Francis Darwin. 
H 
position of maximum stimulation, the plant will tend to curve 
inwards towards the axis of rotation. This is obvious since, on the 
hypothesis, the tendency to bend upwards in position p t is stronger 
than the corresponding tendency in position p 2 . In the same way if 
the horizontal is the position of maximum stimulation the resultant 
curve will be away from the axis. This is what Fitting found, all 
his experiments were in favour of the excitation being greater at 
0 n (horizontal) than at —45°. The experiment resembles a “ tug-of 
war” between opposing stimuli: the proof that one side is stronger 
than the other, is that they haul their opponents over the mark ; so 
in the case of the plants a slight curve in one direction tells us 
which of the opposing excitations is the stronger. 
The fact that the horizontal is actually the position of maximum 
stimulation can only be proved by repeating the experiment with 
alternating positions of 0 n (horizontal) and 22^°, then with 0 n and 11°, 
and so on. Fitting has combined 0 and 22^, proving that the position 
of maximum excitation is not more than 11° from the horizontal. 
Fitting made a very interesting modification of his method by 
using the intermittent klinostat with unequal times. Thus the 
positions 0° and —45°, with equal exposures in the alternating 
positions, cause curvature, but if the plant is kept for fourteen 
minutes in the position of weak stimulation (—45°) and ten minutes 
in that of stronger stimulation (0°) and so on alternately, it does 
not curve, but remains straight. Now 14 : 10 = 1 : 0*71, that is in 
the proportion of sin 90: sin 45. If to nullify the effect of 
alternate exposure in the positions —45° and horizontal, we have to 
make the periods of exposure as sin 90 : sin 45, it is clear that the 
excitations are as sin 45 : sin 90, that is in the proportion of the 
sines of the angles which the plant makes with the vertical. The 
same relation was obtained by alternating other positions, as shown 
in the following table :— 
Angle with vertical. 
Alternate positions ... 60° and 90" ... 30° and 90° ... 15° and 90" 
Times of exposure ... 1:0-87 ... 1:0-5 ... 1:0-26 
Sines. 0-87 : 1 ... 0 5 : 1 ... 0-2 : 1 
It is thus proved that the excitation is directly proportional to 
the time of exposure and to the sine of the angle made by the 
plant with the vertical. These experiments, which have been 
described as made with the intermittent klinostat, give a similar 
result with the continuous instrument. This is a very interesting 
method of proving that the stimulus of gravity is continuously 
perceived by plants rotating on the klinostat. 
