STOMATA. 



71 



tions hereafter to be described, the stoma being in fact a deli- 

 cately balanced valve. A vertical section shows that the outer 

 part of the opening is wider than the narrow passage farther 

 down, and that the space below this widens somewhat towards 

 the intercellular cavity. 1 



1 The following table, compiled from figures given by Weiss, gives the num- 

 ber of stomata on the upper and under sides of the leaves of various plants 

 for the most part readily procurable by students. To show the wide differences 

 in size, the longer and shorter diameters have been added, and, finally, the frac- 

 tion of a square millimeter covered by a single stoma. 



