SLEEP MOVEMENTS 205 



these two extremes intermediate cases occur. In general 

 the movement is closely related to the diurnal change in 

 nature, but it may be carried on in uniform conditions, 

 exhibiting a periodicity due either to the after effects of 

 previous repeated movement, or, perhaps, to an inherent 

 character of the plant. This should be compared with the 

 account of flower movements. 



These sleep movements have of course nothing to do 

 with the repose of animals. What benefit the plant derives 

 through its leaflets folding up, as in the clover, or down, as 

 in the wood sorrel, is a question that has long exercised the 

 ingenuity of naturalists. Darwin (1880) suggested that the 

 vertical position, by diminishing radiation, saves the leaf 

 from damage on cold nights. His evidence in support of 

 this hypothesis was that clover leaves, kept open on cold 

 autumn nights, were seriously damaged, while those in the 

 natural sleep position were not. F. Darwin (1898) points 

 out that, while this may not apply to plants of the tropics, 

 where nyctinastic movements are commonest, sufficient 

 cooling might take place even in these to prevent the 

 ready translocation of starch, and so to interfere with 

 assimilation on the following day. Stahl (1897) held that the 

 chief effect of overcooling is the formation of dew on the 

 leaf surface, and produced experimental evidence to show 

 that dew formation seriously delays the beginning of tran- 

 spiration the next morning. He supposed that the sleep 

 position, by preventing dew formation, increases transpira- 

 tion and through it the supply of salts. We may here note 

 again that, if the avoidance of wetting by dew led to any 

 increase in transpiration in the early hours of the morning, 

 it would also increase the diffusion of carbon dioxide. 



It is possible that dew formation acts directly on the 

 stomata ; and we may note the fact that the stomata of 

 nyctinastic leaves are in many cases open throughout the 

 night. Loftfield states that when a leaf is wetted the 

 stomata usually open widely, and that when it dries they 

 close. If this is the general rule, then the resumption of 

 free gaseous diffusion in the morning would be further 



