1284 
The reactions to small light and gravitational stimuli do not 
noticeably influence each other. 
§ 3. Changes in the phototropic and geotropic reactions under the 
influence of light. 
Up to this point we only used unilateral illumination of fairly 
slight intensity. The question now is whether other results are 
obtained by the application of greater quantities of light. By changing 
the duration of illumination as well as its intensity it is possible to 
bring about modifications in the phototropic reaction which for our 
purpose we may arrange in two different categories: 1. reversal of 
the direction of the curvature and 2. change in the rate of reaction. 
If we wish to know how a definite phototropic curve at its maxi- 
mum extent is combined with a maximal geotropic deviation, we 
have only to determine the length of time after which the maximal 
light-eurve is reached and then to administer the two stimuli with such 
an interval that the curvature maxima coincide. 
In carrying out these experiments it is found to be quite imma- 
terial whether we are concerned with a positive phototropic curvature 
or one in the opposite direction and whether the maximum is reached 
after a shorter or longer time. Summation always takes place. 
This is, however, not the end of the process. If, after stimulation, 
the seedlings are placed on the clinostat and observation is continued 
for a considerable time, then one begins to note deviations, at least 
when there has not been too great an interval of time between the 
two stimulations. By summation of curvatures of the same direction 
apex curvatures finally occur in opposite direction and seedlings in 
which opposite curvatures have been induced, sometimes show 
stronger curvatures. 
The same phenomenon was to be observed when unilateral was 
replaced by omnilateral illumination. 
During illumination which in different experiments was varied in 
strength as well as in duration, the seedlings were rotated at constant 
velocity round their axis. When illumination ceased, they were imme- 
diately placed in a horizontal position and stimulated geotropically 
for some time. 
Already in the first series of experiments differences were obvious. 
The distance from the Osram lamp of 10 candle power amounted in 
this case to 2 metres. The times of illumination are in the following table 
placed in the top line, with the product of intensity and duration 
in metre-candle-seconds (M.C.S.) placed between brackets. The last 
box was not illuminated beforehand. The geotropic induction lasted 
