1045} 
It remains to trace also the course of a phototropie curvature 
with the microscope in the same way as Porowzow did for geotropic 
curvatures. 
In view of the manner of curving, to be described later, the distance 
between the original position of the tip of the coleoptile and the 
new position occupied at any moment during the curvature is chosen 
as the measure of the curvature at that moment. MAtierer') and 
Porowzow also took this distance as a measure of the curvature. 
In the eye-piece there was a net-micrometer, so that it was possible 
to make under the low power a drawing on squared paper of the 
whole apex. 
By comparing these drawings, made every 5 or 10 minutes, it 
was possible to trace the origin of a slight alteration in shape. 
Nutations gave a good deal of trouble, though all specimens in 
which these occurred were absolutely rejected. Since the nutation 
movements are sharply distinguished from a phototropic curvature 
by the change of position of the whole apex with respect to the 
base, it was fairly easy to recognise them. 
In the following curves the abscissa is the time and the ordinate 
the strength of curvature, measured in the afore-mentioned way. 
In one case therefore the curvature began after 12 minutes. Are 
we then to conclude that the curvature begins just at this moment 
and that the reaction time is therefore 12 minutes? I think not: 
it seems to me it would appear that we must come to this conclu- 
sion after a study of the shape of the apex at the beginning of the 
curvature. It is found that while this shape is at first almost exactly 
conical with a somewhat blunted top, the curvature becomes visible 
as a slight mutuai asymmetry of that side of the cone which is 
turned towards the light and that which is turned away from it. 
This asymmetry becomes gradually more marked until the apex be- 
eins to bend forward and the curvature extends further and further 
from the apex. There is no indication of the sudden appearance of 
curvature. In a few cases the shape of the apex favours the perception 
of even a slight asymmetry, but it is very probable that in such cases 
a curvature occurred, even before a deviation was traceable. 
The determination of a reaction-time is therefore experimentally 
impossible, and it is quite conceivable, that the curvature occurs 
immediately on stimulation. 
The passage of a part of the curve which is only visible micro- 
!) A. Maitterer. Elude sur la Reaction géotropique. Bull. Soc. Vaudoise. 1910, 
67* 
