260 Darwin and Pertz. — On the Production 
cannot therefore with any fairness give an estimate of the 
whole number of experiments which have failed and succeeded 
in showing a rhythm. 
The following figures give some idea of the frequency of 
success. 
Out of eighteen experiments on Valerian in 1890-91, nine, 
or one half, showed a rhythm. 
Out of seven experiments on Dandelion in 1890, six showed 
rhythm. 
Out of ten experiments (heliotropic) on Phalaris in 1890, 
seven showed rhythm. 
We have worked at this subject at intervals for three years, 
and we cannot for a moment doubt that what we have seen is 
not an accidental result. To watch the movement of the free 
end of the shoot and to see it reversed exactly at the expira- 
tion of half-an-hour, is an experience so impressive as to 
compel belief. When a shoot is in a thoroughly rhythmic 
state, it is possible to prophecy to a minute at what time the 
reversal will take place. This can only be done with certainty 
when the klinostat is kept going and the alternation of stimuli 
keeps up the rhythm already acquired. We have frequently 
been strongly impressed by a result of this kind. 
Given the fact that a to-and-fro movement of the plant, in 
a rhythm closely approximating to a half-hourly period, takes 
place in a fair proportion of cases, there seem but two possible 
explanations. (1) The theory we hold, namely, that the 
plant learns as it were the rhythm of the alternate stimuli im- 
pressed on it. (2) That the whole affair is a matter of 
chance ; the plants used happen to circumnutate with a half- 
hourly period, which, therefore, seems to have been ‘ learnt 3 
from the tempo of the klinostat. This view, which was sug- 
gested to us by a friendly critic, has thus much in its favour. 
It would certainly account for the cases in which no rhythm 
appeared, because from what we know of circumnutation we 
should not expect to find a half-hourly period except very 
rarely and by the merest chance. If this explanation were 
the true one we ought to have come across a regular rhythm 
