398 MODIFIED CIECUMNUTATIOX. CHAP. VII. 



must extend the same conclusion to a large number of 

 sleeping plants; for the most complicated and the 

 simplest nyctitropic movements are connected together 

 by the finest gradations. But owing to the causes spe- 

 cified in the beginning of this chapter, it is impossible 

 in some few^ases to determine whether or not certain 

 movements should be called nyctitropic. Generally, 

 the position which the leaves occupy at night indi- 

 cates with sufficient clearness, that the benefit thus 

 derived, is the protection of their upper surfaces from 

 radiation into the open sky, and in many cases the 

 mutual protection of all the parts from cold by their 

 being brought into close approximation. It should be 

 remembered that it was proved in the last chapter, that 

 leaves compelled to remain extended horizontally at 

 night, suffered much more from radiation than those 

 which were allowed to assume their normal vertical 

 position. 



The fact of the leaves of several plants not sleeping 

 unless they have been well illuminated during the 

 day, made us for a time doubt whether the pro- 

 tection of their upper surfaces from radiation was in 

 all cases the final cause of their well-pronounced 

 nyctitropic movements. But we have no reason to 

 suppose that the illumination from the open sky, 

 during even the most clouded day, is insufficient for 

 this purpose ; and we should bear in mind that leaves 

 which are shaded from being seated low down on the 

 plant, and which sometimes do not sleep, are likewise 

 protected at night from full radiation. Nevertheless, 

 we do not wish to deny that there may exist cases in 

 which leaves change their position considerably at 

 night, without their deriving any benefit from such 

 movements. 



Although with sleeping plants the blades almost 



