264 On the Production of Rhythm in Plants. 
phenomena. Whether we believe (with Charles Darwin) that 
circumnutation is the basis of growth-curvature, or whether 
with Wiesner we reject such a belief, we must believe in the 
facts of rectipetality, that is, we must believe in a self-regula- 
ting power which keeps growth to a straight line. When a 
normally straight-growing shoot has become curved by excess 
of growth on one side, the regulating power leads to increased 
growth on the concave side, and thus tends to undo the curva- 
ture. Whether or no this has any connection with circumnu- 
tation, it shows a pendulum-like quality in longitudinal growth 
which may well serve as the basis for the induced rhythm 
described by us. 
We are inclined to see in our results a confirmation of the 
main thesis of the Power of Movement in Plants , namely, 
that growth-curvatures are developments or exaggerations of 
circumnutation. We believe that it is because of the connec- 
tion between growth-curvature and circumnutation, that an 
artificial rhythm can be built up by geotropic stimulation. If, 
as the authors of the Power of Movement believe, circumnuta- 
tion can be converted under the action of a single stimulus 
into a one-sided movement, it is not unnatural that under 
intermittent opposite stimuli, a circumnutation should be 
moulded into the half-hourly to-and-fro movement which we 
have described. 
