36 TEXTBOOK OF PLANT PHYSIOLOGY 



This seeming paradox finds its explanation in the peculiarity 

 of the diffusion of gases through small openings. According to 

 Stephan's law, the rate of diffusion through openings of different 

 size is proportional to the area of these openings only when their 

 dimensions are very large. With very small openings, the rate of 

 diffusion is a function of their diameters. 



As the number of stomata in a leaf is very large, usually 

 several hundred per square millimeter, or several thousand per 

 square centimeter, the sum of their diameters, in spite of their 

 small size, is considerably greater than the diameter of the leaf. 

 Therefore, diffusion through the membrane perforated with 

 stomata proceeds almost at the same rate as if this membrane did 

 not exist, and the absorbing cells of the leaf were in direct contact 

 with the atmosphere. 



In order to give an idea of the number and dimensions of sto- 

 mata, the values obtained by Kiesselbach as a result of the inves- 

 tigation of eleven varieties of corn are presented : 



Number of stomata per square centimeter of 



lower epidermis 7,684 



Number of stomata per square centimeter of 



upper epidermis 9,300 



Total for both surfaces 16,984 



Total number of stomata per plant (average) 104,057,850 



Total leaf surface per plant (average) 6,100 sq. cm. 



Average dimensions of one stoma (in 



microns) 25.6X3.5 



Average area of stomatal opening (in square 



microns) 89 



Area occupied by stomatal openings (in 



percentage of total area of the leaf) 0.76 per cent 



Having passed through the stomata, the carbon dioxide enters 

 the intercellular spaces of the leaf and reaches the chlorophyll cells. 

 In the majority of plants, on the upper side of the leaf is situated 

 the palisade tissue, consisting of closely fitting cells which are 

 arranged perpendicular to the leaf surface. As these cells are 

 rich in chloroplasts, the palisade tissue must be regarded as being 

 primarily assimilating tissue. Closer to the lower epidermis lies 

 the spongy tissue with loosely arranged cells, forming large inter- 

 cellular spaces whose dimensions frequently surpass those of the 



