240 FREAKS OF PLANT LIFE. 



what they hold during the day, but with the one point 

 in common, that their upper surfaces avoid facing 

 the zenith, often with the additional fact that they 

 come into close contact with opposite leaves or leaf- 

 lets, clearly indicates, as it seems to us, that the 

 object gained is the protection of the upper surfaces 

 from being chilled at night by radiation. There is 

 nothing improbable in the upper surface needing pro- 

 tection more than the lower, as the two differ in 

 function and structure." ^ 



We are prepared to accept this as the most feasible 

 reason for the night movement of leaves, strengthened 

 as it is by experiments which were made in this 

 direction, and which demonstrated that leaves which 

 had been fixed in a horizontal position during nights 

 when the temperature was below freezing-point, 

 suffered much more from injury by frost than other 

 leaves on the same plant, which were permitted to 

 assume their usual vertical position. 



As it is an important feature to determine what is 

 the probable reason for this "sleep movement" in 

 leaves, we give verbatim the results of one experiment. 

 " We exposed on two occasions during the summer, 

 to a clear sky, several pinncd-open leaflets of Tri- 

 foHiim pratense which naturally rise at night, and of 

 Oxalis purpurea which naturally sink at night (the 



* Darwin, "The Movements of Plants," p. 284. 



