70 
from O—75 M.C.S. the growth retardation curve is more or less 
rectilinear, then the slope decreases, so that it becomes approximately 
logarithmic from 300—700 M.C.S., while a maximum is found at 
about 1600 M.C.S. This value therefore agrees sufficiently well with 
that already found. 
Finally I have also succeeded in finding theoretically the further 
course of the growth retardation curve, which explains both the 
phenomena attending more prolonged unilateral illumination and 
those of unilateral preceded by prolonged ommnilateral illumination. 
growth retardation 
I, 
h 
Fig. III. Diagrammatic representation of 
the growth retardation curve for an intensity 1. 
the growth retardation curve for an intensity 4. 
magnitude of curvature occurring after unilateral illumination with intensity 4. 
Here the growth retardation has not been represented as a function 
of the energy, but as a function of the time, during which there 
was illumination with the same intensity. Since unilateral illumination, 
following an omnilateral illumination of more than 5 minutes’ duration, 
always results in a positive curvature, y, must again be greater 
than vp. This comes about through the renewed rise in the growth 
retardation curve, if the plant is illuminated for more than 5 minutes, 
Since the slope increases here, the threshold value for positive cur- 
vature must again fall. This latter fact agrees with what CLARK and 
Arisz found, namely that the threshold value falls if the plants are 
submitted to a fore-illumination of more than 5 minutes and less 
