Production of Rhythm in Plants. 
105 
Note on the Position of Maximum 
Heliotropic Stimulation. 
Czapek has shown that if an apogeotropic shoot is placed 
at an angle of 45°, the tip being directed obliquely down- 
wards, it receives a stronger geotropic stimulus than if it 
points 45 0 above the horizon. This may be proved to be true 
by the use of our intermittent klinostat. An apogeotropic 
organ being fixed at 45° with the horizontal axis of rotation, 
is subjected (by the periodic rotation of the axis through 180 0 ) 
to opposite and alternate stimuli 1 . If the stimulus is greater 
when the organ points obliquely downwards, the sum of the 
unequal stimuli must tend to bring the organ into line with 
the horizontal axis of rotation. And this was found to be the 
case by one of us 2 , who made the experiment with apogeo- 
tropic grass-haulms. We have now been able to show that 
the same rule applies to heliotropism. 
Our experiments on heliotropism were made with Phalaris 
on a klinostat rotating on a horizontal axis through 180° at 
intervals of half an hour. The plants were fixed 3 so as to 
make an angle of 45 0 with the axis of rotation, and also, 
therefore, 45° with the horizontal light. It follows that the 
plants were alternately pointing obliquely from, and obliquely 
towards the light. 
The following were the results : — 
June 7, 1900 (i-hr. klinostat). Seven Phalaris seedlings: 
after 2 hours all had become slightly more parallel to the 
axis of rotation. 
June 11, 1900. Six seedlings: after 2 hours four were 
more parallel, two unchanged. 
June 12 , 1900. Five seedlings: after 2i hours all more 
parallel. 
1 This is the method used by Czapek ; see Sitzber. K. Akad. Wien, civ, i, p. 1216. 
2 D. F. M. Pertz, in the Annals of Botany, xiii. p. 620. 
3 The plants retained a roughly horizontal position under the influence of the 
alternating geotropic stimuli. 
