747 
Centrifugal Force as Geotropic Stimuli . 
long axis of a Pfeffer’s clinostat and covered by a glass cylinder as shown 
in Pfeffer’s ‘ Physiology of Plants’, vol. iii, p. 169. Up to this time the 
seedlings were constantly held with radicles vertical. The axis was then 
attached to the clinostat. They were allowed to remain motionless during 
the exposition time, after which they were made to rotate at a rate of about 
one revolution in 2 min. The following results were obtained : 
Exposition 
time. 
Temp. 
No. of 
seedlings. 
No. of radicles bent after 
if h. 2 h. 2% h. 3 h. 4 h. 
4§ 
Percentage 
bent. 
Min. Sec. 
10 0 
20° C. 
8 
— 5 
— 66 
’ 
75% 
8 
0 
1 8° C. 
9 
3 — 
5 — — 
8 
89% 
6 
0 
20° C. 
8 
— 4 
~ 5 5 
— 
63% 
5 
0 
1 8° C. 
9 
4 — 
5 — — 
5 
55 % 
4 
30 
1 8° C. 
16 
— 2 
— 6 — 
8 
50% 
3 
0 
20° C. 
8 
— 3 
— 44 
— 
50 % 
Thus the presentation time is about 3 to 4^ min. at 18 0 to 20° C. Also 
Pekelharing (’09) showed that under stimulation greater than gravity the 
presentation time is inversely proportional to the stimulus ; so if, in the 
experiments summarized in Table I, t is greater than the presentation time 
for gravity, T will also be greater than the presentation time for the centri- 
fugal force. 
We thus see that radicles may be exposed alternately to centrifugal 
forces and gravity for much longer periods than the presentation time 
in each direction, and yet show neutrality when CT = mg . t. We have 
therefore to look for some other explanation to account for the discrepancy 
in the results with still longer exposition periods. 
The most probable solution to the difficulty is suggested by the work 
of Jost and Stoppel (T2). These authors showed that under the action of 
high centrifugal forces, though roots at first respond positively, they will 
after a lengthened period of stimulation respond negatively. He found that 
Lupin roots, under the influence of a centrifugal force of 29 mg., will usually 
bend outwards (i. e. positively), but some show a tendency to bend inwards 
after prolonged stimulation. With a centrifugal force of 42 mg. roots some- 
times respond negatively after six hours’ stimulation. With a centrifugal 
force of 70 mg. most roots bend inwards after two hours. 
These results have a very important bearing on the present work. 
Roots at first respond to a high centrifugal force positively, but after 
a while when the stimulation is continued they gradually become less 
active in their response and may end by bending towards the centre 
of rotation. Now we may suppose that in my earlier experiments the 
radicles were not stimulated continuously for sufficient time by the centri- 
fugal force to cause any tendency to negative response. But in the 
experiments summarized in Table II stimulation proceeded long enough 
to allow of a diminution in the activity of response, so that less time 
