ASSIMILATION OF CARBON BY PLANTS 191 



membrane did not exist and the absorbing cells of the leaf were 

 in direct contact with the atmosphere. 



Willstatter showed by experiment that in a saturated atmos- 

 phere when the cells of the leaf mesophyll are protected from 

 drying, photosynthesis proceeds with the same velocity both in 

 uninjured leaves and in leaves with the lower epidermis removed. 

 Thus the epidermis with open stomata does not check in the 

 least the penetration of carbon dioxide into the leaf. Gas 

 diffusion through the stomata, as well as the movements of the 

 stomatal apparatus that regulates the communication of the 

 intercellular spaces with the surrounding atmosphere, will be 

 discussed in the chapter devoted to the water balance of plants. 



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

 stomata, the values obtained by Kiesselbach as a result of the 

 investigation of 1 1 varieties of corn are presented in the following 

 table. 



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 (approxi- 

 mate) 104.057,850 



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



Average dimensions of one stoma, microns 25 . 6 X 3 . 5 



Average area of stomatal openings, square 



microns 89 



Area occupied by stomatal openings (in per- 

 centage of total area of the leaf) . 76 per cent 



Having passed through the stomata, the carbon dioxide enters 

 the intercellular spaces of the leaf and diffuses through the walls 

 of the chlorophyll-bearing cells. In the majority of plants, on 

 the upper side of the leaf is situated the palisade tissue, consisting 

 of closely fitting cells that are arranged perpendicular to the leaf 

 surface. As these cells are rich in chloroplasts, the palisade 

 tissue must be regarded as being the primary assimilating tissue. 

 Closer to the lower epidermis lies the spongy parenchyma with 

 loosely arranged cells, forming large intercellular spaces whose 

 dimensions frequently surpass those of the cells. These cells 

 contain fewer chloroplasts and are less active in assimilation. 

 The spongy tissue is regarded as being especially fit for 



