314 MODIFIED CIECUMNUTATION. Chap. VL 



Kaphanus for only a single night. There are, howerer, 

 some strong exceptions to this rule, as the ootjledons 

 of Grossypium, Anoda and Ipoma3a do not possess pul- 

 vini, yet continue to move and to grow for a long time. 

 We thought at first that when the movement lasted for 

 only 2 or 3 nights, it could hardly be of any service 

 to the plant, and hardly deserved to be called sleep ; 

 but as many quickly-growing leaves sleep for only a 

 few nights, and as cotyledons are rapidly developed 

 and soon complete their growth, this doubt now seems 

 to us not well-founded, more especially as these move- 

 ments are in many instances so strongly pronounced. 

 We may here mention another point of similarity 

 between sleeping leaves and cotyledons, namely, that 

 some of the latter (for instance, those of Cassia and 

 Githago) are easily affected by the absence of light ; 

 and they then either close, or if closed do not open ; 

 whereas others (as with the cotyledons of Oxalis) are 

 very little affected by light. In the next chapter it 

 will be shown that the nyctitropic movements both 

 of cotyledons and leaves consist of a modified form of 

 circumnutation. 



As in the LeguminosaB and Oxalidte, the leaves and 

 the cotyledons of the same species generally sleep, the 

 idea at first naturally occurred to us, that the sleep 

 of the cotyledons was merely an early development of 

 a habit proper to a more advanced stage of life. But 

 uo such explanation can be admitted, although there 

 seems to be some connection, as might have been 

 expected, between the two sets of cases. For the 

 loaves of many plants sleep, whilst their cotyledons do 

 not do so — of which fact Vesmodium gyrans offers a 

 good instance, as likewise do three species of Nico- 

 tiana observed by us; also Sida rhomUfuUa, Ahutilon 

 Barwinii, and Ohenopodium album. On the othei 



