2 62 Darwin and Pertz. — On the Production 
The fact, therefore, that the geotropic or heliotropic rhythms, 
induced by the intermittent klinostat, continue after the 
periodic stimulation has ceased is precisely what we should 
have expected from our knowledge of natural rhythms. 
The comparison need not stop here, we may, without being 
fanciful, compare our results with certain periodic phenomena 
in our own lives. If a man is waked by a knock at his door 
every morning at six o’clock, he will continue for some time to 
awake at approximately the same hour, when he is no longer 
called. In such a case a man says that he wakes at six 
o’clock because he has ‘got into the habit of it.’ If this is a 
fair use of the word habit, which we cannot doubt, then we 
may equally apply the word habit to the periodic phenomena 
of plants, including our klinostat results. 
We think, indeed, that our results may throw a certain light 
on such a periodic act as that of waking at a habitual hour 
in the morning. 
It is always possible to suppose that the man wakes with- 
out being called, because the act of waking has become 
associated or adherent to some other features of the morning 
hours ; that his body (unconsciously) discovers it to be time to 
wake without the particular stimulus implied by a knock at 
the door. It would be impossible to place a man in such uni- 
form conditions that such association or adherence is excluded. 
But in the case of a half-hourly rhythm imposed on a plant 
the case is different, there are no external changes occur- 
ring at half-hourly intervals which can help the plant to ‘ know ’ 
when the half-hour is passed. There must be a sort of 
internal chronometry in the case of the plant, that is to say, 
the reversal of the geotropic curvature must take place at the 
right time because it has been associated and become ad- 
herent to some processes (of nutrition possibly) jn which the 
element of time comes in. What has here been said is the 
merest guess-work ; its speculative character, however, does 
not interfere with the fact that our rhythmic experiments 
point to a time-measuring quality in plants. 
No one has thrown more light on the periodic phenomena 
