47 



the tree will again become single-leadered. 

 Without pruning, a tree may become 

 round-headed from the merest accident ; 

 for instance, an insect, or a bird, or the 

 wind destroying the top bud. A tree on 

 the side of a steep hill may be seen to 

 grow from the opaque side above it, and, 

 after it has reached a certain height, to 

 curve back again. 



So far light would appear to be the 

 principal agent in directing the growth of 

 plants. Yet it is said that if beans are 

 planted in holes through the bottom of a 

 box filled with earth, the stems will grow 

 upward from the light into the earth, and 

 the roots downward towards the light into 

 the air ; and the plants will perish when 

 they cease to derive nutriment from their 

 seed-leaves or cotyledons. Nay, so deter- 

 mined is the downward tendency of the 

 root, that when I have inverted a newly- 

 grown bean in water, so as to have the 

 roots in the air, covered with a dark cup, 



