174 VEGETABLE MATTER OF THE SOIL. 



without manure, yield luxuriant crops. The carbon 

 in such cases must also have been derived from the 

 air. De Saussure found that two beans, when veg- 

 etated in the open air on pounded flints, doubled 

 the weight of the carbon which they originally 

 possessed. 



(c) " Some plants grow when suspended in the 

 air without contact with the soil. 



(d) " When lands are impoverished, we lay them 

 down to grass, and the longer they lay undisturbed 

 the richer in vegetable matter does the soil become. 



" In certain extreme cases, as in those of plants 

 growing in soil perfectly void of organic matter, 

 this conclusion" (that they draw all their carbon 

 from the atmosphere) " must be absolutely true. 

 But is it as strictly true of the more usual forms of 

 vegetable life, or in the ordinary circumstances in 

 which plants grow spontaneously, or are cultivated 

 by the art of man 1 Has the vegetable matter of 

 the soil no connection with the growth of the tree ? 

 Does it yield them no regular supplies of nourish- 

 ment '? Is nature working in vain when preparing 

 all this vegetable mould in the soil] 



" The consideration of one or two facts will show 

 that our general conclusion must be modified. 

 Plants in certain circumstances will grow in a soil 

 containing no sensible quantity of organic matter ; 

 but it is also true, generallij, that they do not luxu- 

 riate, or readily ripen their seed in such a position. 



