248 Darwin and Pertz—On the Production 
spindle. It is not found necessary to carry on the ex- 
periment in the dark ; the axis of the plant being directed 
towards the window, heliotropic disturbance is sufficiently 
eliminated. An index is fixed to the free end of the shoot 
and its displacement in a vertical plane observed by means of 
a horizontal microscope 1 , or simply with a millimeter scale. 
The movements are at first irregular, and it is only after a 
varying period has elapsed that the half-hourly rhythm is 
produced. The following may serve as examples of the 
behaviour at first : — 
Exp. I. May 17, 1889. Valerian . 
p. 
Minutes. 
Movement. 
I 
30 
Down 
II 
30 
D 
I 
D 
III 
14 
Up 
14 
D 
4 
D 
IV 
2 3 
U 
2 
D 
It will be well to give a few explanations with regard to 
Exp. I, so that the following records may be comprehensible. 
The vertical columns headed P, minutes , and movement , are 
divided into spaces by horizontal lines, each space represent- 
ing half an hour : the first column ‘ P 5 simply gives the 
number of these half-hourly periods. It will be noticed that 
in the two last spaces observation was only carried on during 
twenty-nine minutes, made up by 14+14+1 and 4 + 23 + 2 
minutes ; this is because it is not always possible to record 
accurately either just before or just after the moment at 
which the klinostat makes its semi-turn. 
1 Each division of the micrometer eye-piece being equivalent to 0,024 mm * 
