1289 
direct reversal of the reaction would only be possible through a 
reversal of the polarity of the cells, a hypothesis far from simple and 
hitherto not susceptible of experimental verification. For this reason 
I may direct attention to another possibility. 
In the cells which are given preliminary illumination, phototropic 
reactions are in progress which cannot lead to curvatures, because 
they keep each other in equilibrium. In consequence of the geotropic 
stimulation this equilibrium is now upset. It might be imagined, for 
example, that the geotropic stimulation displaces or destroys a 
substance necessary for the pbototropie reaction. The resulting cur- 
vature would in reality therefore be phototropie. It is not impossible 
that also in the experiments of Jost and Miss Stopper an omnilateral 
stimulation (perhaps hydrotropic) thus expresses itself as a curvature. 
§ 4. Changes of the geotropie and phototropic reactions 
under the influence of gravity. 
‘According to Martr-MarrHe Risz *) an omnilateral gravitational 
stimulus has no effect on the sensitiveness with respect to a subse- 
quent unilateral one. From the data in the second part of her paper, 
however, one may deduce that this is not correct. For Miss Risz 
there proves, that the component of gravity in the direction of the 
organ?) weakens the reaction. After an omnilateral gravitational 
stimulus acting at an angle of 90° and having therefore a longitudinal 
component equal to 0, the reaction must be stronger than when the 
plant is placed vertically for the stimulation (longitudinal compo- 
nent + me.). This was indeed the case in my own experiments. 
Some boxes rotated for a certain time round the horizontal axis 
of the clinostat and were subsequently subjected to a unilateral 
gravitational stimulus simultaneously with a control box. I examined 
the curvatures forty minutes after the end of stimulation. 
Differences such as those found here, are also observable when 
the boxes are placed for some time in the reverse position (longi- 
tudinal component — mg.) 
Duration of unilateral geotropic stimulus 30 minutes. 
Previously placed on the clinostat during. 
1) Marie-Martue Risz. Uber den Einflusz allseitig und in der Längsrichtung 
wirkender Schwerkraft auf Wurzeln. Jhrb. f. wiss. Bot. LI. 1913. 
2) | already pointed out in 1912 the importance of this component. Die 
rotierende Nutation und der Geotropismus der Windepflanzen. Rec. d. Tray. Bot. 
Néerl. IX. p. 298—301. 
