STRUCTURE AND ACTION OF STOMATA 197 



formed by the splitting of epidermal cells, the halves of which 

 become guard cells with the power of movement in such manner 

 as to open or close the pores. The origin of the transpiratory 

 openings among the lower forms may not be explained in this 

 manner in all instances, however. The action of the guard cells 

 may be best shown by Fig. 99. These cells are attached to the 

 epidermal cells in such manner that the lower side away from the 

 surface of the leaf is free, and the wall nearest the pore is com- 

 paratively thin, while the surface wall, and the one parallel to it, is 

 very much thickened. Any increase of the turgidity of the guard 

 cell tends to force the suiface and inner walls farther apart, and to 

 diminish the convexity of the thin wall extending toward the center 

 of the opening. Decrease in turgidity causes the reverse action, 

 and the stoma is closed. This action is somewhat modified by 

 the behavior of the neighboring epidermal cells, which play a very 

 important part in controlling the transpiratory openings in some 

 species. The transpiratory openings of some of the lower forms 

 are permanently open and may not be controlled. Wilting of the 

 leaf, compression of the stem, dry atmosphere, electric stimula- 

 tion (strong), darkness, and strong mechanical stimulation cause 

 the closing of the pores of the stomata, while light and heat as 

 well as prolonged darkness cause them to open. 1 It is to be said 

 that the stomata do not close their pores so tightly that some 

 gaseous diffusion may not take place through the diminished 

 opening. Stomata are generally found on the lower (outer) sur- 

 face of leaves although they occur on both sides of many forms 

 and on the upper surfaces of floating aquatic leaves. The open- 

 ings are in the form of more or less narrow slits having the max- 

 imum measurements of .03 mm. in diameter, and an area of .0046 

 sq. mm., and a single leaf may be furnished with many millions 

 of these organs. 



271. Structure and Action of Stomata. Cut sections of the leaf 

 of Iris, or Amaryllis, Avena, and Caltha palustris and note the 

 structure of the stomata as seen in cross section and suiface view. 



1 Darwin, F. Observations on stomata. Phil. Trans. Roy. Soc., 190 : 531. 1898. 



