CHAP. VI. SLEEP OF COTYLEDONS. 311 



left cotyledon. This seems to be the object gained by these 

 singular movements. Altogether 20 seedlings were examined on 

 successive nights, and in 19 of them it was the left cotyledon 

 alone which became twisted, with the true leaf always so twisted 

 that its upper surface approached closely and fronted that of the 

 left cotyledon. In only one instance was the right cotyledon 

 twisted, with the true leaf twisted towards it ; but this seedling 

 was in an abnormal condition, as the left cotyledon did not rise 

 up properly at night. This whole case is remarkable, as with 

 the cotyledons of no other plant have we seen any nocturnal 

 movement except vertically upwards or downwards. It is the 

 more remarkable, because we shall meet with an analogous case 

 in the leaves of the allied genus Melilotus, in which the ter- 

 minal leaflet rotates at night so as to present one edge to the 

 zenith and at the same time bends to one side, so that its upper 

 surface comes into contact with that of one of the two now ver- 

 tical lateral leaflets. 



Concluding Remarks on the Nijditropic Movements o/ 

 Cotyledons. The sleep of cotyledons (though this is a 

 subject which has been little attended to), seems to be 

 a more common phenomenon than that of leaves. We 

 observed the position of the cotyledons during the day 

 and night in 153 genera, widely distributed through- 

 out the dicotyledonous series, but otherwise selected 

 almost by hazard; and one or more species in 26 of 

 these genera placed their cotyledons at night so as 

 to stand vertically or almost vertically, having gene- 

 rally moved through an angle of at least 60. If we 

 lay on one side the Leguminosae, the cotyledons of 

 which are particularly liable to sleep, 140 genera 

 remain ; and out of these, the cotyledons of at least one 

 species in 19 genera slept. Now if we were to select 

 by hazard 140 genera, excluding the Leguminosse, and 

 observed their leaves at night, assuredly not nearly 

 so many as 19 would be found to include sleeping 

 species. We here refer exclusively to the plants 

 observed by ourselves. 



