126 



INTAKE AND DISTRIBUTION OF GASES 



ings in a membrane than through larger openings; and by- 

 having the minute openings frequent enough it is possible for it 

 to pass through them as rapidly as if no membrane whatever 

 were interposed. Physiological experiments show further that 

 when the stomata are closed carbon dioxide does not enter the 

 leaf rapidly enough for food-making. (See Chapter IX.) 



A better conception of the frequency of the stomata will be 

 gained by a comparison of a and b, Fig. 64. a, which is a square 



of 5 mm. on the side, contains 100 dots. 



jiiiHijH Imagine this figure reduced to the size of b, 



while retaining the same number of dots, and 



it will be conceived how small and numerous 



Fig. 64.--Diagram ^j^^ stomata must be when there are 100 of 



indicating the t r e- 



quency of the stomata. them in a square millimeter. Then conceive 



a, a square of s mm. on ,- i • i i 



the side containing 100 ot 300 or cven 700 oi them in the same area! 

 dots; these would have jj^ ^.j^^g ^^^^y q^^j. '^gg^ q£ ^^ye sizc and frequency 



to be crowded into b, ^ '■ -^ 



to approximate the of the stomata bccomcs somewhat definite. 



average frequencv of rr-ii i_-r r^i ^ j. • j. 11 



stomata in leaves." The chicf usc of the stomata IS to allow 



carbon dioxide gas to enter by diffusion from 

 its very dilute solution in the atmosphere. Oxygen also enters 

 through the stomata, but so far as this gas is concerned they 

 seem to be unnecessary, for respiration in which oxygen is 

 employed seems to go on perfectly well when the stomata are 

 closed or artificial^ plugged with vaseline, etc. Respiration can 

 still take place under such circumstances because there are 20 

 parts of oxygen in 100 parts of atmosphere, or about 500 times 

 as much oxygen as carbon dioxide. 



Assuming that the chief function of the stomata is to admit 

 carbon dioxide for food-manufacture, it is evident that the 

 conditions which most favor food construction should also induce 

 the opening of the stomata; and this is found to be the case, for 

 when the cells are turgid and the sun is shining — two conditions 

 essential to the manufacture of food by the chloroplasts in the 

 leaves — the stomata stand open, while in darkness and when 

 turgidity is low in the leaves, they close. Their behavior under 

 these conditions admits of a simple physical explanation: The 



