44 



THE PHYSIOLOGY OF STOMATA. 



3 — 



2 



a.t 



r.t. ' 



and for maximum evaporation rate, will be the maximum. No more water- 

 vapor may then escape than does so under these conditions. Such fluctua- 

 tions in transpiration rate as may occur can never transgress the upper and 

 lower limits, and in this degree the stomata of the plant must affect the loss 

 of water. But this is far from attributing to stomata the intimate regulatory 



function, as is said to 

 obtain under a very 

 low relative humidity. 

 Repeated observations 

 have convinced me that 

 so long as a water-sup- 

 ply is available at the 

 roots of Verbena and 

 ocotillo, there is no such 

 adaptive change. If the 

 plant wilts from any 

 cause, the stomata, suf- 

 fering like any other 

 cell, though not quite 

 as rapidly, close, but 

 this is purely a passive 

 behavior. 



The low rate of trans- 

 piration induced by a 

 high relative humidity 

 may occur in any state, 

 open or nearly closed, of 

 the stomata. In a state 

 9ajn. 12 3p.m. 6 9 4a.m. 7 of nature, and so long 



Fig. 12. — Curves of absolute (a. /.) and relative transpiration (r. t.) of aS the plant is normal, 

 two plants of ^»a a/»a to for 24 hours. A curve of evaporation k . probab l e that Sel- 

 rate (e) is also plotted. (From data supplied by B. E. Livingston.) r 



dom, if ever, are the 

 stomata used to their full capacity. This is shown by the readily observed 

 fluctuations in the rate of transpiration which may be induced, accompanied 

 by little or no change in the form of the stomata. So long as these fluctu- 

 ations in transpiration rate may occur, the regulatory effect of the stomata 

 must be small, and this in inverse ratio to the amplitude of these fluctuations. 

 Curves of transpiration constructed from data taken at short intervals always 

 show such positive and negative changes in successive periods. Some investi- 

 gators have, indeed, referred them to stomatal activity (e. g., Curtis, 1895), 

 but without evidence. The low rates of nocturnal transpiration are, it is 

 highly probable, not due to stomatal closure, since very narrow openings have 



3 — 



e 



i.t. 



r.t. ~- 



