216 M. Macaire on Vegetable Physiology, 



the practice of fallowing. Apiece of fallow ground will, al- 

 most to a certainty, be covered with a crop of weeds. These 

 being plants of a different nature, do not unfit the soil, but pre- 

 pare it for a succession of the same crop as that which preceded 

 them. But science or experience has taught the enlightened 

 farmer to substitute useful plants in the room of weeds, and 

 thus to keep iiis ground in profitable activity. 



Various reasonings have been employed to account for the 

 necessity of this rotation. 1^^, That different plants absorb 

 different juices from the same soil, and that a piece of ground 

 exhausted by culture may still be rich for another class of ve- 

 getables. But it is known to physiologists, that plants absorb 

 all the soluble substances that the soil contains, whether inju- 

 rious to their growth or not. 9,d, That the roots of different 

 plants being of different lengths, extend into different layers 

 of the soil, and thus derive from it appropriate nourishment. 

 But the roots of all plants, at the period of germination, must 

 be in the same stratum, and, of course, be equally dependpnt 

 upon it ; and, besides, the culture of the farmer turns up and 

 mixes the various layers of the soil together, so as to render 

 them, in all probability, homogeneous. It is known also, that 

 plants of the same family^ such as clover (trefoil) and lucerne, 

 do not prosper in succession, although their roots are of very 

 different lengths. The true explanation of the necessity of ro- 

 tation appears to be founded on the fact stated by Brugmans, 

 and more fully exposed by De Candolle, that a portion of the 

 juices which are absorbed by the roots of plants, are, after the 

 salutiferous portions have been extracted by the vessels of the 

 plant, again thrown out by exudation from the roots, and de- 

 posited in the soil. It is probable the existence of this exuded 

 matter, which may be regarded, in some measure, as the excre- 

 ment of the preceding crop of vegetables, that proves injurious 

 to a succeeding vegetation. It has been compared to an attempt 

 to feed animals upon their own excrements. The particles 

 which have been deleterious to one tribe of plants cannot but 

 prove injurious to plants of the same kind, and probably to 

 those of some other species, while they may furnish nutriment 

 to another order of vegetables. 



The author endeavoured to subject these theoretic views to 



