THE STEM AND BRANCHES. 165 



just in proportion as plants grow tall and spread- 

 ing, carry masses of foliage, and are exposed to 

 heavy winds, do they need to form a stout and 

 woody stem, w^hich shall support the constant 

 weight of the leaves, or even bear up under the 

 load of snow which may cover the boughs in win- 

 try weather. Thus, a tapering tree like the Scotch 

 fir requires a comparatively smaller stem than an 

 oak, because its branches do not spread far and 

 wide, while its single leaves are thin and needle- 

 like; whereas the oak, with its massive boughs 

 extending far and wide on every side, and cov- 

 ered with a weight of large and expanded absorb- 

 ent leaves, requires a peculiarly thick and but- 

 tressed stem to support its burden. Both in girth 

 and in texture it must differ widely from the loose 

 and swaying pine-tree. Every stem is thus a 

 piece of ingenious engineering architecture, adapt- 

 ed on the average to the exact weight it will have 

 to bear, and the exact strains of wind and weather 

 to which on the average it may count upon being 

 exposed in the course of its life-history. We see 

 the result of occasional failure of adaptation in 

 this respect after every great storm, when the 

 corn in the fields is beaten down by hail, or the 

 fir-trees in the forest are snapped off short like 

 straw by the force of the tempest. But the sur- 

 vivors in the long run are those which have suc- 

 ceeded best in resisting even such unusual stresses ; 

 and it is they that become the parents of after 

 generations, which of course inherit their powers 

 of resistance. 



Most stems, at least of perennial plants, and 

 all those of bushes, shrubs, and forest trees, are 

 strengthened for the purpose of resisting such 

 strains by means of a material which we call 



