LIGHT. 53 



mata were 70 per cent open at 8 a. m., 90 per cent at 9 a. m., and at 

 maximum the following hour. At noon they were still wide open, 

 but closed to 20 per cent at 2 p. m., remaining at this opening 1 hour. 

 Then they opened to 90 per cent at 5 p. m., when the series closed 

 (fig. 28). The starch index resembles that of poplar series 6, no 

 change having occurred because of mid-day closure. The amount 

 of starch in the guard-cells was decreasing rapidly at the start of 

 the series and reached a minimum at 10 a. m. No change was 

 observable the next hour, but after this the amount increased uni- 

 formly and gradually to the end of the series. Starch was on the 

 increase while the stomata were closing, but hardly fast enough to 

 produce as great closure as found. It was still increasing at the 

 same rate when the stomata reopened, contrary to expectations. It 

 is evident that reduced concentration of the surrounding cells, or 

 increased density in the guard-cells due to some other factor, caused 

 this behavior. 



In some series there was at times a certain relationship between 

 changes in the starch index and mid-day closure, while at other times 

 no relation existed. Thus, in alfalfa during series 10, the stomata 

 were opening rapidly at the start and the amount of starch was 

 decreasing almost as rapidly. The observable minimum in the starch 

 index was reached at 7 a. m. and persisted until 10 a. m. In the 

 meantime, the stomata opened to the maximum at 6 a. m., closed 

 to 40 per cent at 8 a. m., and reopened to 60 per cent at 9 a. m. 

 changes in starch-content showing no relation to this. After 10 a. m. 

 the stomata closed gradually, and completely at 2 p. m. This is 

 correlated with a similar slow rise in the starch-content of the guard- 

 cells. They opened to 25 per cent at 3 p. m., accompanied by a 

 slight drop in the amount of starch, and then closed to 10 per cent 

 at 6 p. m., without change of starch-content. They remained sta- 

 tionary for 2 hours, while the starch index increased perceptibly. It 

 was still increasing, though more slowly, when night opening started, 

 but between 11 a. m. and 1 p. m., when the stomata showed the 

 greatest increase, it remained stationary. Between 1 and 3 a. m., 

 when they closed again, the starch index increased rapidly once more, 

 and decreased very much the next hour, when morning opening 

 started (fig. 29). Hence, at times there existed a good correlation 

 between the changes in starch-content and stomatal movement and 

 at other times none whatever. It seems clear that the entire subject 

 is more complex than either Iljin or Lloyd has supposed, and must 

 be the subject of renewed investigation. 



The time required to produce response to light of various inten- 

 sities was first studied at Salt Lake City in June and July 1916. 

 In order to determine the reduction necessary to induce closure of 

 the stomata, 4 control cabinets, used in the sulphur-dioxide experi- 



