179 
therefore, have to know 1st either the optimum or the maximum 
of the curve representing the strength of the curvature, and 224 in 
what way the sensibility can be represented as a function of the 
energy. 
When we have found out what changes the sensibility in the an- 
terior, and in the posterior side undergoes, it will at once be clear 
how the light is distributed over those two halves. Since more data 
are available relative to the optimum than to the maximum I started 
in my calculation from those relative to the optimum. The accu- 
rateness of the latter is, however, also very small, so that the value 
found for the proportion should be taken only as a first approxi- 
mation. 
The changes in the sensibility to light could be deduced from the 
data obtained by Arisz*) in his experiments on phototropie disposi- 
tion. As most important among those changes | may mention 
1st that the sensibility undergoes a gradual decrease in consequence 
of the illumination and is ultimately entirely destroyed; the required 
quantity of light should, however, be supplied within a fixed time 
(ef. 2nd); Joad that the sensibility is ere long augmented. again, the 
rate of the increase being greater according as the intensity of the 
illumination is smaller, the sensibility reaching at last a higher 
value in weak illuminations than in strong ones; 3rd that this is 
also the case after the light has been put out and that in this case 
the sensibility regains at last its original value; and 4: that the 
velocity of the increase reaches its highest value only after some 
time has elapsed. In order to determine the relation between the 
changes of the sensibility to light and those of the rate of growth 
we need only to compare, in a given case, the changes undergone 
by the difference in sensibility in the antagonistic sides with the 
growth of the curvature. 
From the foregoing it may be concluded: 1% that the magnitude 
of the curvature at a certain moment, is determined by the changes 
undergone by the difference in sensibility of the anterior and the 
posterior side from the beginning of the illumination up to a point 
of time which is separated from the chosen moment by the length 
of the latent period, i.e. the interval between the beginning of the 
illumination and the initiation of response; and 2"¢ that the direc- 
tion of the curvature is determined by the sign of this difference 
in sensibility. It follows from this that an antiphototropic curvature 
will appear if the sensibility at the anterior side exceeds that at the 
LW El ARISZ, Untersuchungen über den Phototropismus. Rec. d. trav. bot 
Néerlandais. Vol. XII. 1915. 
KAN 
