1283 
effect of gravity.') It is of little importance in this case whether the 
stimuli are applied immediately after one another or with a certain 
interval between them. The speedy falling off of the curvature as 
soon as it has reached its maximum, is not observable in plants 
which turn on a horizontal axis. On the other hand the phototropic 
reaction goes on for so much longer than the geotropic one (a 
difference of hours) that it is experimentally impossible to make the 
maxima coincide. We must in this case therefore limit ourselves to 
establishing that the curvature of seedlings to which both stimuli 
have been applied, according as they have acted to reinforce or 
counteract each other, equals the sum or the difference of the 
curvatures which are shown by two groups of controls of which one 
is only illuminated and the other only stimulated geotropically. This 
is found to be possible at any moment, chosen arbitrarily. In the 
following example the geotropic stimulation was administered 50 
minutes after the phototropic and the record was made 5 hours 
after the commencement of the experiment. 
2 | 3 | 4 5 
7.7 (100) °8.5(100 -20min) 
11.1 (100--20 min.) | 3.3 (20 min.) 
Since it might be considered objectionable when dealing with 
such marked curvatures to take the horizontal deviation of the apex 
as a measure, I have in addition determined the angle of the 
curvature. For the sake of simplicity I considered the curvature as 
a circular arc, to which the lines bisecting the base and the 
apex — which latter at this moment has become straight again, — are 
tangents. The supplement of this arc gives an idea of the distance 
travelled. This amounts to: 
There is here therefore also a complete summation. 
The experiments which are described in this section, lead to the 
following conclusion : 
1) In very small curvatures, it is principally the longitudinal component which 
opposes the reaction. Cf. § 4. 
85 
Proceedings Royal Acad. Amsterdam. Vol. XVII. 
