DISCUSSION OF RESULTS. 45 



DISCUSSION OF RESULTS. 



The plants described are but a few of those investigated, but they 

 are representative in some measure of all. Their stomatal behavior 

 provides a basis for classifying the stomata of all the species into three 

 general groups, typified by barley, alfalfa, and potato. Naturally, 

 the stomatal movement in each species varies in some respects 

 from that in the type, as has been shown. Barley represents the 

 cereal type with stomata of peculiar construction and great sensitive- 

 ness, which show no opening at night, no matter how slight the day 

 opening. Alfalfa represents the group which, under normally favor- 

 able conditions, have open stomata during the day and closed stomata 

 at night, but as conditions become less favorable show increasing 

 night opening and extended mid-day closure. Potato belongs to the 

 group of plants that normally have open stomata at night under 

 favorable conditions, and close them only as water-content decreases 

 or evaporation becomes greater. Naturally, it is possible by an 

 unusual grouping of factors to vary the behavior in many plants to 

 such an extent that they will show almost any kind of movement. 

 For this reason it is unsafe to carry out experiments in the greenhouse 

 alone, as the factors present differ greatly in degree from those in the 

 field, and the stomatal movements of greenhouse plants, in conse- 

 quence, are distinctly different. Hence greenhouse experiments are of 

 value only in connection with similar ones made in the field. 



Barley has never been found in the course of the experiment to have 

 open stomata at night correlated with day closure. Many series 

 indicate that highly favorable conditions would probably cause 

 barley or corn to show stomatal behavior resembling that of wheat 

 in series 26, i. e., opening with the appearance of daylight, closing 

 gradually during the afternoon, and remaining closed all night. 

 Under conditions only slightly less favorable, the stomata of barley 

 have been found to close in the afternoon, and on a hot, dry day were 

 but partially open an hour or two after sunrise. In all cereals the 

 tendency seems to be to operate with many closed stomata at nearly 

 all times. Even under very favorable field conditions all the stomata 

 are wide open for only an hour or two. In the case of corn, sorghum, 

 and Sudan grass, very warm or even hot weather, brilliant sunshine, 

 and a high water-content seem to be the optimum conditions. For 

 wheat, oats, and barley, cool and rather humid weather and less 

 sunshine are best. This has a practical bearing on the spread of 

 wheat rust, as the parasite gains entrance to the leaves of the host 

 through the stomatal openings and in part explains how it can make 

 most headway during such weather. Millet occupies a position 

 between barley and corn in respect to optimum conditions. The 

 cereal type of stomatal behavior may be characterized as showing 



