>3° /ngvar Jorgensen and Walter Stiles. 
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 annuus , 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 Escoinbe 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 7 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 annuus was capable of increasing in dry 
weight by about *17 milligrams per hour per sq. cm. which corresponds 
to an intake of carbon dioxide of 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. 
