INDUCED RHYTHM IN STOMATA. 75 



than by attributing it to induced rhythm. For reasons which I have insisted 

 upon above it would be improper to infer stomatal periodicity from the trans- 

 piration curve. It is, on the other hand, very difficult to draw conclusions 

 in these instances from measurements of the stomata, since the error in 

 counting and measuring may well be greater than the total movement which 

 it is the endeavor to measure. This, I think, is shown by the data in table 

 42, obtained by placing verbena cuttings in the dark room. 



The increase in size indicated in I and II, on the second morning in each 

 case, might be interpreted as indicating a slight opening. The differences 

 are not great, however, and must be accepted with caution, for the reason 

 stated above. The fact that a few stomata on July 12 (III) were rather wide 

 open, is not surprising, as occasional extremes are frequent. Disregarding 

 the exceptional stomata, the personal error and the variation between leaves 

 may be as great as the slight differences obtained, and thus we are forced to 

 discard the data as unconvincing. On the other hand, the almost complete 

 closure at the end of 12 hours (III) indicates that, if a tendency to rhythm is 

 present, it is almost, if not quite, inappreciable by our means of measurement. 

 Being dissatisfied with observations of this kind, I have attempted to get 

 evidence of the sort employed by Francis Darwin. 



Francis Darwin's method of examining stomata, by means of his hygro- 

 scope, after exposure to illumination and to darkness for the given period in 

 the morning and afternoon, was modified in the fixation of pieces of epidermis 

 at brief intervals during the observational periods and taking measurements. 

 My method of making preparations should be mentioned here. The piece of 

 epidermis to be examined, after sufficient fixation in absolute alcohol, was 

 mounted in the same medium, and observed with a Leitz No. 7 objective and 

 a micrometer eyepiece. The whole area of epidermis was rapidly surveyed 

 to find the largest and smallest stomatal openings and the measurements of 

 these taken. The average of these extremes was then applied, by eye, to a 

 large number of stomata, and by this means a close estimate was made of the 

 average size of the openings. If the measurements of the extremes are, say, 

 o and 8 micra, and the average opening found to be 4 micra, these extremes are 

 given. Not infrequently, however, one or two in a hundred or more stomata 

 are found to be, say, 12 micra, while the remainder range between, say, 4 

 and 8 micra. These few exceptions are disregarded in plotting the data, 

 though I have generally mentioned the extreme cases, putting the figures in 



parenthesis. 



EXPERIMENTS. 



The experiments were made with well-irrigated plants which showed no 

 signs of wilting and chiefly with Ampelopsis quinque folia, a "mesophytic" 

 vine which is readily grown for ornamental purposes at Tucson. Other 

 plants (Solatium sp., Iris sp.,) gave purely negative results. The transverse 



