430 



Minnesota Plant Life. 



primrose, or in the mullein. In other plants the shapes of the 

 leaves are modified by their mutual relations to each other upon 

 the general stem tract. Thus the one-sidedness of elm or 

 hackberry leaves will be found to be dependent upon the rela- 

 tive positions of the leaves upon the twig. The side of the leaf 

 which is less protuberant is the one that is shaded by the leaf 

 above. The sizes of leaves vary considerably with their illumina- 

 tion. Thus the leaves of plants in shady places are generally 

 larger than those of plants growing in the sun. The trillium 

 and jack-in-the-pulpit leaves, for example, are considerably 



FIG. 214. Two-leafed wood-lilies. These plants have the broad leaves of shade plants and 

 the white, conspicuous flowers. After photograph by Hibbard. 



broader than the ordinary leaf of their class. Some grass leaves 

 belonging to species growing in the deep woods are decidedly 

 broader than ordinary, and the forest-dwelling asters and gold- 

 enrods are conspicuous for their broad leaves, quite different in 

 shape from the willow-like or linear leaves of the sun-loving 

 varieties. 



It is well known that light retards that actual increase in 

 size of plant organs which alone should be called growth. It 

 is well to distinguish between growth, meaning by this, increase 



