Minnesota Plant Life. 



423 



under a heavy branch the trunk of the tree is especially strength- 

 ened to bear the weight. And so, too, against the massive 

 roots of a tree, the trunk is often buttressed because it is against 

 these points that the pressure is exerted, when the tree top is 

 pushed laterally by the wind. Plants with comparatively slen- 

 der branch systems may mature large fruits if they let these 

 fruits lie upon the ground. This is the device adopted by the 

 gourd family, and the gigantic pumpkins and squashes which 

 are so often exhibited could scarcely be borne, except under 

 very exceptional conditions, upon plants that carried the whole 

 weight of such enormous bodies themselves. Some plants, it 

 is true, manage to suspend extremely heavy fruit areas. Thus, 

 the banana forms a bunch of fruits weighing in the aggregate 

 some scores of pounds. But it is borne close to the main trunk 

 and is hung in such a way that the strain is not unbearable. 

 Cocoanuts, which form heavy fruits at a considerable height, 

 have them placed close to the center of the tree, not out near 

 the tips of the branches like those of the apple. 



The relation between the sizes of leaves and their exposure 

 to the winds has already been mentioned. It is not possible for 

 large, thin and delicate leaves to maintain themselves under 

 climatic conditions in which heavy winds are prevalent. If large 

 leaves occur in windy districts, their edges are strengthened by 

 special adaptations of the leaf network. Very beautiful ex- 

 amples of strengthening devices for the edges of leaves may be 

 seen in milkweeds, basswoods or catalpas. In the latter tree, 

 especially, the edges of the large, heart-shaped leaves are faced 

 by arch after arch of network, making the margin of the leaf 

 very strong against any lateral tearing agent. 



A consideration of the points which have been thus briefly 

 presented will show how reasonable it is that the longest 

 stemmed plants in the world should be oceanic, for such stems 

 have water on every side to support them. It will be equally 

 apparent how reasonable it is that the most massive plant struc- 

 tures and the strongest should be the trunks of trees on land, 

 for these, of all plant organs, have the greatest need of strength 

 if they are to perform their work and meet the forces to which 

 they are subjected. The difference of elasticity between differ- 

 ent plants growing under different conditions, or between two 



