84 UPWAED GEOWTH OF THE HEAD, AND pt. ii. 



ing outward of their bark from all sides, like the mast 

 How light of a ship. But light, by increasing the giving off of 



acts on this x o ./ o cj o . 



turgesence. water (transpiration), decreases swelling ; and when it' 

 falls unequally on plants, the forces on the illumined 

 side will be weakened, and the shoot, according to the 

 formation of its cellular tissue, will be turned either 

 towards or from the light. Thus ivy, or the tendrils of 

 climbing plants, are turned from the light towards any 

 opaque body, while the generality of plants are turned 

 from the opaque body towards the light. The whole 

 of a plant kept in a room will grow sideways towards 

 the window. The same plant placed where the light 

 comes from above will immediately turn straight up. 

 But though this principle is not so striking out of 

 doors, it is perpetual and universal. 



As the light falls equally on the leader of a tree, it 

 goes up straight. In proportion as the leader intercepts 

 the hght from above, the branches grow towards the 

 light at the sides ; and if one outstrips its neighbours, 

 the light from above turns its end up. If the lower 

 tier of branches outgrows the tier above it, in turning 

 up, it will take its neighbour with it. That tier will 

 turn the tier above ; and so in succession all the 

 branches will grow upward. This growth is often 

 seen in beech-trees. 



It is this principle which prevents the boughs of a 

 tree from growing against one another. In proportion 

 to their opacity they grow one from the other. 



Notwithstanding the original downward vertical 



