g6 THE PHYSIOLOGY OF STOMATA. 



as the opening, so that by about 4 p. m. many stomata are closed and the 

 range of size is between o and 3 or 4 micra. I have attempted to represent 

 these facts graphically in diagram, fig. 32, which will serve to bring at once to 

 the mind the whole course of action of the stomata. On the ordinates have 

 been laid off the limits of size, and a curve is laid off so as to embrace, in 

 a general way, the average sizes. Such a curve is true within a reasonable 

 limit of error. Indeed, it would be of little profit to attempt a more exact 

 representation, inasmuch as two sets of material would vary from each other 

 to as great an extent as the error involved. 



The conclusion to be drawn from the facts summarized in this curve is 

 that the opening and closing of the stomata do not proceed rapidly upon the 

 advent of day and of night, respectively, but are on the contrary rather slow 

 processes. The evidences indicate that the opening, as we shall see elsewhere, 

 is inaugurated as a response to the stimulus of light during the early hours of 

 the day. The cause of the closure, which, it should be especially noticed, 

 occurs early in the afternoon, and, as I have shown, independently of wilting, 

 of rate of transpiration, and not on the advent of darkness, is less obvious, 

 but I believe is explicable on the theory of processes within the cell of the 

 nature of enzymatic activity. Inasmuch as this general question must be 

 treated beyond at greater length, it will be unnecessary to speak of it further 

 at this point. 



The only attempt which, so far as I am aware, has been made to represent 

 the behavior of stomata in graphic form is that recorded in Francis Darwin's 

 paper (1898, p. 595). A comparison of his curves with that here given dis- 

 covers certain differences, which may indeed be due to the different plants 

 used by him, and the widely different conditions of experimentation. Dar- 

 win's curves appear to me, however, to resemble transpiration curves much 

 more closely. 



The rise of the curve in the morning and its fall in the evening are "fairly 

 steep," indicating a correspondingly rapid opening and closure of the stomata; 

 that is, within the space of an hour or a trifle over. This conclusion is based 

 on the readings of the hair hygroscope of Darwin's contrivance, a device 

 which undoubtedly responds to changes in transpiration, but does not at all 

 prove that the movements of the stomata are responsible for the changes in 

 the rate of transpiration. My own experiments compel me to question the 

 conclusion that these rather sudden changes, caused by temporary meteoric 

 changes, are the expression of stomatal action. Rather would I hold that no 

 stomatal movements take place at all, or at any rate, if changes occur, 

 they are far too small to be recorded. I show elsewhere that wide variations 

 in the transpiration of a plant may occur without any observable stomatal 

 movement, a result which makes it unsafe to infer stomatal action from the 

 rate of transpiration. 



