450 LECTURE XVII. 



and to epinasty, and when the movement is one of torsion, to 

 these factors more or less interfered with in their operation by 

 the mechanical moment of the heavy lamina. Wiesner also 

 attributes these movements to negative geotropism, positive 

 heliotropism, epinasty, weight of lamina, etc. 



It would seem to be almost impossible to account for all 

 these phenomena, especially those exhibited by the Compass- 

 plants, in the way proposed by de Vries and by Wiesner, even 

 admitting that all the agents above enumerated are in opera- 

 tion. We have, however, seen reason to come to the conclusion 

 that dorsiventral organs are not endowed with either positive 

 or negative heliotropism, and we have learned from F. Darwin's 

 experiments that the movements in question are performed by 

 leaves when rotating on the clinostat, that is, when the effect 

 of gravity is eliminated. These movements, then, cannot be 

 due in any degree to negative geotropism. Under these 

 circumstances the only course left to us is to accept Frank's 

 suggestion that these organs possess diaheliotropic irritability. 



In concluding this subject we may advantageously sum up 

 in a concise form the conclusions to which a full consideration 

 of all the available facts has led us, We have found that 

 dorsiventral organs are photo-epinastic and diaheliotropic. 

 It is clear that when the photo-epinasty of an organ is well- 

 marked, its diaheliotropism will be less so. Thus Frank 

 observed that in Lysimachia Nummularia when growing on 

 sloping ground, the shoots were directed downwards and 

 were closely appressed to the sloping surface, so that their 

 long axes made an angle of considerably more than 90 with 

 the direction of the incident rays of light. In Polygonum 

 aviculare he observed, on the contrary, that, under the same 

 circumstances, the shoots proceeding from the lower side of the 

 plant raised themselves from the soil so that their long axes 

 made an angle of 90 with the direction of the incident rays. 

 The explanation of these facts seems to be that in the former 

 case the photo- epinasty overcomes the diaheliotropism, whereas 

 in the latter the diaheliotropism overcomes the photo-epinasty. 

 The same thing was observed by F. Darwin in the course of his 

 experiments with leaves. As already mentioned, the leaves of 



