NATURAL HISTORY OF THE FOREST. 145 



mixed forest depend primarily on two qualities in 

 combination> namely, its relative rapidity and per- 

 sistence of height growth, and its relative require- 

 ments for light, while the manner of seed production, 

 seed transportation, and character of seed are addi- 

 tional factors. 



In those natural forests which are composed 

 mainly or entirely of one species, a comparatively 

 rare occurrence, the presumption is that climatic 

 or soil conditions are such that other species do 

 not find them congenial, at least, not when they 

 must contend for root and air space. 



One, by a prolific production of seed, has an 

 advantage over another which produces seed only 

 every three or four years. The heavy nut of the 

 walnut, or the acorn or beechnut, needs squir- 

 rels, mice, birds, and water to extend its territory, 

 while the light-winged seeds of birch and poplar, 

 carried by the winds, make these trees almost 

 ubiquitous. The seed of the willow loses its power 

 of germination within a few hours or days ; hence 

 it is confined mainly to the borders of streams, 

 where favorable opportunities for sprouting exist. 

 The acacia and others of the leguminous tribe, 

 like the black locust, preserve their seed alive 

 for many years ; nay, the seed of the former will 

 often lie buried in the ground for years, until a 

 fire that destroys all other vegetation breaks their 

 hard seed coat and calls to life the dormant germ : 

 the cones of some pines remain closed, and release 



