TRANSPIRATION AND STOMATAL MOVEMENT. 



33 



weighed, the weight being compared to that of the whole piece. The meas- 

 urements are presented in table 17. 



It will be seen that the largest areas here used, those of pores with dimen- 

 sions 6 by 19.5 and 9 by 16 micra, are fully as great as the area obtained for 

 the stomatal pore of Helianthus. The largest possible pore dimensions, which 

 do not exceed 10 by 20 micra, give us an area of 157.5 square micra, equal to 

 a circle of a diameter of 14.24 micra, quite in excess of the stoma of Helianthus. 



TABLE 17. Measurements of stomatal pores, and areas determined by the method of pro- 

 portional areas of weighing and by computation, together with the diameters of circles 

 of equal areas. 



The depth of the Fouquieria pore-tube is from 9 to 1 5 micra ; so it may be 

 concluded that, aside from slight differences possible in the form of the tube 

 (fig. 3), the diffusion capacity of the stomata of Fouquieria is certainly not 

 inferior to that of Helianthus stomata. How far the geometrical form of the 

 area may modify the calculation we can not say, but, as in the case of the pore- 

 tube, it may be assumed to be but little. Since the stomata of both plants 

 conform closely to a very common type of this apparatus, the main results 

 of the comparison will not be vitiated by the trifling differences which must 

 of course exist. 



By means of a potbmeter experiment, the maximum rate of transpiration 

 for a Fouquieria shoot 27 cm. long, having a leaf surface of 299 sq. cm., was 

 found to be 312 c.c. per square meter per hour,* a rate between one-sixth and 

 one-fifth the stomatal diffusion capacity of Helianthus and, on the basis of 

 the above comparison, approximately that of the Fouquieria leaf. 



*It should be noted that due caution has been exercised in obtaining this rate, allowance 

 having been made for the probable inaccuracy of the potometer method. The rate without 

 the correction was 390 c.c. per square meter per hour. By weighing (vide anf-e) a correction 

 for absorbtion of 10 per cent was determined to be necessary during the earlier hours of 

 the experimental period. In order to be well within the truth a correction equal to the 

 volume of the twig used has been applied. This correction proved to be about 20 per cent. 

 The average rate per hour for 24 hours was found to be about 90 c.c. per square meter 

 per hour, this being a high rate for a xerophyte. As has been shown, however, the leaves 

 under consideration are not markedly xerophytic. 



