32 THE PHYSIOLOGY OF STOMATA. 



Diaphragms perforated at regular intervals with holes 0.38 mm. in diameter may be so 

 arranged as to produce but little obstructive influence on the diffusive flow of a gas when the 

 total area of the apertures amounts only to about 10 per cent of the area of the septum, and 

 that nearly 40 per cent of the full diffusive flow may be maintained when the number of the 

 apertures is so far reduced as to represent an area of only 1.26 per cent of the full area of 

 the septum. 



The structure of a typical herbaceous leaf illustrates in a striking manner all the physical 

 properties of a multiperforate septum.* Regarded from this point of view, it is shown that 

 the stomatic openings and their adjuncts constitute even a more perfect piece 0} mechanism 

 than is required [italicizing mine] for the supply of carbon dioxid for the physiological needs 

 of the plant, and instead of expressing surprise at the comparatively large amount of the 

 gas which an assimilating leaf can take in from the air, we must in future wonder that the 

 intake is not greater than it actually is. 



The large amounts of water-vapor which pass out of the leaf by transpiration are well 

 within the limits of diffusion. 



The authors, assuming circular openings! m the application of their mathe- 

 matical deductions, stated that the maximum observed rate of transpiration 

 for a plant of Helianthus was one-sixth of the possible rate or "mAy c. c. 

 per square meter per hour, in view of the number of stomata and their 

 distribution. 



It will be of interest to compare the leaves of Helianthus and of Fouquieria 

 splendensvfith respect to the physical conditions which are related to stomatal 

 diffusive capacity. J Brown & Kscombe found that in Helianthus there are 

 33,000 stomata per square centimeter of leaf surface. The area of the pore 

 was calculated to be 0.0000908 square millimeter, which is equal to the area 

 of a circle of 0.0107 millimeter diameter. The depth of the pore is 0.014 

 millimeter. In Fouquieria splendens there are j$ (=320) per square mil- 

 limeter, or 32,000 per square centimeter of leaf. The depth of the stomatal 

 pore is 9 to 15 micra. Their areas in various conditions were found by 

 making careful drawings to scale of openings of different dimensions on stand- 

 ard ruled paper. The figures so obtained (figs. 2 and 5) were cut out and 



*In their longer paper (IQCXD a) Brown & Escombe say that the perforations, if 8 or 9 

 diameters apart, do not interfere with each other, in which case they act as separate tubes; 

 and this condition is approximated in the leaf. 



tOf areas equal to those of the elliptical stomata, since the evaporation from an elliptical 

 surface is equal to that from a circular surface of the same area (Stefan) . The area of a stoma 



was taken to be l^M 11 ! x breadth X * (= 1 . 6-) as sufficiently close. Since, however, the 



openings are not true ellipses (fig. 2) and are sometimes (Verbena*) quite aberrant (fig. 5), 

 the area may perhaps better be deduced empirically, at least in many cases. 



J This comparison does not extend to the form of the intercellular spaces, which are 

 narrower, it is quite probable, in Fouquieria. It will be evident, however, that this does 

 not vitiate the comparison, since, in spite of the small intercellular spaces, the maximum 

 transpiration rate is fully as great as in Helianthus. The probable greater thickness of the 

 leaves of Fouquieria may tend to offset the shorter transverse dimensions of the intercellular 

 spaces by their greater extent. (Bergen, 1904.) 



