412 MODIFIED CIRCUMNUTATION. CHAP. VII 



in gaining quickly its proper nocturnal position by 

 a direct course. In several other cases, for instance, 

 when a leaf after describing during the day one or 

 more fairly regular ellipses, zigzags much in the 

 evening, it appears as if energy was being expended, 

 so that the great evening rise or fall might coin- 

 cide with the period of the day proper for this 

 movement. 



The most complex of all the movements performed 

 by sleeping plants, is that when leaves or leaflets, 

 after describing in the daytime several vertically 

 directed ellipses, rotate greatly on their axes in the 

 evening, by which twisting movement they occupy 

 a wholly different position at night to what they do 

 during the day. For instance, the terminal leaflets 

 of Cassia not only move vertically downwards in the 

 evening, but twist round, so that their lower surfaces 

 face outwards. Such movements are wholly, or almost 

 wholly, confined to leaflets provided with a pulvinus. 

 But this torsion is not a new kind of movement 

 introduced solely for the purpose of sleep; for it 

 has been shown that some leaflets whilst describing 

 their ordinary ellipses during the daytime rotate 

 slightly, causing their blades to face first to one side 

 and then to another. Although we can see how the 

 slight periodical movements of leaves in a vertical 

 plane could be easily converted into the greater yet 

 simple nyctitropic movements, we do not at present 

 know by what graduated steps the more complex 

 movements, effected by the torsion of the pulvini, 

 have been acquired. A probable explanation could 

 be given in each case only after a close investigation 

 of the movements in all the allied forms. 



From the facts and considerations now advanced we 

 may conclude that nyctitropism, or the sleep of leaves 



