570 SUMMAKY AND CHAP. XIT. 



opposite direction might gradually be acquired through 

 natural selection.* 



Although so many movements have arisen through 

 modified circumnutation, there are others which 

 appear to have had a quite independent origin; but 

 they do not form such large and important classes. 

 When a leaf of a Mimosa is touched it suddenly 

 assumes the same position as when asleep, but Briicke 

 has shown that this movement results from a different 

 state of turgescence in the cells from that which 

 occurs during sleep ; and as sleep-movements are cer- 

 tainly due to modified circumnutation, those from a 

 touch can hardly be thus due. The back of a leaf of 

 Drosera rotundifolia was cemented to the summit of 

 a stick driven into the ground, so that it could not 

 move in the least, and a tentacle was observed during 

 many hours under the microscope; but it exhibited 

 110 circumnutating movement, yet after being mo- 

 mentarily touched with a bit of raw meat, its basal 

 part began to curve in 23 seconds. This curving 

 movement therefore could not have resulted from 

 modified circumnutation. But when a small object, 

 such as a fragment of a bristle, was placed on one side 

 of the tip of a radicle, which we know is continually 

 circumnutating, the induced curvature was so similar 

 to the movement caused by geotropism, that we can 

 hardly doubt that it is due to modified circumnu- 

 tation. A flower of a Mahonia was cemented to a 

 stick, and the stamens exhibited no signs of circum- 

 nutation under the microscope, yet when they were 

 lightly touched they suddenly moved towards the pistil. 

 Lastly, the curling of the extremity of a tendril when 



* See the remaiks in Frank's 91, &c ), on natural selection in 

 *Die wagerechte Idchtung von connection with geotropisni, helio- 

 Pnauzentheilen ' (1870, pp. 90, tropism, &c. 



