98 Darwin and Pertz. — On the artificial 
is represented along vertical lines (ordinates), and it will be 
seen that the times of rotation are at 10.38, 11.8, 11.38. The 
upward or downward curvature of the plant is represented 
by the line in the diagram travelling to the left or to the 
right. The letter D (for downward) being on the right in 
Period 97, it follows that until 10.38 movement to the left 
means an upward curvature, movement to the right a down- 
ward curvature. Thus from 10.12 a.m. to 10.22 the curvature 
was upwards ; at 10.22 (as shown in Table I) the curvature is 
suddenly reversed, and the plant curves downwards. In 
Period 98, owing to the rotation of the axis at 10.38, D must 
be placed on the left ; in this way it is indicated that after 
10.38 the hypocotyl was bending upwards, but the continuity 
of the curve-line from 10.22 to 10.51 shows that, considered as 
a growth-curvature, it is a single act ; it is in fact a single unit 
in what is practically half-hourly rhythm. 
In Period 99 the turning-point of the curve comes at 11.21, 
almost exactly half an hour after 10.51, as shown in the table 
and in the diagram. At 11.38 the klinostat was not allowed 
to turn : this is indicated by a thick curve-line, also by the 
absence of a numbered period and the absence of the symbol 
D in the space 11.38-12.8. At 11.38 the plant was placed 
with the original plane of curvature vertical, to avoid as far as 
possible geotropic stimulus in that plane. In spite of the 
freedom from alternate stimuli, the reversal of curvature took 
place at 11.47J, that is, minutes before the proper moment. 
By referring to Table I it will be seen that after the sharp 
turn at 11.472 the movement continued unchanged in 
direction until 12.30, when the observations were discon- 
tinued. In the diagram there is only room for the curve 
up to 12.10. 
Quarter-hourly Period. (Geotropism.) 
Six experiments were made, on cut stalks of a Valerian, 
with a klinostat rotation through 180° at intervals of 
15 minutes. In one experiment no curvature of any sort 
occurred, but in the other five cases a regular rhythm was 
