Cahp. II. CIECUMNUTATION OF COTYLEDONS. Ill 



time they are expanded almost horizontally. The 

 circumnutating movement is thus at least partially 

 periodic, no doubt in connection, as we shall hereafter 

 see, with the daily alternations of light and darkness. 

 The cotyledons of several plants move up so much at 

 uight as to stand nearly or quite vertically ; and in 

 tliis latter case ihey come into close contact with one 

 another. On the other hand, the cotyledons of a 

 few plants sink almost or quite vertically down at 

 night ; and in this latter case they clasp the upper 

 part of the hypocotyl. In the same genus Oxalis the 

 cotyledons of certain species stand vertically up, and 

 those of other species vertically down, at night. In 

 all such cases the cotyledons may be said to sleep, 

 for they act in the same manner as do the leaves of 

 many sleeping plants. This is a movement for a 

 special purpose, and will therefore be considered in a 

 future chapter devoted to this subject. 



In order to gain some rude notion of the proportional 

 number of cases in which the cotyledons of dico- 

 tyledonous plants (hypogean ones being of course 

 excluded) changed their position in a conspicuous 

 manner at night, one or more species in several 

 genera were cursorily observed, besides those described 

 in the last chapter. Altogether 153 genera, included 

 in as many families as could be procured, were thus 

 observed by us. The cotyledons were looked at in 

 the middle of the day and again at night ; and those 

 were noted as sleeping which stood either vertically 

 or at an angle of at least 60^ above or beneath the 

 horizon. Of such genera there were 26 ; and in 21 of 

 them the cotyledons of some of the species rose, and 

 in only 6 sank at night; and some of these latter 

 cases are rather doubtful from causes to be explained 

 in the chapter on the sleep of cotyledons. When 

 6 



