of Rhythm in Plants. 263 
of plants than Pfeffer, whose admirable ‘ Periodische Bewe- 
gungen 5 is known to all physiologists. He uses the pendulum 
as an illustration : a pendulum is kept in action by the 
periodic application of force, and continues to swing in 
its proper rhythm for some time after the force has ceased 
to be applied. This, of course, is no explanation of the 
similar state of things in plants, but such an analogy is 
more useful than the statement that the rhythms in plants 
are explicable as the result of 4 latent period 5 and ‘ after- 
effect. 5 The pendulum illustration fixes the attention on the 
right point, namely, the condition of equilibrium in the 
organism. A pendulum is a machine specially constructed to 
swing ; and the organism must have a faculty of repetition, a 
power of swinging as it were, or it could not be periodic. 
This repeating power may be that fundamental property of 
living matter, which stretches from inheritance on one side to 
memory 1 on the other — a region too wide for the limits of 
our present paper. 
Pfeffer has shown in a beautiful manner 2 the resemblance 
between a rhythmic plant and a pendulum. Acacia lophantha , 
like other sleeping-plants, becomes motionless in continued 
daylight ; if it is now darkened the periodic movement of 
the leaves begins, but this movement does not continue 
nearly so long as if the plant had, before the artificial dark- 
ening took place, been exposed to normal alternations of day 
and night. The first case corresponds to a stationary pen- 
dulum set in action by a single touch ; the second to an 
oscillating pendulum receiving a similar touch synchronously 
with its swing. 
The rhythmic condition induced in our experimental plants 
has a special interest for us because of its possible bearing 
both on rectipetality and on circumnutation. One of us has 
sought to show elsewhere 3 that it is possible to look at these 
two forms of movement as different aspects of the same 
1 See Mr. S. Butler’s Life and habit. 
2 Pflanzenphysiologie, ii. p. 263. 
3 F. Darwin, Presidential Address, British Assoc., Section D (Cardiff, 1891). 
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