50 THE VITAL FUNCTIONS. 



tiveness of the soil.* It also gives a satisfactory 

 explanation of the curious phenomenon oi fairif 

 rings, as they are called ; that is, of circles of dark 

 green grass, occurring in old pastures : these Dr. 

 Wollaston has traced to the growth of successive 

 generations of certain /M7^^^, or mushrooms, spread- 

 ing from a central point-t The soil, which has 

 once contributed to the support of these fungi, be- 

 comes exhausted or deteriorated with respect to the 

 future crops of the same species, and the plants, 

 therefore, cease to be produced on those spots ; 

 the second year's crop consequently appears in the 

 space of a small ring, surrounding the original 

 centre of vegetation ; and in every succeeding year, 

 the deficiency of nutriment on one side necessarily 

 causes the new roots to extend themselves solely in 

 the opposite direction, and occasions the circle of 

 fungi continually to proceed by annual enlarge- 

 ment from the centre outwards. An appearance 



* There is also another reason why it is advantageous to observe 

 a certain rotation in crops ; namely, that different kinds of plants 

 differ considerably in their respective capacities to extract from the 

 soil, and from the atmosphere, some of the principal elements of 

 their nourishment ; and in particular carbon and nitrogen. This 

 fact is well established b^ Boussiiigault, who, by a series of careful 

 experiments, found that a crop of potatoes, raised from a given 

 extent of ground, had, in the course of their growth, obtained from 

 the atmosphere a quantity of carbon nearly live times greater than, 

 and a quantity of nitrogen twice as great as tliat which they had 

 derived from the soil and the manure which had been laid upon it. 

 On the other hand, wheat, and the Cerealia generally, derive by far 

 the largest proportion of their nourishment from the earth, and but 

 a comparatively small quantity from the atmosphere. Hence the 

 latter tend to impoverish the soil in a much greater degree than the 

 former. Ann. Sc. Nat. serie 2. xi. 41. 



t Phil. Trans, for 1807, p. 133. 



