Chat. VIL MODIi'IED CIECUMNUTATION. 317 



CHAPTEE VII. 



MomriED CiEccMNUTATioN : Ntotitropio ok Sleep Movejibn'h of 

 Leaves. 



Conditiotis necessary for tliese nrnvements — List of Genera and Families, 

 which include sleeping plants — Description of the movements in 

 the several Genera — Oxalis : leaflets folded at niu;ht— Averr"hoa ; 

 rapid movements of the leaflets — Porlie'lii: leaflets close ■when 

 pLmt 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, eflect 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 — Nyctitropism consists 

 of modified circumnutation, regulated by the alternations of light 

 and darkness — SJiape of first true leaves. 



We now come to the nyctitropic 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, ouly 

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



