DAILY MOVEMENT IN CEREALS. 39 



nasturtium, and similar thin-leaved plants exhibited striking irregu- 

 larities in their stomatal movement. 



Under extreme conditions of temperature, evaporation, and usually 

 low water-content, the stomata of the onion, as well as those of 

 alfalfa, were open all night and closed during the day. On the other 

 hand, a very high water-content causes the stomata of onion to 

 remain continuously open throughout a 24-hour period, opening 

 somewhat wider upon sunrise and closing very slowly during the 

 night, but not to less than 75 per cent as a rule. Upon the appearance 

 of sufficient light at dawn such closure ceases, and the stomata again 

 open to maximum just after sunrise. However, even such a plant, 

 during a very hot and dry day, will show considerable midday closure, 

 this sometimes being complete. Hence, evaporation may cause the 

 stomata of onion to be closed during the day and open only at night, 

 more or less regardless of the water-content, but as this decreases 

 the effect of evaporation increases, and this reversal of normal 

 behavior in the stomata becomes the more usual occurrence. 



THE DAILY MOVEMENT IN CEREALS. 



Cereals are alike not only in the structure and mechanism of their 

 stomata, but in the behavior of these as well. Certain peculiarities 

 which distinguish them from other plants must be dependent upon 

 the unique structure of their stomata, but other characteristics found 

 in the behavior are perhaps due to differences in the plant as a whole. 

 The failure to show night opening must be largely a matter of 

 stomatal mechanism, but the rare occurrence of maximum opening of 

 all the stomata of a cereal is probably the result of peculiarities 

 in other parts of the plant. Moreover, the rarity of maximum 

 opening is perhaps a regional phenomenon, as the conditions under 

 which wide opening does occur show that it is probably common in a 

 more humid region, such as the northeastern States. 



The stomatal movement described is the average of that found in 

 the upper and lower surfaces. The difference between upper and 

 lower stomata was slight when it did occur and was clearly due to 

 the light intensity. Hence, when a leaf was blown about and alter- 

 nately illuminated on the two sides, as usually was the case, no 

 difference in the movement of stomata on the two surfaces could be 

 detected. The two sets of epiderm were collected from each plant 

 and the apertures were calculated from both surfaces to furnish 

 the graphs. In photographing, however, only the lower surface was 

 used, since this represented the apertures for both surfaces. 



A set of barley epiderm as well as of alfalfa was collected in series 

 1 (fig. 2). The stomata of barley were but 12 per cent open at 9 a. m., 

 when the series started, and were closing, while those of alfalfa were 

 opening rapidly. At 10 a. m. barley showed less than 3 per cent 



