54 



EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



ments, were set up on as many plots. These cabinets were 6 feet 

 square, 4 feet high, and consisted of a light wooden framework 

 covered with sheets of celluloid. A constant current of air was 

 driven through a 6-inch sheet-iron pipe from an electric blower to the 

 top of the cabinet, and given a whirling motion by stationary radial 

 vanes at the vent of the pipe (plate 10). The air escaped from the 

 cabinet through an adjustable slot 3 inches wide around the bottom. 

 The first cabinet was used unchanged, the second shaded by a large 

 muslin-covered screen, the third covered with muslin tacked on above 

 the celluloid, and the fourth with oil fabric or imitation leather, the 

 black side turned in. Of the light incident, 60 per cent entered the first 

 cabinet, 8.5 per cent the shaded cabinet, and only 2 per cent entered 

 the muslin-covered one. The fourth cabinet showed only the faintest 

 traces of light. 



IOO 



20 

 10 



\ 



Z 



\ 



HZ 



56 7 & 9 10 II NOON I 



34-56 



8 9 10 II MT. I 



Fio. 29. Series 10, showing movement of upper stomata of alfalfa (A) and 

 starch index of guard-cells (B). 



The cabinets were placed on alfalfa plots to determine the readiness 

 with which the stomata would respond to reduction of light during 

 the forenoon when they had just opened, and again in the afternoon 

 when they were closing. On July 3, 1916, the cabinets were placed 

 upon well- watered plots for 90 minutes, at 3 p. m., when the stomata 

 were 80 per cent open. After half an hour the opening was 75 per 

 cent in all except the dark cabinet. At the end of an hour it was 60 

 per cent in the open and in the unshaded and shaded cabinets, 40 

 per cent in the muslin-covered cabinet, and 15 per cent in the dark 

 cabinet. At the end of the experiments the opening was 50 per cent 

 outside as well as in the first cabinet, 45 per cent in the shaded 

 cabinet, 20 per cent in the muslin-covered one, and closed in the 

 dark cabinet. The reduction of the light to 60 per cent had no effect 

 and to 8.5 per cent but little. Reduction to 2 per cent caused notice- 

 able increase in the rate of closure, while darkness caused the stomata 

 to close in less than an hour and a half. Long-period experiments of 

 6 to 24 hours made at various other times showed that the reduction 

 of light in the unshaded cabinet was not sufficient to cause changes. 



