304 MODIFIED CIRCUMNUTATION. CHAP. VH 



its fellow. The movement of another leaflet, when asleep, 

 was traced between 6 P.M. and 10.35 P.M., and it clearly cir- 

 cumnutated, for it continued for two hours to sink, then rose, 

 and then sank still lower than it was at 6 P.M. It may lie 

 seen in Ihe preceding figure (167) that the leaflet, when the 

 plant was subjected to a rather low temperature in the house 

 descended and ascended during the middle of the day in a 

 somewhat zigzag line; but when kept in the hot-house from 

 9 A.M. to 3 P.M. at a high but varying temperature (viz., between 

 72 and 83 F.) a leaflet (with the petiole secured) circumnutated 

 rapidly, for it made three large vertical ellipses in the course of 

 the six hours. According to Brongniart, Marsilea pubescens sleeps 

 like the present species. These plants are the sole cryptogamic 

 ones known to sleep. 



Summary and Concluding Remarks on the Nyctitropio 

 or Sleep-movements of Leaves. That these movements 

 are in some manner of high importance to the plants 

 which exhibit them, few will dispute who have ob- 

 served how complex they sometimes are. Thus with 

 Cassia, the leaflets which are horizontal during the 

 day not only bend at night vertically downwards with 

 the terminal pair directed considerably backwards, but 

 they also rotate on their own axes, so that their lower 

 surfaces are turned outwards. The terminal leaflet 

 of Melilotus likewise rotates, by which movement one 

 of its lateral edges is directed upwards, and at the 

 same time it moves either to the left or to the right, 

 until its upper surface comes into contact with that ot 

 the lateral leaflet on the same side, which has like- 

 wise rotated on its own axis. With Arachis, all four 

 leaflets form together during the night a single 

 vertical packet; and to effect this the two anterior 

 leaflets have to move upwards and the two posterior 

 ones forwards, besides all twisting on their own axes. 

 In the genus Sida the leaves of some species move at 

 night through an angle of 90 upwards, and of others 



