CHAP. VI. SLEEP OF COTYLEDONS. 315 



hand, the cotyledons of some plants sleep and not the 

 leaves, as with the species of Beta, Brassica, Geranium, 

 Apium, Solanum, and Mirabilis, named in our list. 

 Still more striking is the fact that, in the same genus, 

 the leaves of several or of all the species may sleep, 

 but the cotyledons of only some of them, as occurs 

 with Trifolium, Lotus, Gossypium, and partially with 

 Oxalis. Again, when both the cotyledons and the 

 leaves of the same plant sleep, their movements may 

 be of a widely dissimilar nature : thus with Cassia the 

 cotyledons rise vertically up at night, whilst their 

 leaves sink down and twist round so as to turn their 

 lower surfaces outwards. With seedlings of Oxalis 

 Valdiviana, having 2 or 3 well-developed leaves, it 

 was a curious spectacle to behold at night each leaflet 

 folded inwards and hanging perpendicularly down- 

 wards, whilst at the same time and on the same plant 

 the cotyledons stood vertically upwards. 



These several facts, showing the independence of 

 the nocturnal movements of the leaves and cotyledons 

 on the same plant, and on plants belonging to the 

 same genus, lead to the belief that the cotyledons have 

 acquired their power of movement for some special 

 purpose. Other facts lead to the same conclusion, 

 such as the presence of pulvini, by the aid of which 

 the nocturnal movement is continued during some 

 weeks. In Oxalis the cotyledons of some species 

 move vertically upwards, and of others vertically 

 downwards at night ; but this great difference within 

 the same natural genus is not so surprising as it 

 may at first appear, seeing that the cotyledons of all 

 the species are continually oscillating up and down 

 during the day, so that a small cause might determine 

 whether they should rise or sink at night. Again, the 

 peculiar nocturnal movement of the left-hand coty- 



