79 



and development of the different parts of 

 plants, cannot be doubted, though we 

 are quite in the dark about it. And as 

 regards the peculiar inorganic matters 

 absorbed from the soil by particular plants, 

 while the land is bearing one sort of crop 

 it maybe lying fallow, and collecting them, 

 by disintegration, for another sort. 



I believe Sir Humphry Davy first re- 

 marked, on the assumption that the up- 

 ward and downward growth of plants is 

 vertical, that woods and crops growing on 

 the side of a hill would derive no greater 

 advantage from the additional space than 

 if they grew on the horizontal surface of 

 its base. But it must be recollected, that 

 as the plants on the side of a hill rise tier 

 above tier, with the same light and aeration 

 from above they have a greater side light 

 and aeration ; they are in fact placed head 

 above head, like people in a race stand, 

 where but for this arrangement the 

 spectators would have good opportunity for 



