IRRITABILITY. 503 



position which entails a horizontal direction of the axis of 

 growth. The degree of the plagiotropism induced by light 

 depends, in the first instance, upon the relative irritability of 

 the organ to light and to gravity, and further upon the angle 

 of incidence and the intensity of the rays. 



In endeavouring to illustrate the foregoing considerations 

 by reference to examples, we will begin with the case in 

 which the organs grow fully exposed to light, so that the rays 

 may be regarded as falling vertically downwards upon them. 



In the case of diageotropic dorsiventral organs, as ex- 

 amples of which we may mention the runners of Fragaria 

 lucida, by the lateral branches of Conifers and other trees 

 (p. 470), and by some leaves, the angle of deviation from the 

 vertical, under the assumed conditions of illumination, is either 

 a right angle or is greater than a right angle, so that the 

 direction of growth is either horizontal or obliquely de- 

 scending. The horizontal direction of growth is to be at- 

 tributed to an equality between diageotropism and dia- 

 heliotropism on the one hand, and photo-epinasty on the 

 other: the obliquely descending direction, to the pre- 

 ponderance of photo-epinasty over diageotropism and dia- 

 heliotropism. A striking instance of the latter is afforded 

 by F. Darwin's observation, that a cherry-leaf, exposed to 

 light falling vertically downwards upon it, curved downwards 

 epinastically. 



The angle of deviation of negatively geotropic dorsi- 

 ventral organs is, in some instances, lateral branches for ex- 

 ample, less than a right angle, so that the direction of growth 

 is obliquely ascending ; in others, as the shoots of Polygonum 

 aviculare (p. 450), the angle of deviation is a right angle, 

 so that the direction of growth is horizontal ; in others, 

 again, as the shoots of Lysimachia Nummularia (p. 450), 

 the angle of deviation is greater than a right angle, so that 

 the direction of growth is obliquely descending. In the 

 first case, the direction of growth is the resultant effect of the 

 antagonistic operation of negative geotropism on the one 

 hand, and of photo-epinasty and diaheliotropism on the 

 other; in the second case, diaheliotropism co-operates with 



