EE 
than 20 minutes. If the fore-illamination is more than 20 minutes, 
the growth retardation curve has become a straight line, since the 
threshold value now remains constant; however prolonged the fore- 
illumination with this intensity is, there is no further change in dispo- 
sition. Conversely there will be no question of “disposition’’, “change 
of sensitiveness”, in a process where the effect increases in a rectilinear 
manner with increasing strength of stimulus. 
Now since also after unilateral continued illumination (for longer 
than about 5 mins.) a positive curvature is again obtained *), the 
growth retardation curve, for an intensity m times as great, will 
run more steeply, i.e. for the same abscissa (time) there will be a 
greater ordinate (retardation of growth). If we take, however, the 
growth retardations of different intensities with equal duration of 
illumination, and plot these against the intensities, the slope of the 
resulting carve will of course greatly decrease at higher intensities, 
as a simple consideration will show. With this two facts agree: 
firstly that the threshold value after prolonged fore-illumination with 
high intensities comes to lie higher than after prolonged fore-illumi- 
nation with low intensities; secondly, that prolonged unilateral illu- 
mination with a high intensity gives a feebler curvature than illu- 
mination with a low intensity during the same period. 
We see therefore that the phototropic curvature is determined by 
the reactions of the separate longitudinal strips of the front and 
back respectively. Formerly the curvature was regarded as the direct 
result of a single condition of stimulation, the phototropic, which 
was considered to be induced as such. According to the view set 
out above, the curvature must be regarded as the resultant of the 
effects arising from the conditions of stimulation, which exist on the 
side, towards which the ultimate curvature will take place, and on the 
opposite side. These conditions of stimulation express themselves in 
photo growth reactions; the difference between the two reactions is 
expressed by the phototropic curvature. 
The cause of that which was formerly called “disposition” lies 
in the peculiarities of the growth retardation curve. These peculia- 
rities occur to some extent in every process in which the reaction 
is not directly proportional to the stimulus. A tangent galvanometer 
also becomes less “sensitive” at greater strengths of current. The 
“disposition” at a given point of the growth retardation curve, 
whether we take energy or time as abscissa, depends therefore on 
the magnitude and the sign (+ or —) of the angle of slope, and of 
1) See footnote 2 p. 60. 
