( 65 ) 
Botany. “Investigations on the relation between the presentation 
time and intensity of stimulus in geotropic curvatures.” By Miss 
f C. J. Pekelharing. (Communicated by Prof. F. A. F. C. Went). 
p The investigations of Blaauw 1 ) and of Froschel 2 ) have shown 
that in the case of phototropic stimulation by light of various intensities, 
the presentation time is inversely proportional to the intensity of 
the light. 
It is not known whether a similar connexion exists for other 
kinds of stimuli; for geotropic curvatures, however, Bach’s observa¬ 
tions") seem to indicate that this is indeed the case. 
I Bach examined the influence of various external conditions on the 
presentation time for geotropism. Some of his observations are con- 
| cerned with the inertia forces resulting from small and large centripetal 
: accelerations *), and with the influence of the angle at which the plant 
is fixed. As was shown by Fitting’s experiments*), the plant is 
subjected in the latter case to a force equal to mg X the sine of 
the angle which the plant makes with the normal. 
Bach himself does not deduce from his figures any relationship 
between the duration of the stimulus and its intensity. If such a 
deduction be attempted, and (for the sake of agreement with Blaauw’s 
method) the presentation time be expressed in seconds — if further¬ 
more a mean is taken for the centrifugal forces which are given 
for each experiment, then the following table results: 
With centrifugal force. 
Acceleration 
1.07 g 
5.25 g 
Presentation time 
48Q" 
270" 
Product in ^-seconds 
513 
472 
540 
10.65 g 
19.40 g 
27.35 q 
0.71 g 
0.60 g 
60" 639 
30" 582 
15" . 410 
600" 426 
J 500" 900 
0.40 g ' 1800" 720 
0.14 g 3000" 420 
*) A. H. Blaauw, a. Proceedings of the Roy. Acad, of Sciences, Amst. Sept. ’08. 
b. Die Perception des Lichtes. Recueil des Travaux Botaniques Neerlandais. 
Volume V. 1909. 
2 ) P. Froschel. Sitzungsberichte d. K. Akad. der Wiss., Wien, 117. 1908. 
s ) H. Bach. Jahrb. f. Wiss. Bot. XLV. 1907, p. 86—91. 
4 ) I will call these centrifugal forces. 
6 ) H. Fitting. Jahrb. f. Wiss. Bbt. XLI. 1905, p. 282. 
5 
Proceedings Royal Acad. Amsterdam. Vol. XII. 
