*5 
The Analysis of Geotropism. 
Upon this fact depends Fitting’s method of determining the 
perception or excitation time. Most observers have employed a 
continuous klinostat rotating in about 20', but Fitting finds the result 
to be the same with rotations of 12", 6" and even 1". Take the case of 
an oblique klinostat rotating in 2", here the plant will be for not more 
than 1" in each of the alternate positions of unequal stimulation. 
And since under these conditions a curvature is the result, it is 
certain that exposure of 1" produces excitation. 1 The same result 
followed with rotation of 1"; there can be little doubt therefore that 
excitation is practically instantaneous. 
By the same method it was strikingly. shown that perception 
time and reaction time have nothing to do with each other. Thus 
grass haulms when left horizontal do not curve for many hours, yet 
when placed on an oblique klinostat rotating once in 6" they give a 
geotropic result, shewing a perception time of not more than 3". 
Some very interesting results were obtained by combining 
angles which are nearer together than 0 n and 45°. Thus +3° and 
—9°, differing by 6 n , produced no curvature; but with larger angles 
differing also by 6° a result did follow, thus +55° and -—61° gave a 
curvature. Even with + 86" and —88° a result was obtained. In 
this case the axis of the klinostat was only 1° from the horizontal. 
This suggests that when Miss Pertz believed herself to be experi¬ 
menting with a horizontal klinostat she was accidentally using an 
oblique one, and that the curvatures she obtained when the plants 
were supposed to be +45° and 45°, really occurred with plants at 
say 44° and 46". 2 
Carelessly considered, it would seem that in Fitting’s 
experiments the plant is more sensitive when it is nearly vertical 
than when it is horizontal, i.e., that it is less sensitive when it is in 
the optimum position. But probably it is a case of the Weber- 
Fechner law. Our eyes distinguish between two sources of light 
when they differ by \%. So that if the light is very faint, 1% is a 
very minute absolute difference, whereas with a bright light 1% is a 
big amount. This is probably true, mutatis mutandis, of Fitting’s 
results. 
Another subject of investigation was the plant’s power of 
distinguishing between exposures of different duration. For this 
1 With these rapid rotations an effective amount of centrifugal 
force is produced, but this was discounted by the arrangement 
of the experiment. 
2 In the Annals of Botany, XIX, p. 569, Miss Pertz shows that 
with a carefully levelled klinostat the results confirm those 
of Fitting. 
