42 



Then I tried outside the thermostat in the dark room, 

 stimulating 4'30" at 20° C. (which is more than the pré- 

 sentation -time at 20° C.) and found that of 70 seedlings 

 not a single one curved. Now either the condition of the 

 dark room or that of the seedlings was necessarily the 

 cause. In order to settle this I made several experiments 

 in the dark room of the hothouse and there the reaction 

 was very good. So it was évident that the dark room 

 constituted a bad influence and now for the first time 

 I thought of the paint of the thermostat and perhaps of 

 other parts too, which might spoil the air of the dark 

 room when heated. The thermostat was emptied and 

 heated on a stove with a permanent tire, for a week, during 

 which time I investigated whether the air in the dark 

 room was improving. 



The success was complète: On February 10"" the 

 thermostat (which had then been in use for two months) 

 was removed from the dark room and the same day the 

 electric fan was worked for halfanhour. On February 11"" 

 ventilation by means of the electric fan was started at 

 9 o'clock in the morning; at 10.15 a batch of plants was 

 stimulated during 4'30", resuit: 36% curved; the ventila- 

 tion was continued until 3.35, when a second batch of 

 plants was stimulated during 4'30", resuit: 74 o/o curved, 

 Hence it was out of the question, that the dark room had 

 been spoiled by the thermostat and that the ventilation 

 had rectified it again. 



Aftei- thèse experiments I always used the electric fan 

 freely; and moreover made regular control-experiments in 

 order to discover any impurity of the air. 



Thèse control-experiments proved at the same time that 

 the heating of the thermostat had been a sovereign 

 remedy, for a decrease of the reaction -faculty of the 



