CH. II.] THE EOOT. ]17 



The rest han-ow or caanmock (Ononis arvensis and 

 Ononis spinosaj is also indicative of barrenness and 

 lightness of soil. I have gathered them on such 

 land where clay pits were in the immediate vicinity. 

 But where the \>voom.( Spa rtiumscopariunij abounds, 

 we may be certain that no clay will be found ^\-ithin 

 reach of its deeply penetrating roots. It thrives only 

 in hungiy deep gi'avels or sands. 



The common nettle ( Urtica urens) I have always 

 considered indicative of fertility, and this will be 

 found, I believe, to be the fact, except in accidental 

 cases. Thus was I once startled from such conclu- 

 sion by seeing immense beds of this plant gro-uing 

 upon that most sterile of sandy soils, the rabbit war- 

 rens near Brandon, in Norfolk. But the explanation 

 of this was easy upon remembering that the urine of 

 the rabbit contains an excess of the salts of potash, — 

 salts absolutely necessary for the nettle's A^gorous 

 growth, — salts found, also, near old brick walls, and 

 inducing the nettle to be so constant an attendant on 

 the dwellings of man in temperate latitudes^. 



Wherever the elm (Ulmus campestris) grows 

 * Vauquelin found in the urine of the rabbit. 



Carbonate of lime. 



magnesia, 

 potash. 



Sulphate of lime. 



Chloride of potassium. 



Urea. 



Mucus and sulphur. 



Sulphate of potash. 



Its milkiness arises from carbonate of lime, — (Tfiot/uon's 

 AniiKKiL Ckeiiiistry, 496.) 



