160 



reaction-time was lengthened as the air-pressure was 

 reduced. Then he investigated separately the perception 

 and reaction. As measure of the influence of pressure on 

 the sensory phase he used the presentation-time. The 

 roots were placed horizontally in air of known pressure 

 for a time and then transferred to the chnostat. If, after 

 an interval, 50 Vo of the roots showed a curvature, they 

 were assumed to hâve been stimulated during the presen- 

 tation-time. Thèse experiments showed that the presentation- 

 time is prolonged when the pressure of air is reduced; 

 this indeed is already marked at 0.74 of an atmosphère, 

 then it decreases gradically to 0.21 of an atmosphère, 

 below which the decrease is very rapid. The experiments 

 further showed that under thèse conditions also the 

 reaction-time is prolonged. A continued stay under reduced 

 pressure lengthened the reaction-time more than the pre- 

 sentation-time. Therefore the cause of the delayed reaction 

 cannot be wholly due to the lengthening of the sensory 

 phase, but also of the motor phase. 



He finally carried out experiments by exposing to 

 normal pressure and allowing the reaction to take place 

 under reduced pressure. Hère also a lengthening of the 

 reaction-time took place. 



Finally Kenkel ^) mentions experiments on the influence 

 of reduced air- pressure on geotropism and phototropism. He 

 followed exactly the same method as C o r r e n s ^) and 

 like him found for varions plants, that with a definite 

 oxygen-content, at which the geotropic reaction can still 

 take place, the heliotropic no longer occurs, or is at 

 least diminished. 



') J. Kenkel. Ueber den Einflusz der Wasserinjektion auf Geotro- 

 pismus und Heliotropismus. Inaugural-Dissertation, Munster 1913. 

 -) Correns. le, p. 138. 



