Centrifugal Force as Geotropic Stimuli. 755 
where 5 is the value of the stimulating force in terms of mg , and t is the 
number of seconds during which it acts. Thus the conclusion is reached 
that in any given radicle a constant amount of response takes place to 
a stimulus of a given number of mg. sec. units , however this number may be 
made up y so long as the time of exposure to the stimidus is not long. 
This last reservation will be understood by reference to Tables I and II, 
from which it will be seen that for Helianthus radicles the statement is true 
so long as each single exposure to the stimulus of gravity does not exceed 
fifteen minutes, and the exposure to a centrifugal force is correspondingly 
small. The fact that the law does not hold good for longer induction 
periods becomes explicable, as shown above, from the discovery of Jost and 
Stoppel (T2) that roots respond negatively to prolonged stimulation by 
high centrifugal forces. 
This result is in contradiction to Pekelharing’s conclusions. She found 
that the geotropic response in coleoptiles displaced 17 T from the normal 
was less than that in coleoptiles displaced 5 0 from the normal, though in 
the former position the stimulus acting at right angles to the coleoptiles 
was greater than in the latter. She continues : 4 aus diesen Mitteilungen 
geht aber hervor, dass die alten Untersuchungen liber die Reaktionskriim- 
mungen nicht mehr zu gebrauchen sind, und meine Erfahrung warnt vor 
der Hypothese dass gleich grosse Reize aucli gleich grosse Krummungen 
verursachen wiirden. Augenscheinlich hangen diese Krtimmungen nicht 
nur von Reizintensitat und Reizdauer, sondern auch noch von einem oder 
mehreren, uns bis jetzt unbekannten Faktoren ab.’ (Italics are mine.) 
The hypothesis, italicized in the above quotation, against which Pekel- 
haring warns us, has, however, been proved to be true for the case of 
centrifugal forces by the results of the present research. 
Summary. 
1. A comparison of the value of different degrees of centrifugal force 
as geotropic stimuli was obtained by the method of continued alternation 
of short-timed exposures to gravity and a centrifugal force in opposite 
directions. A machine was constructed to accomplish this alternation by 
which centrifugal forces up to 21 mg. could be obtained. The centrifugal 
‘ wheel ’ rotated in the horizontal plane and was driven by an electric motor. 
A description of this machine is given in the text. 
2. The seedlings were pinned to the back of a box, which automatically 
took up a nearly horizontal position when the ‘ wheel * was rotating. Thus 
the radicles on the outer side of the box were subjected to a higher cen- 
trifugal force than those towards the inner side. If the exposures to the 
centrifugal force were neutralized by the alternated opposing exposures to 
gravity at a certain point in the box, the radicles at this point would not 
