[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 



219 



Fig. 77. Five plants of Venus's fly-trap (Dionaea) two of which are in bloom, and 

 four plants of sundew (Dwsera). Photo by G. S. Growl. 



the convex side is stretched more than the wall on the concave side. 

 The convex wall is the more extensible probably because it is thinner. 



Increase in turgor of the guard cells is the result of the entrance of 

 water by osmosis. The energy back of the expansion of guard cells, there- 

 fore, may be traced through diffusion to the energy of molecular motion. 

 The concentration of the water in the guard cells is largely dependent 

 upon the transformation of starch to sugar and vice versa, which in turn 

 depends upon the acidity of these cells. In the guard cells that have been 

 studied, a slight decrease in acidity results in the digestion of starch to 



