26 i 
of Rhythm in Plants . 
of say twenty or forty minutes’ period. But this was not so, 
whenever a regular rhythm was set up, its period was half-hourly. 
Nor would such a theory account for the oscillations occurr- 
ing in a vertical plane, whereas if they are geotropic in origin 
they must obviously occur in that plane. 
Moreover, the movement induced by the Idinostat is pre- 
cisely the kind of result that might be expected from what we 
know of intermittent stimulation in plant-physiology. The best 
example of what we mean is the series of movements produced 
by the alternation of day and night Take for instance the 
nyctitropic movements of leaves. The most striking feature 
about these movements is that the turning-points, the places 
where a reversal of movement occurs, are not at the begin- 
nings of the periods, but in the middle of them 1 . Thus 
a leaf begins, in the day, to assume the night-position and in 
the early morning before it is light it begins to return towards 
the day-position. A graphic representation of the movement 
is a wavy line in which the crests and valleys of the waves 
occur in the afternoon and early morning. Precisely the 
same is true of the geotropic rhythm, as may be seen in 
Fig. i 2 * . Thus, whether we have a cycle of twenty-four, or of 
one hour, the result is the same, and it is not a little remarkable 
that it should be possible to make, with a klinostat and a 
dandelion stalk, a working model of the natural periodic 
movements of plants. 
It is one of the most familiar facts about the periodic 
phenomena in plants, that the rhythm continues after the 
conditions which have built up the rhythm have ceased to 
act. Thus in the 4 sleep ’ of flowers and of leaves, it is well- 
known that in constant darkness the periodic movements 
continue for some time. The same is true of the periodic 
variations in growth-rate. 
1 See the graphic representation of sleep-movements in Pfeffer’s Die periodischen 
Bewegungen der Blattorgane, 1875. 
2 It should be noted that in the majority of our heliotropic experiments the 
moment of reversal of movement coincides much more closely, often exactly, with 
times at which the klinostat turns. 
