180 THE 'STORY OF THE PLANTS. 



the oak, with its massive boughs extending fax* 

 and wide on every side, and covered with a 

 weight of large and expanded absorbent leaves, 

 requires a peculiarly thick and buttressed 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, adapted 

 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 survivors in the long run are 

 those which have succeeded 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 

 wood. And what is wood ? Well, it is an 

 extremely hard and close-grained tissue, manu- 

 factured by the plant out of its ordinary cells 

 by a deposit on their walls of thickening matter. 

 This process of thickening goes on in each cell 

 until the hollow of the centre is almost entirely 

 filled up by the thickening material, leaving only 

 a small vacant space in the very middle. The 



