540 



LECTURE XX. 



Among the Oxalidaceae the sleep-movement consists in the downward 

 sinking of the leaflets, the leaflets becoming at the same time somewhat 

 folded on themselves in the genus Oxalis ; in Averrhoa the leaves simply 

 hang vertically downwards. Among the Leguminosae the leaflets, in some 

 cases, simply sink vertically downwards (Phaseoleae) ; in others, they sink 

 down whilst the main petiole rises (terminal leaflet of Desmodium, Acacia 

 Farnesiand) ; in others, they sink downwards and twist on their axes so 

 that their upper surfaces are in contact beneath the main petiole (Cassia) ; 

 in others, again, they rise and bend backwards towards the insertion of 

 the petiole (Coronilla rosea)\ in others, they rise, and the main petiole 

 rises also (Lotus, Cytisus, Trigonella, Medicago), whereas in Mimosa 

 pudica the leaflets rise and bend forwards, whilst the main petiole falls ; 

 in some, finally, the leaflet (Melilotus) or the whole leaf (Lupinus) turns 

 through an angle of 90 so that its surfaces are vertical. The sleep- 

 movement of the leaflet of Phyllanthus Niruri resembles that of Cassia : 

 the leaves of Porlieria hygrometrica sink downwards : those of Thalia 

 dealbata and Maranta arundinacea (Cannaceae) rise up : in the Grass 

 Strephium floribunduni) the leaves on the upright shoots rise up at night, 

 whereas those on the horizontal shoots twist through an angle of nearly 

 90 so that the tips point towards the apex of the shoot the surfaces being 

 vertical: in Marsilea the leaflets rise up, the two upper leaflets being 

 embraced by the two lower. (See Darwin.) 



FIG. 58 (after Darwin). 



Tracing of the nyctitropic movement of a leaflet of 

 Averrhoa bilimbi. 



