SLEEP MOVEMENTS 207 



leaflets have the under surfaces exposed, and the stomata 

 are twice as numerous on the upper " protected " surface. 



The general result is that there is an unmistakable 

 tendency for the stomata to be more numerous on that side 

 of the leaflet which is covered in the sleep position. It is 

 of interest that this tendency is much more marked when the 

 under surface, and less marked when the upper surface, 

 is covered. As the normal distribution of stomata is 

 greater on the under surface, in the latter case we have a 

 partial reversal, in the former an exaggeration, of the normal 

 distribution. There can be no doubt that Erban's results 

 point to some sort of protection of the stomata, and it remains 

 for critical studies of the transpiration and assimilation 

 conditions in such leaves to discover the precise effect of 

 this protection. When these leaves assume the vertical 

 position as a result of strong insolation, we are safe in 

 assuming that a reduction in transpiration will result. 

 There is always the possibility that day sleep is the primary 

 phenomenon, and that the much more striking and regular 

 night sleep is secondary, the result simply of the mechanical 

 properties and of the chemical processes in the pulvinus. 



§ 34. Functions of Transpiration 



Of the various features which have been described as 

 tending to promote transpiration it cannot be said, in any 

 case, that there is very good evidence of eff'ective action. 

 Nor can we draw from them much support for the theory 

 that transpiration is of primary importance to the plant. 

 At the beginning of this chapter we said that transpiration 

 might perform two useful functions — it might increase the 

 supply of salts, and it might reduce overheating. We may 

 now reconsider these two possibilities. 



Salt Supply. — ^In the behaviour of aquatic plants we 

 have the only convincing evidence in support of the salt 

 supply theory, but statistics of the ash contents of various 

 leaves are suggestive. Czapek has gathered the results 

 obtained by various investigators for ten different plants, 



