CHAP. VIII. PARAHELIOTROPISM. 446 



the Anoda and of some species of Ipomoea, it was proved 

 that the downward movement did not depend on the 

 weight of the cotyledons ; but from the fact of the move- 

 ment being so much more strongly pronounced after 

 the cotyledons have grown large and heavy, we may 

 suspect that their weight aboriginally played some part 

 in determining that the modification of the circum- 

 nutating movement should be in a downward direction. 

 The so-called Diurnal- Sleep of Leaves, or ParaJielio- 

 tropism. This is another class of movements, dependent 

 on the action of light, which supports to some extent 

 the belief that the movements above described are 

 only indirectly due to its action. We refer to the 

 movements of leaves and cotyledons which when 

 moderately illuminated are diahelio tropic ; but which 

 change their positions and present their edges to the 

 light, when the sun shines brightly on them. These 

 movements have sometimes been called diurnal sleep, 

 but they differ wholly with respect to the object 

 gained from those properly called nyctitropic ; and in 

 some cases the position occupied during the day is the 

 reverse of that during the night. 



It has long been known* that when the sun shines brightly 

 on the leaflets of Piobinia, they rise up and present their edges 

 to the light ; whilst their position at night is vertically down- 

 wards. We have observed the same movement, when the 

 sun shone brightly on the leaflets of an Australian Acacia. 

 Those of AmphicarpcKa monoica turned their edges to the sun ; 

 and an analogous movement of the little almost rudimentary 

 basal leaflets of Mimosa alUda was on one occasion so rapid that 

 it could be distinctly seen through a lens. The elongated, uni- 

 foliate, first leaves of Phaseolus Roxburgh stood at 7 A.M. at 20 

 above the horizon, and no doubt they afterwards sank a little 

 lower. At noon, after having been exposed for about 2 h. to 



* Pfeffcr gives the names and dates of several ancient writers in his 

 ; Die Periodischen Bewegungen,' 1875, p, 62. 



