( 1025 ) 
minated with different intensities, was not equally great in every 
case; with weaker light it was considerably less. 
It was found that slighter curvatures exist than those observed by 
Braauw and by Froéscurn. This was a quite unexpected result, for 
although Braauw had very cautiously spoken of those curvatures 
which were only just macroscopically visible, he still believed he 
was working with a threshold of stimulation. 
This applies still more to Fröscurer, who attached great value to 
the smallest product still giving a curvature, as a measure of photo- 
tropic sensitiveness, for comparison with that of other plants. 
Has the deviation of the apex valued by Braauw as still 
showing curvature, any special value, to which alone the rule of 
products applies, or would it be found, that to a smaller or greater 
energy of stimulus there also corresponds a smaller or greater 
curvature? In general, that a definite quantity of energy causes a 
definite extent of curvature ? 
In order to obtain an answer to this question a larger number 
of experimental data had to be available. 
The tedious observation with the microscope was superseded by a 
much simpler apparatus. A photographic lens, magnifying 2 times, 
projected the image of the seedlings on a glass plate, upon which a 
scale of half-millimetre squares had been photographed. The position 
of the apices was read by means of a simple lens. 
The advantage of this arrangement is that in addition to a greater 
number of plants, the whole coleoptile can always be observed. 
The deviation of the apex from its original position before the be- 
ginning of the curvature, chosen at the moment when this distance 
is greatest, was taken as the measure of the magnitude of the cur- 
vature. Since from the beginning gravity opposes the curvature, 
there comes a moment when the apex under the influence of photo- 
tropism moves no further from the vertical, because phototropism 
is neutralised by gravity. Although this point will probably give no 
accurate idea of the sensitiveness, it is here only necessary to have 
a fixed point of the curvature-proces. 
Out of many observations made, | here bring together the fol- 
lowing, which hold good for seedlings of an average length of 22.5 m.m. 
at a temperature of about 17.5 degrees Cent. 
The light energy was obtained by various Combinations of inten- 
sities by stimulation periods of various lengths. (from 2 to 240 sec.) 
The intensities were determined by a Weper') Photometer. 
1) Prof. H. SNELLEN, Director of the Dutch Eye Hospital was kind enough to 
place this photometer at my disposal. 
