55 2 LECTURE XX. 



to Darwin, under the influence of bright sunlight the leaflets of 

 Robinia rise up, instead of sinking down as they do at night : 

 the leaflets of A mphicarpcea monoica twist so as to turn their 

 edges to the sun, whereas at night they sink down vertically; 

 those of Mimosa albida twist in a similar manner instead of 

 enfolding each other as they do at night. These cases are 

 clearly analogous to those of the leaves of the Compass- 

 plants and others which, under the influence of full exposure 

 to bright sunlight, assume, in the course of their growth, a 

 position such that their surfaces are in a vertical plane 

 (p. 446). 



The sensitiveness of motile leaves to the action of gravity 

 can be readily observed by hanging a plant possessing them 

 upside down in the dark. If this be done with Mimosa pudica, 

 for instance, it will be seen that the primary petioles raise 

 themselves so as to form an acute angle with the inter- 

 nodes which, in the normal position, are below them. The 

 primary petioles are clearly negatively geotropic. This 

 upward movement is accompanied usually by a twisting 

 either of the primary or of the secondary petioles which 

 brings the leaflets into such a position that their normally 

 upper surfaces are again directed towards the zenith. The 

 leaf-blade of Mimosa appears, then, to be diageotropic. 

 Pfeffer, however, does not accept this view, but regards the 

 twisting which brings the leaf-blade into its normal position 

 with reference to the vertical as a purely mechanical phe- 

 nomenon. The normal position of the leaf-surfaces is the 

 position of stable equilibrium of the leaf; when the leaf is 

 reversed it is brought into a position of unstable equilibrium, 

 and the result is twisting until the position of stable equilibrium 

 is again obtained. He observed in the case of a leaf of 

 Phaseolus that, the plant lying horizontally, there is no 

 torsion of the petiole when the lamina is cut off, but that 

 torsion at once ensues when the lamina is replaced by a piece 

 of paper of about the same weight. The views held with 

 regard to the diageotropism of motile leaves stand in the 

 same relation to each other as those respecting that of grow- 

 ing dorsiventral organs (p. 473), and all that can be said is 



