1175 
The favourable influence of longer preliminary warming at 27.5° 
and 30°, also the unfavourable influence of a longer exposure at 
32.5° and 35° is represented graphically in figure 1 in which the 
abscissae show the duration of preliminary warming, and the ordinates 
the energy in M. CG. S. 
It is further clear from the figure that there is a transition point 
between the favourable and unfavourable influence; the amount of 
M. C. S. is here constant. 
Figure 2 represents graphically the energy in M. C. S. which 
causes a curvature of 2 m.m., as a function of temperature. The 
abscissae represent temperature, and the ordinates luminar energy 
in M. C. S. As the drawing is much reduced the lines representing 
longer preliminary warming are omitted for the sake of clearness ; 
only the line for one hour’s warming has been drawn. 
200 
160 
120 
Evidently we are here concerned with an optimum-curve. The 
optimum is at 30°. 
Finally there is the question whether van ‘Tr Horr’s rule applies 
to phototropism. The energy in M. C. S. decreases to the optimum 
because perception takes place more rapidly. To determine the 
temperature-coefficients, the ratios of the quantities of luminar energy 
must not be taken, but the ratios of their reciprocal values, as was 
K 
done by Ruraers*) for geotropism. For this reason = etc. is taken, 
20 
iS 
and not — etc. 
10 
The following temperature-coefficients are then found: 
) A. A. L. Rureens. Proceedings Royal Acad. Amsterdam, Vol. XIII. 
