Chap, xi Sensitivity and Stimulation 479 



We have already seen that roots manifest both helio- 

 tropic and geotropic sensitiveness. Their endowment with 

 contact irritability as well was first observed by Sachs in 

 1873, when he described a curvature resulting from their 

 coming into contact with particles of soil. The root bent 

 in such a way that the stimulated side became concave. 

 This curvature can be explained on purely mechanical 

 principles. It follows a pressure upon the part of the root 

 which is the seat of more or less active growth and is the 

 result of the inhibition or diminution of growth at the part 

 so touched, the other side of the root continuing to grow 

 and so becoming convex. The effect is thus more akin 

 to the result of an injury than to that of a stimulus. In 

 1880 Darwin observed what Wiesner later called the Dar- 

 winian curvature, which is a definite result of stimulation, 

 the tip being diverted away from the obstacle touching it 

 by a bending of the axis at a point higher up. Detlefsen, 

 in 1882, and Burgerstein, in 1883, opposed Darwin's explana- 

 tion of the movement as a response to contact stimulation, 

 claiming that it is preferably to be regarded as traumatic, 

 for injury of the same part by cutting, branding, or treat- 

 ment with caustics, produces the same effect, though often 

 giving rise to more irregular movements. The criticism does 

 not appear to be very destructive, as it seems quite possible 

 that the differences between slight contact and severe injury, 

 chemical or mechanical, may be only of degree and not 

 of kind. 



Darwin's investigations showed clearly that there is a 

 sensitive area, and a motor one, which differ in position, 

 and that the movement is the result of stimulation and 

 not of direct mechanical injury of the tissues. The explana- 

 tion which Darwin gave of the greater growth of the convex 

 side has been accepted generally, but a detailed study 

 of its mechanism was carried out in 1896 by McDougal. 

 To this point we shall return in connexion with the general 



