1166 
consisted in placing the experimental plants in complete or partial 
vacuum, and was that employed by most investigators before him. 
The dependence of the geotropic stimulation process on oxygen was 
examined by partially exhausting the vessel containing the seedlings 
and then placing them in a horizontal position. The seedlings were 
then observed for 6—12 hours, to see whether curvature took place. 
Thus he found for instance that seedlings of Helianthus were still 
capable of reaction, when the oxygen was reduced to traces ; Sirapis 
seedlings on the other hand required at least 4—5°/, of oxygen to 
develop ‘a curvature. When no curvature took place, there was 
also no after effect in ordinary air. CoRRENs concludes from these 
experiments that oxygen is necessary for the execution of a geotropic 
process. 
His heliotropie experiments were so arranged that the seedlings 
were continuously exposed in the receiver to unilateral day light, 
and he concludes that oxygen is necessary also for heliotropic stimu- 
lation. The quantities of oxygen which just permit of heliotropic 
curvature differ, however from the minimum guantities allowing a 
geotropie reaction. Thus for a phototropie reaction of Simapis seedlings 
the oxygen could not be reduced below 6 percent. 
Geotropic curvatures are therefore, according to CORRENs, executed 
by the same objects at a lower pressure than phototropic ones. In 
my opinion CoRRENs is not justified in drawing this conclusion from 
his experiments since he compares stimulation intensities, the result 
of which perhaps are curvatures of very different degree. Geotropic 
and phototropic stimuli can only be compared, if we employ stimuli 
of such intensity that they produce maximal curvatures of the same 
strength. 
ARPAD Paár *) published a paper dealing exelusively with the 
influence of rarefaction of the air on the geotropie stimulation 
process and therein considered separately the perception and the 
reaction, the former by determining the presentation-times under 
normal pressure and after evacuation to various extents, the latter 
by causing perception to take place at normal pressure and allowing 
the reaction to occur under reduced pressure. His experiments led 
him to the view that with diminution of pressure the presentation- 
times and the reaction-times are lengthened. . 
It should be noted in this connection that he of course still 
adhered to the old conception of presentation- and reaction-times and 
!) ArPÁp PaAt. Analyse des geotropischen Reizvorgangs mittels Luftverdiinnung 
Jahrb. f. wiss. Bot. L. pag. 1. 1912, 
