10 
Francis Darwin. 
LECTURES ON THE PHYSIOLOGY OF MOVEMENT 
IN PLANTS. 1 
By Francis Darwin. 
[Text-Fig. 1.] 
III.— The Analysis of Geotropism. 
HE attempt to obtain a clear insight into the nature of 
geotropic curvatures is comparatively modern. We owe 
much to the well known researches of Czapek 2 and recently we have 
had a no less interesting work by Fitting 3 to which I propose to 
call your attention to-day. 
Among earlier researches in this direction may be mentioned 
the Power of Movement in Plants, where the attempt was, for the 
first time, made to distinguish between the motor and perceptive 
elements that go to make up geotropic and heliotropic curvatures. 
The relation between excitation and reaction is one to which 
Fitting devotes a considerable amount of attention. An external 
agent,—a stimulus,—produces some sort of upset or disturbance in 
the protoplasm. This, which the Germans call Erregung, and we 
Excitation, has not necessarily any relation to movement. A 
slight excitation may be produced, and may die away and cease to 
exist, without leading to movement. We must, however, believe 
that even a passing excitation begins to prepare the explosion in 
the motor apparatus which is the result of stronger excitation. 
We may once more use the well worn illustration of a gun fired 
by hanging a weight to the trigger. A minute weight would not 
let off the gun, but since an accumulation of minute weights will 
do so, each must have its effect. Here the weights represent 
excitation, and the firing of the gun the motor reaction. 
Let us imagine that the weight which pulls the trigger is 
water added drop by drop : in that case, if the previous drop has 
evaporated before its successor is added, the gun will never 
explode. We shall see that this sort of thing actually occurs in 
intermittent stimulation, if one unit of excitation dies out before 
1 A Course of Advanced Lectures in Botany given for the 
University of London at the Chelsea Physic Garden in the 
October term, 1906. 
3 Pringsheim’s Jahrbiicher, XXVII., 1895, XXXII., 1898, and 
elsewhere. 
3 Pringsheim’s JahrbQcher, XLL, 1905. 
