SUCCESSION OF CROPS. 123 



rinac«ous seeds ; and the agriculturist cannot be at too 

 much pains to free his grounds from weeds of that nature^ 

 which so readily impoverish them^ especially from the 

 wild mustard, sinapis arvensis., with which cultivated fields 

 are so often covered. 



Principle 3. Plants of different kinds do not exhaust 

 a soil in the same manner, 



^ The roots of plants of the same genus or family, grow 

 in the soil in the same manner ; they penetrate to a simi- 

 lar depth, and extend to corresponding distances, and ex- 

 haust all that portion of the soil with which they come in 

 contact. 



Those roots which lie nearest the surface, are more di- 

 vided than those that -penetrate deeply. The spindle or 

 tap roots, and all those that penetrate deeply into the earth, 

 throw out but few radicles near the surface, and conse- 

 quently the plant is supplied with nourishment from the 

 layers of soil in contact with the lower part of the root. 

 Of the truth of this I have often had proof, and I will 

 mention an example. If, when a beet or turnip is trans- 

 planted, the lower portion of the spindle be cut off, it wiH 

 not grow in length, but in order to obtain its supplies of 

 nourishment from the soil, it will send out radicles from 

 its sides, which will enable it to obtain the necessary sup- 

 plies from the upper layers of the soU ; and the root wiH 

 become roundish instead of long. 



Plants exhaust only that portion of the soil which comes 

 in contact with their roots ; and a spindle root may be able 

 to draw an abundance of nourishment from land, the sur- 

 face of which has been exhausted by short or creeping 

 roots. 



The roots of plants of the same and of analogous spe- 

 cies always take a like direction, if situated in a soil 

 which allows them a free developement ; and thus they pass 

 through, and are supported by, the same layers of earth. 

 For this reason we seldom find trees prosper that take the, 

 place of others of the same species ; unless a suitable pe- 

 riod has been allowed for producing the decomposition of 

 the roots of the first, and thus supplying the earth with 

 iresh manure. 



To prove that different kinds of plants do not exhaust 

 the soil in the same manner, it is perhaps sufficient for me 

 to state, that the nutrition of vegetables is not a process 

 altogether mechanical; that plants do not absorb indis- 



