ON THE SUCCESSION OF CROPS. 267 



the organs, or presenting them to the suckers of the 

 roots by which they are absorbed ; thus the progress of 

 vegetation tends constantly to impoverish the soil, and, 

 if the nutritive juices in it be not renewed, it w^ill at 

 length become perfectly barren. 



A soil well furnished with manure may support sev- 

 eral successive crops, but each one will be inferior to 

 the preceding, till the earth is completely exhausted. 



Principle 2. All plants do not exhaust the soil 

 equally. Plants are nourished by air, water, and the 

 juices contained in the soil ; but the different kinds of 

 plants do not require the same kinds of nourishment in 

 equal degrees. There are some that require to have 

 their roots constantly in water ; others are best suited 

 with dry soils ; and there are those, again, that prosper 

 only in the best and most ri<3hly manured land. 



TIt^ grains and the greater part of the grasses push 

 up long stalks, in which the fibrous principle predomi- 

 nates : these are garnished at the base by leaves, the 

 dry texture and small surface of which do not permit 

 them to absorb much either of air or water : the prin- 

 cipal nourishment is absorbed from the ground by their 

 roots : their stalks furnish little or no food for animals ; 

 iso that these plants exhaust the soil without sensibly 

 repairing the loss, either by their stalks, which are cut 

 to be applied to a particular use, or by their roots, which 

 are all that remain in the ground, and which are dried 

 and exhausted in completing, the process of fructifica- 

 tion. 



Those plants, on the contrary, that are provided with 

 large, fleshy, porous, green leaves, imbibe from the 

 atmosphere carbonic acid and water ; and receive from 

 the earth the other substances by which they are nour- 

 ished. If these are cut green, the loss of juices which 

 the soil has sustained by their growth is less sensibly 

 felt, as a part of it is compensated for by their roots. 

 Nearly all the plants that are cultivated for fodder are 

 of this kind. 



