I. THE DAILY MARCH OF STOMATAL MOVEMENT. 



These experiments were originally undertaken to determine when 

 the stomata of the more important crop-plants of the Great Salt 

 Lake region were open. Experiment had shown that usually five 

 times the S0 2 concentration required to injure pHnts during the day 

 was necessary to produce a like degree of injury at night. This was 

 explained as a result of the condition of the stomata, but exact in- 

 formation was needed as to when stomata opened and closed during 

 a 24-hour period. This was especially desirable, as prevention of 

 smelter injury to vegetation was at that time based upon the "sea 

 captain" plan of smelter operation, by which the manager kept in- 

 formed of the approach of unfavorable conditions when plants were 

 especially susceptible to S0 2 injury, and reduced operations within 

 the smelter accordingly. 



At the outset it was thought probable that for each species the 

 daily march of stomatal movement was the same from day to day 

 as long as conditions were not extreme. Lloyd (1908 : 108) states 

 that in Fouquiera "the stomata open and close rather slowly and 

 maintain a maximum opening for about 3 hours from 9 a. m. to 12 

 day. This maximum can not be said to be strictly constant, but the 

 differences are slight and within the personal error of observation. 

 Stomatal closure occurs in the early afternoon, advancing steadily 

 until nightfall. It is difficult to correlate this with changes in the 

 surrounding media." Nevertheless, it was felt that such similarity 

 of behavior on days of unlike weather conditions required demon- 

 stration for each species concerned. Largely for this reason, the 

 same plants were used in successive series on days of different weather 

 conditions. By this means it was soon found that similarity of 

 behavior was by no means the rule, and that stomatal movement 

 differed in the same species from day to day in accordance with the 

 physical conditions. 



THE DAILY MOVEMENT IN ALFALFA. 



The first series, consisting of strips of epiderm collected hourly, 

 was begun at 9 a. m. May 8, 1916, and finished at 5 p. m. the same 

 day. Light, temperature, and humidity readings were made during 

 this period, as well as in all the other series. The curve for stomatal 

 movement in the upper epiderm of alfalfa follows that of sunlight, 

 except for the sharp dip commencing at noon. The curve for sun- 

 light shows that the day was cloudless and totally free from haze. 

 At 9 a. m. both were at 60 per cent of maximum; at 10 a. m. they 

 reached 90 per cent; at 11 a. m. they were at 99 per cent; and at 12 

 noon both reached maximum. Sunlight continued at maximum for 



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