104 Darwin and Pertz. — On the artificial 
effect is non-existent) the curvature will cease at the turn 
of the klinostat, and the plant will be able to receive the new 
stimulus, and will therefore after a period of quiescence curve 
in the opposite direction. We can see no reason why this 
imaginary state of things should not build up a rhythmic 
condition, so that the plant would tend to nutate at half- 
hourly intervals after the klinostat had been stopped. We 
can conceive the natural circumnutation of the plant being 
moulded to a half-hourly rhythm under these conditions — 
that is to say, without the existence of after-effect. 
All we can do is to compare our results with other 
periodic phenomena. For instance, when the stimulus is 
given by the alternation of day and night, we get the diurnal 
periodicity of sleeping plants, which, as in our experiments, 
continues after the stimulus has ceased. It seems to us that 
such natural rhythms, as well as our artificial phenomena, are 
intelligible only as modifications of a fundamental rhythmic 
faculty in plants. Such a faculty exists as circumnutation, 
and we may point out that the possibility of regulating 
artificially the rhythmic growth of a plant is in entire agree- 
ment with the fundamental idea of ‘The Power of Movement 
in Plants/ namely that growth-curvatures are modifications 
of circumnutation. 
It is unfortunate that the word after-effect has been used in 
two senses: — (i) To designate the continuance of curvature 
after the cessation of the stimulus, which may be most 
conveniently classed with the phenomena of the motor 
mechanism, although it is also a character of the percipient 
element. (2) To designate such a case as the continuance 
of periodic movements in a sleeping-plant in continuous 
darkness. This should be classed with habit or memory, 
and is a phenomenon of the percipient or quasi-nervous 
element in plants. Precisely the same may be said of the 
artificial rhythm above described. 
