60 PLANT MIGRATION 
lower half, which when dry is much twisted, like a 
rope (Fig. 11). The covering of the seed itself is 
furnished with stiff short hairs pointing upwards. The 
whole structure when mature is cast off by the parent. 
The curiously twisted appendage is hygroscopic, and 
readily responds to wetness by untwisting and to dry- 
Fic, 11.—FRUIT OF STORKSBILL (ERODIUM). 32. 
a, Mature, twisting beginning ; b, separate fruit, fully twisted. 
ness by twisting. Should it be thus caused to un- 
twist when the upper end is free from obstruction the 
latter will revolve slowly like the hand of a clock. But 
should it meet with an obstacle in the course of its 
revolutions, such as a blade of grass, the motion is 
transferred to the lower end, which revolves like an 
