CHANGES IN CONTENTS OF STOMATA. 97 



According to the same data, Darwin concludes (loc. cit., p. 594) that 



The maximum opening of the stomata is not reached till the day is well advanced, i. e., 

 at times between 1 1 a. m. and 3 p. m. But the stomata reach a fair aperture, i. e., two- 

 thirds of the maximum reading, at from half an hour to two and a half hours after sunrise. 

 The time of closure is usually from three-quarters of an hour to one hour after sunset, but 

 Narcissus was shut .... more than half an hour before sunset, and .... 

 Cineraria was shut ten minutes after sunset. 



Aside from minor details, my own conclusions differ from those of Darwin 

 in ( i ) the maintenance of the maximum opening for a rather definite period ; 

 (2) the early closure, 2 to 3 hours before sunset ; and (3) the slower movement, 

 aside from that of the mid-day period in Darwin's plants, whether opening or 

 closing. To be sure, the maximum opening "is not reached till the day is 

 well advanced" according to Darwin, which maximum is, however, not main- 

 tained. In making the comparison, however, if we regard the "fair aperture " 

 in Darwin's curves as the maximum, then the chief facts are the rather rapid 

 opening and closing of the stomata and the marked period of maximum 

 aperture. 



Table 47 contains typical data for Verbena ciliata, giving the range and 

 average size of the stomatal apertures. The measurements, in micra, were 

 made in connection with a study of the transpiration, the results of which 

 are given elsewhere (experiment 140, p. 40). 



TABLE 47. Measurements of stomatal pores, Verbena ciliata, 1905. (Fig. 32.) 



OIL. 



The variation in the amount of starch in the guard-cells has its comple- 

 ment in the variation in the oil content. The occurrence of oil in stomata 

 has been several times noted, in Aloe (Strasburger, Practicum, pp. 167, 168), 

 in Galtonia (Leitgeb), and I have myself found them in Buxus, Agave, lilac, 

 privet (apparently not noted by Pirotta), and in another species of Verbena.* 

 This substance is undoubtedly of quite common occurrence in stomata. In 



* Occurs also in the chlorenchyma cells (as, too, in the pomegranate) and in some other 

 plants in the epidermal cells. 



