TRANSPIRATION AND STOMATAL MOVEMENT. 35 



diameter of a circle of equal area would beg. 4. At n h i5 m a.m. the pores were 

 found to be smaller by not more than 2 micra in transverse measurement, 

 reducing the area to 49.61 square micra, equal to that of a circle with a diameter 

 of 7.8 micra. The length of the pore-tube was 12 micra. The ratio of these 

 two observed rates of transpiration is 1.02, while that of the diffusive capa- 

 cities of the stomata is 0.62 approximately. If the rate of transpiration 

 followed the diffusion capacity of the stomata, the change at approximately 

 the sizes given would be in the neighborhood of from unity at 9 h i5 m to 0.60 

 at 1 1 h 1 5 m a. m. That is, from the data as they stand, the change of trans- 

 piration rate should, it is obvious, have been decreased, and the calculation 

 shows that this decrease should have been of about 40 per cent. During the 

 period, however, on account of exposure to darkness, the rate had fallen in 

 one hour (9 h i5 m to io h i5 m ) in the ratio of 0.33 — twice as much as can be 

 accounted for by the ratio of stomatal diffusion at the beginning and end of 

 the 1 -hour period. 



There can of course, be no doubt at all that complete closure of stomata, 

 if this occurs, reduces transpiration to or near to the cuticular rate, so that 

 thick or peculiarly impregnated cuticles and plugged stomata, etc., such as 

 have been described by various authors, would act as a constant, more or 

 less effective, preventive of water loss. Our problem relates, however, to 

 stomatal movement, and, further, takes no account of anatomical elabora- 

 tions, as pits, partial diaphragms, canaliform passages, etc., such as may be 

 believed to influence the total amount of water loss, though perhaps not to 

 regulate it,* during successive intervals of time. Conservation depends upon 

 anatomical peculiarities, regulation upon physiological activity. 



It has been the purpose of this part of the study to determine if consider- 

 able differences in the rate of transpiration are or are not constantly accom- 

 panied by corresponding stomatal movements. When a change in the dimen- 

 sions of the stomata have been observed, the possible change in their 

 diffusion capacity may be known by the change in linear dimensions by 

 the use of a formula of Brown & Escombe's, by which the proper correction 

 for the length of stomatal tube is introduced. This, compared with the actual 

 change in transpiration rate, will give us the data desired. 



For the sake of convenience in estimating the relative diffusive capacity 

 of the stomata of Fouquieria splendens\ at different degrees of opening, I have 

 arranged a table of calculations based upon the formula of Brown &Escombe 

 above mentioned. In this table the longitudinal diameter {2d) has been 

 taken as a constant at about the average stomatal-pore length, namely, 15 



*The effect of such structures would be represented, in a mathematical expression of the 

 physical processes in which they are involved, as a constant, and this would exclude any 

 regulatory function. 



fl have depended upon the data below for Verbena ciliata, as they are approximately 

 true for this plant also. 



