRITABITITY 27 



ment, so widely different from that of the 

 shoot, it is to be said that roots have taken 

 on fewer functions, and have always been sur- 

 rounded by much more uniform conditions 

 than the shoot, and in consequence have met 

 the necessity of a much narrower range of 

 adaptive modifications. But, while the rapid- 

 ity of variation of outward conditions affect- 

 ing roots has been much less than that of the 

 shoot, yet the greater number of the factors 

 concerned and the inequalities of diffusion and 

 distribution of the nutritive substances are 

 much greater than those affecting the shoot. Nutritive 

 Water and food substances lie below the sur- ^^c*®^ ^ ^^^ 

 face of the substratum and the root has de- 

 veloped a highly marked form of geotropism, 

 which enables it to penetrate the soil. Water 

 and food substances are by no means so uni- 

 formly distributed as sunlight and carbon di- 

 oxide, however. While water exhibits a fairly 

 horizontal distribution in quantity, yet so far 

 as its actual availability is concerned, differ- 

 ences corresponding to the physical character- 

 istics of the soil are to be found. The vertical 

 distribution is modified in the same manner. 

 The mineral food substances present no sys- 

 tem or uniformity of distribution whatever. 

 As a matter of fact the masses of food sub- 

 stances may and do lie in all possible direc- 

 tions from the absorbent zone of the apical 



