v Movements of Plants 91 



on unequal distribution of longitudinal growth. Moreover, 

 his experiments on the horizontal runners of the straw- 

 berry, and those of Elfving on rhizomes "paved the way 

 for the theory that there are a variety of different organisa- 

 tions (or, as we now say, irritabilities) in growing plants ; 

 and that, whether a plant grows vertically upwards, or down- 

 wards, or horizontally, depends on the individual and highly 

 sensitive constitution of the plant in question." Frank's 

 views were, as we have seen, accepted by the authors of 

 The Power of Movement in Plants, although Frank's 

 particular interpretation of the irritabilities as due to 

 " polarities " was not. 



Francis Darwin then points out how the development of 

 our present views on irritability was delayed by the in- 

 sufficient theory of light-seeking, as implied, for instance, 

 in De Candolle's explanation, that curvature towards the 

 light was simply due to the more rapid growth of the 

 shaded side. Fuller acquaintance with the facts, e.g. of 

 plants which curve away from the light, showed that this 

 again too mechanical theory was insufficient, and led on to 

 the idea, repeatedly expressed in The Power of Movement, 

 that light and gravitation act merely as landmarks by which 

 the plant can direct itself. Pfeffer, Sachs, and Vines, and 

 other botanical physiologists are at one in regarding 

 growth-curvatures as phenomena of irritability, as responses 

 to gravitation, light, and other stimuli. 



So far the general question of irritability. Mr. Darwin 

 then proceeds to discuss that of mechanism. 



" The first step in advance of Hofmeister's views was 

 the establishment of the fact that the curvatures under 

 consideration are due to unequal growth that is to say, 

 to greater longitudinal growth on the convex than on the 

 concave side." Frank made important contributions to the 

 subject, and Sachs thoroughly demonstrated that the con- 



