CHAP. VIL MODIFIED CIRCUMNUTATION. 317 



CHAPTER VIL 



MODIFIED CIRCUMNUTATION : NYCTITROPIC OR SLEEP MOVEMENTS OP 

 LEAVES. 



Conditions necessary for these movements List of Genera and Families, 

 which include sleeping plants Description of the movements in 

 the several Genera Oxalis : leaflets folded at night Averrhoa : 

 rapid movements of the leaflets Porlieria: leaflets close when 

 plant kept very dry Tropseolum : leaves do not sleep unless well 

 illuminated during day Lupinus: various modes of sleeping 

 Melilotus : singular movements of terminal leaflet Trifolium 

 Desmodium: rudimentary lateral leaflets, movements of, not de- 

 veloped on young plants, state of their pulvini Cassia : complex 

 movements of the leaflets Bauhinia: leaves folded at night 

 Mimosa pudica : compounded movements of leaves, effect of dark- 

 ness Mimosa albida, reduced leaflets of Schrankia: downward 

 movement of the pinnae Marsilea : the only cryptogam known to 

 sleep Concluding remarks and summary Nyctitropisrn consists 

 of modified circumnutation, regulated by the alternations of light 

 and darkness Shape of first true leaves. 



WE now come to the iiyctitropic or sleep move- 

 ments of leaves. It should be remembered that we 

 confine this term to leaves which place their blades 

 at night either in a vertical position or not more than 

 30 from the vertical, that is, at least 60 above or 

 beneath the horizon. In some few cases this is 

 effected by the rotation of the blade, the petiole not 

 being either raised or lowered to any considerable 

 extent. The limit of 30 from the vertical is obviously 

 an arbitrary one, and has been selected for reasons 

 previously assigned, namely, that when the blade 

 approaches the perpendicular as nearly as this, only 

 half as much of the surface is exposed at night to the 



