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large number of experiments in air in order to obtain a standard, 
from which possible deviations might be measured. In these experi- 
ments I used stimuli of arbitrary intensity. Thus the geotropie 
stimulation consisted in placing the seedlings during a quarter of an 
hour in a horizontal’ position, which therefore means an intensity 
of stimulus of 900 mgs. Phototropie stimulation took place by ex- 
posure during eight seconds to a lamp, placed at a metre’s distance 
from tlie middle of the box, with the seedlings arranged so 
that intensity of the light falling on them was 5 metre candles. 
This therefore corresponds to a stimulus of 40 metre-candle-seconds. 
I now determined the maximal curvatures corresponding to the two 
stimuli and the intervals of time between’ the beginning of stimu- 
lation and the attainment of maximum curvature, i.e. the reaction 
times. The result is, that for both stimuli the maximal curvature is 
2 mm. and that the geotropic reaction time is 65 minutes, the 
phototropic 75 minutes. 
§ 3. Znfluence of oxygen deprivation on perception. 
In order to see whether Avena seedlings are able to perceive a 
stimulus in an oxygen-free atmosphere, I first left them for some 
time in the thermostat, while a current of nitrogen was passing. 
When the objects had in this way been deprived of oxygen for 
some time the stimulus was administered, when they were still in 
a nitrogen atmosphere; immediately afterwards the nitrogen current 
was stopped and ordinary air was sucked through the thermostat 
by means of an aspirator. In considering the length of the preli- 
minary sojourn (fore-period) in nitrogen, given below, it must be 
remembered that this includes the 14—2 hours, necessary to free 
the thermostat completely from oxygen. During the reaction time 
the seedlings were therefore in air; when this time was up the 
seedlings were removed from the thermostat and their curvatures 
were measured. 
I. Geotropic experiments. 
Two or three hours of preliminary sojourn in nitrogen had not 
the slightest effect. A subsequent stimulus of 900 mgs. expressed 
itself by a maximal reaction of 2 mm. in the air. A fore-period in 
nitrogen of 5 hours was clearly evident by a diminished response, 
and after 6 hours there was no reaction at all. 
