ASSIMILATION OF CARBON BY PLANTS 



189 



leaf is covered by an epidermis containing no chloroplasts and 

 along its outer wall by a cuticle that does not permit mass move- 

 ment of gases (Fig. 54). Carbon dioxide penetrates into the 

 leaf and reaches the absorbing cells mainly through open stomata. 

 The cuticle allows carbon dioxide to pass through it, but only in 

 solution in the cuticular substances. The stomata are numerous 

 small openings in the epidermis, whose structure is known from 

 anatomical studies of plants. That the carbon dioxide enters 

 the leaf mainly through the stomata may be shown by a simple 

 experiment. If on a certain portion of the leaf the stomata are 



CTpper Epidermis 



Palisade CeUs 



Cuticle 



Spongy 

 Ti$sw 



Lower Epidermis 



^Stoma 

 ^ Guard CeUs 



Fig. 54.^ — Cross section of a sunflower leaf (after Smith et al.). 



coated with vaseline, and the leaf is then exposed to light and 

 afterward treated with iodine, the blue color reaction will be 

 observed only in those portions where the stomata remained 

 open (Fig. 55). 



It is well known that the degree ^of opening of the stomata is 

 not always the same. In many plants, they are able to widen 

 and narrow their apertures or close them entirely. These 

 changes in the degree of opening have an influence on photo- 

 sjrnthesis. Therefore, environmental conditions that induce the 

 closing of the stomata, such as lack of moisture, also check 

 photosynthesis. During periods of drouth, plants may give 

 indications of starvation. They cease to develop and even lose 

 in dry weight. This explains the poor development of plants 

 in dry regions, as well as the low yields in years of drouth. 



