CHAP. VI. USE OF SLEEP MOVEMENTS. 285 



that one variety of the cherry has the petals of its 

 flowers much curled backwards, and after a severe 

 frost all the stigmas were killed ; whilst at the same 

 time, in another variety with incurved petals, the 

 stigmas were not in the least injured. 



This view that the sleep of leaves saves them from 

 being chilled at night by radiation, would no doubt 

 have occurred to Linnaeus, had the principle of radia- 

 tion been then discovered ; for he suggests in many 

 parts of his * Somnus Plantarum ' that the position of 

 the leaves at night protects the young stems and 

 buds, and often the young inflorescence, against cold 

 winds. We are far from doubting that an additional 

 advantage may be thus gained ; and we have observed 

 with several plants, for instance, Desmodium gyrans, 

 that whilst the blade of the leaf sinks vertically down at 

 night, the petiole rises, so that the blade has to move 

 through a greater angle in order to assume its vertical 

 position than would otherwise have been necessary ; but 

 with the result that all the leaves on the same plant 

 are crowded together as if for mutual protection. 



We doubted at first whether radiation would affect 

 in any important manner objects so thin as are many 

 cotyledons and leaves, and more especially affect dif- 

 ferently their upper and lower surfaces ; for although 

 the temperature of their upper surfaces would un- 

 doubtedly fall when freely exposed to a clear sky, yet 

 we thought that they would so quickly acquire by 

 conduction the temperature of the surrounding air, 

 that it could hardly make any sensible difference 

 to them, whether they stood horizontally and radiated 

 into the open sky, or vertically and radiated chiefly 

 in a lateral direction towards neighbouring plants and 

 other objects. We endeavoured, therefore, to ascer- 

 tain something on this head by preventing the leaves 



