VEGETABLE EXCRETIONS. 45 



species, it does not therefore follow that they are incapa- 

 ble of supplying salutary nourishment to other kinds of 

 plants: thus, it has been observed that the Salicaria flou- 

 rishes particularly in the vicinity of the willow, and the Oro- 

 banche, or broom-rape, in that of hemp. This fact has also 

 been established experimentally by M. Macaire, who found 

 that the water in which certain plants had been kept was 

 noxious to other specimens of the same species, while, on 

 the other had, it produced a more luxuriant vegetation in 

 plants of a different kind. 



This fact is of great importance in the theory of agricul- 

 ture, since it perfectly explains the advantage derived from 

 a continued rotation of different crops in the same field, in in- 

 creasing the productiveness of the soil. It also gives a satis- 

 factory explanation of the curious phenomenon oi fairy rings, 

 as they are called, that is, of circles of dark green grass, oc- 

 curring in old pastures: these Dr. Wollaston has traced to the 

 growth of successive generations of certain fungi, or mush- 

 rooms spreading from a central point.* The soil, which has 

 once contributed to the support of these fungi, becomes ex- 

 hausted or deteriorated with respect to the future crops of 

 the same species, and the plants, therefore, cease to be pro- 

 duced on those spots: the second year's crop consequently 

 appears in the space of a small ring, surrounding the origi- 

 nal 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 direc- 

 tion, and occasions the circle of fungi continually to proceed 

 by annual enlargement from the centre outwards. An ap- 

 pearance of luxuriance of the grass follows as a natural con- 

 sequence; for the soil of an interior circle will always be 

 enriched and fertilized with respect to the culture of grass, 

 by the decayed roots of fungi of the preceding years 5 growth. 

 It often happens, indeed, during the growth of these fungi, 

 that they so completely absorb all nutriment from the soil 

 beneath, that the herbage is for a time totally destroyed, 



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



