270 



EVOLUTION OF PLANTS 



usually almost circular in outline. Stomata are de- 

 veloped upon the exposed surface whose outer cell- 

 walls are also cutinized, but are absent from below. 



Many submersed aquatics have the roots imperfectly 

 developed, serving merely as organs of attachment, or 

 they may be quite absent, as in the common bladder- 

 weed, Utricularia. As these plants absorb most of their 



food from the substances dis- 

 solved in the surrounding 

 water, the roots are much less 

 important than in plants whose 

 upper members are exposed to 

 the air. However, the devel- 

 opment in rootless forms of 

 special contrivances for pro- 

 curing nitrogenous food, such 

 as the traps of Utricularia 

 (Fig. 58), would indicate that 

 the roots, even of these sub- 

 mersed forms, are still of im- 

 portance in absorbing nitrog- 

 enous compounds from the 

 mud in which they are fast- 

 ened. Where plants float 

 upon the surface, like the 

 duckweed (Lemna), or Sal- 

 vinia, there may be either 

 true roots developed, or root- 

 like organs which replace 

 them. 



Most aquatic plants are entirely free from hairs or 

 scales, so that the surface is smooth. Exceptions to 



FIG. 59. A, two leaves of an 

 aquatic buttercup (Ranun- 

 culus Purshii) ; L, aerial 

 leaf; w, submersed leaf; 



B, twig of horse-chestnut, 

 showing a winter-bud pro- 

 tected by thick scales, sc ; 



C, plant of pine-sap (Mono- 

 tropa), a colorless sapro- 

 phyte with rudimentary 

 leaves, sc. 



