The Path of Gaseous Exchange. 63 



conclusion that "the interference of the density shells of small 

 holes set at 10 diameters or more apart is small, each hole beyond 

 this limit acting almost independently according to the diameter law." 



Now in the leaf of Helianthus uniiuns, for example, the 

 stomata on the under surface are actually about 8 diameters apart. 

 The stomata themselves open into cavities in which shells of 

 diffusion may form. The under surface of such a leaf is therefore 

 a multiperforate septum in which the perforations are so far apart 

 that practically each single opening can exercise its full efficiency 

 as regards diffusion through it, without interference from its 

 neighbours. We may, therefore, expect the diameter law to hold, 

 and the rate of diffusion of carbon dioxide through the stomata to 

 be proportional to the linear dimensions of the stomata. 



Assuming the stomata to be circular in shape instead of 

 elliptical as they actually are, Brown and Escombe have worked 

 out the quantity of carbon dioxide capable of diffusing into the 

 leaf under various conditions. Under the most favourable circum- 

 stances, when the stomata are wide open and the carbon dioxide 

 in the air outside the leaf is in constant motion so as to maintain 

 the greatest possible pressure of carbon dioxide there, we have the 

 following data : — 



Diameter of stoma, 0-00107 cm. 



Length of tube, 0-0014 cm. 



Number of stomata per sq. cm., 33,000. 



Area of cross section of stoma, 9-08 x 10.' sq. cm. 



Under these circumstances the theoretical value for the quantity 

 of carbon dioxide absorbed by the leaf is 2-578 c.c. per sq. cm. per 

 hour. 



If, on the other hand, the air outside the leaf is perfectly still 

 the maximum quantity of carbon dioxide entering the leaf is 2-095 c.c. 

 per sq. cm. per hour. 



These values are far higher than the observed quantities of 

 carbon dioxide taken in by the leaf. Thus Thoday (1910) found 

 a leaf of Helianthus annun.i was capable of increasing in dry 

 weight by about - 1 7 milligrams per hour per sq. cm. which corresponds 

 to an intake of carbon dioxideof only about 0-14 c.c. of carbon dioxide 

 per hour measured at normal temperature and pressure. 



Hence, the stomata, in spite of the relatively small area of the 

 whole leaf surface they occupy, could yet allow the diffusion 

 through them of many times as much carbon dioxide as actually 

 passes through. There is then, every reason to regard the results 

 of Blackman and Brown and Escombe as affording definite proof 

 that the path of intake of carbon dioxide into the assimilating 

 aerial leaf of higher plants is mainly through the stomata. 



