Production of Rhythm in Plants. 103 
In ‘ The Power of Movement in Plants ’ it was shown that 
heliotropic curvature may be brought about by modified 
circumnutation, i. e. by a revolving movement in which the 
curvature towards the light is greater than that in the 
opposite direction. Just in the same way, in our experi- 
ments, heliotropic curvature was produced by asymmetrical 
nutation. 
General Remarks. 
It is often said that periodic phenomena are due to after- 
effect. But this, though true in a certain sense, is too vague 
a statement to be called an explanation. It is known that 
geotropic and heliotropic curvatures continue long after the 
stimulus has ceased to act. So that it might at first appear 
as if the curvatures in one direction were the after-effect of 
a given half-hour’s stimulus, and the opposite curvatures were 
the effect of the next ensuing half-hour. But we are unable 
to construct a scheme of this sort which fits the facts. The 
after-effect in a curving shoot which has been stimulated for 
half an hour lasts a long time, and we cannot see how the 
series of opposite curvatures, each lasting half an hour , could be 
caused by the combination of such after-effects. After-effect 
in the ordinary sense is the result of the last stimulus received, 
and we know of nothing to make us believe that the latent 
after-effect of an antecedent and opposite stimulus can be held 
to account for the sharp reversal of curvature which we find 
to occur. 
We believe, moreover, that an artificial rhythm may be 
imagined to be produced without what is ordinarily described 
as after-effect. Suppose an apogeotropic shoot to be placed 
horizontally ; after some 1 5 minutes of stimulation it will 
begin to curve upwards, and will continue so to do for 
another 15 minutes. At this point the klinostat rotates 
through 180 0 , and the stimulus is replaced by an equal 
and opposite one. According to our assumption (that after- 
