ioo Darwin and Pertz . — On the artificial 
the curvature of the plant was reversed at the end of the 
i hr., and again of the next £ hr. These movements could 
not be due to physical drooping. 
Four of these showed the 
P Vi 
Quarter-hourly Period. (Heliotropism.) 
Six 1 experiments were made on the heliotropic curvature 
of Phalaris canariensis with the quarter-hourly rhythm. 
stimulated rhythm,’ and two 
showed also c unstimulated 
rhythm,’ i. e. a rhythm con- 
tinued after the klinostat 
stopped. One of these made 
a single turn, and the other 
made two such reversals of 
curvature. 
Exp. Ill, Fig. 13, May 10, 
1900. Phalaris . Quarter- 
hourlyperiod. (Heliotropism.) 
The experiment is remark- 
able as showing that the 
plant may acquire a rhythm 
in a very short time, e. g. 
after four periods of £ hr. 
each. It should be noted 
that in heliotropic diagrams 
the letters D and L mean 
Dark and Light, and that 
they change sides in each 
period precisely as the letters 
D , U (for Down and Up) alternate in the geotropic diagram, 
Fig. 12. Here again the curve changes direction syn- 
chronously with the rotation of the klinostat, but in this 
case there can be no question of a purely physical droop 
(as in Exp. II), since the axis of rotation is vertical. Neither 
in heliotropic nor geotropic experiments is this synchrony 
1 Several experiments were vitiated by the slow or oblique growth of the seed- 
lings, and are therefore omitted. 
P IV 0 
L 
p III L \ 
D 
\ 
PI! ° 
L 
L 
\ D 
P i 
I 
Fig. 13. 
