62 
fore-illumination, can be satisfactorily explained by assuming that 
at every point of the circumference the energy of the unilateral 
after-illamination is simply added to that of the fore-illamination. 
The retardation at a given point of the circumference will 
therefore depend on the total energy received from the two 
illuminations together. If the energy difference between the 
anterior and posterior sides is then sufficiently great to give the 
difference of growth retardation required for a visible curvature, 
the threshold value will be reached. I have nowhere assumed a 
change in sensitiveness, in contradistinction to BREMEKAMP'), who 
attempts to explain the increase in the threshold value by the 
hypothesis, that the plant has become less sensitive as a result of 
the unilateral fore-illumination and thus imagines a change in the 
perception-basis. 
If we apply unilaterally 6 M.C.S., the front will receive 6, and 
b 
of this the back — M.C.S.; m is greater than 1, since the back 
m 
receives less than the front. If we had, however, previously given 
a M.C.S. omnilaterally, the later front would have received of this 
4 M.C.S., the later back also a M.C.S.; a unilateral after-illu- 
n n 
mination will then supply to the front in addition 6 M.C.S., alto- 
gether therefore Ee: + 6 M. C.S; the back receives altogether 
n 
a b 
— + — M.C.S. In both cases the absolute energy difference between 
n m 
: ’ b m—1 
the front and back remains constant, 1.e. b>— — = 6—— M. C.S. 
m m 
Now the resultant curvature entirely depends on the 
difference of growth retardation, which corresponds 
m—1 
to an energy difference of 5 M.C.S. If the growth retar- 
m 
dation curve were a straight line, there would always correspond 
1—1 
to an energy difference of b— M..C.S. one and the same differ- 
m 
ence of growth retardation, whatever place of the abscissa we may 
consider; the threshold value would then necessarily be constant. If 
the growth retardation curve is, however, a curve with decreasing 
slope, the growth retardation difference corresponding to an abscissa 
—Ì 
difference of 6- M.C.S. will become the smaller, the gentler 
m 
1) BREMEKAMP l.c. 
