LUTHER BURBANK 



ences of warmth and moisture being supplanted 

 by the chill and drought that presaged the onset 

 of perpetual winter, a premium was put on the 

 conservation of plant energies. Whereas before 

 the elements favored the tree that could raise its 

 head highest and thrust out the most luxuriant 

 growth of spreading leaves to absorb the carbon 

 from the heavily laden atmosphere, the time now 

 came when the tree that had a smaller system of 

 branches to nourish and a less expansive leaf 

 system had better chance of maintaining existence. 



So in the lapse of ages, the conditions becoming 

 more and more hard, the trees that varied in the 

 direction of smaller size and narrower leaves had 

 an ever-increasing advantage. These survived 

 where their more rank-growing and luxuriant- 

 leaved fellows perished. 



Thus generation after generation natural selec- 

 tion operated to modify the size of the trees and 

 to develop a race of trees with narrow leaves, 

 which ultimately were reduced to the form of 

 needles. 



Such leaves, offering the largest possible sur- 

 face in proportion to their bulk, could gain nour- 

 ishment from an impoverished atmosphere, and 

 at the same time would obstruct the rays of the 

 sun but little, so that the entire foliage of the tree 

 might secure a share of the all-essential light 



[284] 



