NYCTITBOPIC MOVEMENTS. 409 



1063. The ample revolving movement is not confined to stems, 

 but is observed in some modified branches and leaves, for ex- 

 ample in certain ten- 

 drils, etc. A single 



instance will serve to 

 show the remarkable 

 nature of the move- 

 ment in the case of 

 the tendrils of Echi- 

 nocystis lobata, as de- 

 scribed by Darwin : 1 

 "These are usually 

 inclined at about 45 



1OJ. 



above the horizon, but 



they stiffen and straighten themselves so as to stand upright in 

 a part of their circular course ; namely, when they approach and 

 have to pass over the summit of the shoot from which they arise. 

 If they had not possessed and exercised this curious power, they 

 would infallibly have struck against the summit of the shoot and 

 been arrested in their course. As soon as one of these tendrils 



with its three branches be- 

 gins to stiffen itself and rise 

 up vertical!}-, the revolving 

 motion becomes more rapid ; 

 and as soon as it has passed 

 over the point of difficulty, 

 its motion coinciding with 

 that from its own weight 



causes it to fall into its previously inclined position so quickly 

 that the apex can be seen travelling like the hand of a gigantic 

 clock." 



1064. Nyctitropic, or sleep, movements. The foliar organs of 

 man}- plants assume at nightfall, or just before, positions unlike 

 those which they have maintained during the day. In many 

 cases the drooping of the leaves at night is suggestive of rest, 

 and the name given by Linnaeus to this group of phenomena, 

 namely, "the sleep of plants," seems appropriate. But in numer- 

 ous cases the nocturnal position is one of obvious constraint, 

 and considerable force has to be expended in lifting the leaf to 



1 Power of Movement in Plants, 1880, p. 266. 



FIG. 181. Leaf of Coronilla rosea at night, f Darwin.) 



Fm. 182. Leaf of White Clover. A, day position ; B, night position. (Darwin.) 



