102 Darwin and Pertz. — On the artificial 
klinostat was so arranged that the plants were illuminated 
on one side for 28 minutes and then on the opposite side for 
32 minutes. Twelve experiments were made, eleven with 
Phalaris , and one with an oat seedling ; out of these only 
two failed to show stimulated rhythm, and out of these 
ten successful cases, three showed ‘ unstimulated rhythm ’ 
after the klinostat had been stopped. We hoped to find 
that the plants would show an inequality in the period of 
their rhythm corresponding to the inequality of the alternate 
stimuli, but the results are disappointing. If we call the 
number of minutes that elapse between two successive 
reversals of curvature, the length of the period, we can 
indicate the amount of regularity of the rhythm by writing 
in a row the lengths of successive periods. In an ideally 
regular half-hourly rhythm we should have— 30, 30, 30, 30, 
30. When we compare with this the result of an experiment 
made with alternate stimuli of 28 and 32 minutes, we seem 
to get the desired result. Thus in Exp. XXIX, the figures 
were — 28, 30, 27, 32, 27-5. In Exp. L we got — 23,37, 22, 28, 
30, 30. In Exp. LI — 25, 37, 17, 40, 19. In all these there 
is a clear indication of the double or unequal rhythm. But 
unfortunately it is possible to find similar results produced 
by the ordinary half-hourly stimulation. Thus in Exp. II 
(Annals of Botany, VI, p. 250) with a regular half-hourly 
geotropic stimulus we got — 18, 35, 14, 29. This is an 
unusually irregular rhythm for an experiment of this class, 
but it clearly renders it impossible to come to any conclusion 
as to the special character of the rhythm produced by unequal 
stimulation. 
The experiments are nevertheless interesting in another 
way, for there was an undoubted difference in the degree of 
curvature in the two directions. That is to say that after 
some hours of alternate illuminations of 28 and 32 minutes 
the plants became perceptibly concave on the side which 
received the longer light-stimulus 1 . 
1 The same effect was seen with the radicles of Sinapis , which gradually bent 
away from the more illuminated side. 
