IRRITABILITY. 505 



is due to diaheliotropism is to be considered, for we know 

 that photo-epinasty is not affected by the direction of the 

 incident light. The most marked plagiotropic effect will be 

 produced when photo-epinasty and diaheliotropism co-operate, 

 and when the deviation of the organ takes place downwards 

 in a vertical plane. This will be produced when the median 

 plane of the organ, that is the longitudinal plane passing 

 through its dorsal and ventral surfaces, is vertical, and when 

 the incident rays lie also in a vertical plane. The most 

 effective angle of incidence of the rays in this plane is that in 

 v/hich they fall upon the apex of the organ, at angles be- 

 tween the vertical and the horizontal, the plagiotropic effect 

 increasing as the direction of the incident rays approaches 

 the horizontal. For instance, when a dorsiventral organ is 

 placed with its apex pointing vertically upwards, and light 

 falls vertically upon it, it will tend plagiotropically towards 

 the horizontal, and if its photo-epinasty is sufficient to over- 

 come the diaheliotropic and geotropic effects, its direction of 

 growth may be obliquely descending. Again, when a dorsi- 

 ventral organ is placed horizontally, with its morphologically 

 superior surface uppermost, and light falls horizontally on its 

 apex, it will tend downwards in consequence of photo-epinasty 

 and diaheliotropism, provided that they can overcome the 

 geotropic effect, until the long axis of the organ is directed 

 vertically downwards, until, that is, the highest possible de- 

 gree of plagiotropism is attained. An illustration of this is 

 afforded by F. Darwin's observations on the leaves of Ra- 

 nunciilus Ficaria, to which allusion was made in a previous 

 lecture (p. 448). 



When, however, the rays of light do not fall upon the 

 organ in a vertical plane, parallel to its own median plane, 

 but in planes more or less inclined to this, the plagiotropic 

 effect gradually diminishes as the plane of incidence is more 

 and more removed from the vertical towards the horizontal, 

 until, when the plane of incidence is horizontal, that is, when 

 the rays of light fall upon the flanks of the organ, no pla- 

 giotropic effect is produced. Under these circumstances the 

 organ simply twists round its long axis in consequence of 



