72 
the direction in which the latter is changing. We can ascertain the 
sign by arranging that «, and #, approximate closely; then we can 
see from the curvature whether y.—y, is positive (effect: positive 
curvature) or whether it is negative (effect: negative curvature). In 
unilateral illumination «, and «, lie too far apart and may be on 
dissimilar parts of the curve, so that the latter is very difficult to 
draw. According as #, and «, approximate more closely, the angle 
of slope can be found with greater accuracy. Since we have found 
that the growth retardation curve shows an ascending, then a des- 
cending and subsequently again an ascending portion, the “disposition” 
must therefore have three phases; finally after 20 mins., it is not 
possible to bring about a “disposition change” by further illumination 
with this intensity. These three phases have been repeatedly discus- 
sed in the literature and explained as processes, involving a change 
in the perception basis. From the above discussion it is evident, 
however, that disposition is a concept devoid of specific properties 
and is simply an expression of peculiarities of the growth retardation 
curve. Every growth retardation curve has a maximum at about 
1400 M.C.S. and a minimum at about 5 minutes. The growth retar- 
dation curves of two intensities, which are in the ratio 1: m and 
with which 5000 M.C.S. can be applied within 5 minutes, intersect 
at two points; the first point is the energy threshold for the negative 
curvature (+ 5000 M.C.S.), the second is the time threshold for the 
second positive curvature (5 minutes). It will, however, be easier 
and more accurate to construct the growth retardation curves, both 
energy curve and time curve, from the numbers for photo growth 
reactions than from interpolations according to the above mentioned 
method, which is only qualitative, but not absolutely quantitative. 
Phototropism retains its value, however, since the study of photogrowth 
reactions presents difficulties in so far as we are here certainly 
concerned with the conduction of stimuli, while we only observe 
the reaction of the whole plant. Since in phototropic curvature the 
various zones curve, one after the other, we get this conduction of 
stimuli here separated into its successive phases. The difference in 
growth retardation will also be more constant as phototropic curva- 
ture, than when it is found by examining the photogrowth reactions 
of two different plants, in one for a quantity of energy J, in the 
other for a quantity of energy m. For in unilateral illumination 
both experiments are carried out on the same plant, one on the 
front, the other on the back. The difference, the resultant of the 
two reactions, will thus be less influenced by individual variability. 
Utrecht, April 1919. 
