1898] RECENT PROGRESS IN ROOT-PHYSIOLOGY 389 



the adjoining base of the root curve upwards towards the shoot 

 part of the plant — as Czapek expresses it ; whilst the side-roots- 

 arising nearer the root-apex are indefinite, and may bend towards 

 the shoot or towards the main root. In one and the same individual 

 rootlets may be seen bending in both directions. 



Another manifestation of irritability by roots has lately received 

 renewed attention from Professor Spalding (14). In his work on the 

 " Power of Movement in Plants," Charles Darwin first examined the- 

 effect of attaching small pieces of cardboard and the like to the tip 

 of the root by means of shellac or gum-water, or of touching this 

 region of the organ with dry silver nitrate (caustic), or of slicing off 

 a small fragment from one side of the apex. The usual result was- 

 that the root executed a curvature, in the growing region, of such a 

 nature that the convexity was on the side of the object or cut. 

 Darwin concluded that the phenomenon was one of irritability and 

 that the apex was sensitive to the stimulation of touch. These 

 experiments of Darwin led to a long discussion, which, although 

 fruitful in many respects, yet left the main issues undecided. It 

 was clearly shown by Wiesner that the curvature was due, not to 

 simple contact, but to injury. Detlefsen believed, on the grounds of 

 his researches, that these curvatures — often called 'Darwin's curva- 

 tures '■ — had no connection with the irritability of the organ but 

 were purely mechanical results of the injury. Wiesner, likewise,, 

 favours a mechanical interpretation, but many of his experiments, 

 and statements may equally well be taken as upholding the opposite 

 view, which invokes the aid of protoplasmic irritability. Pfeffer, on 

 the other hand, regards mechanical explanations as insufficient, and 

 he suggests the name of ' Traumatropism ' for the phenomena result- 

 ing from such one-sided injuries. By this term he brings the pro- 

 cesses in question in line with the manifestations of geotropism and 

 heliotropism. 



In 1894 Spalding carried out a careful research on these- 

 traumatropic curvatures, and justified the name by bringing forward 

 good evidence for the belief that the phenomena were due to irrita- 

 bility. Most of his experiments were made by branding the root- 

 apex with a small, heated, glass rod. A root so treated executes a 

 curvature in the growing region, with the convexity towards the in- 

 jured spot, and also exhibits a second bend directly over the damaged 

 region but in the opposite direction to the first curve. Darwin had,, 

 likewise, observed this second bend and rightly — as Spalding points 

 out — attributed it to mechanical actions. Eoot-tips touched on one- 

 side with silver nitrate or metallic copper behaved in a similar manner., 

 Detlefsen connected his mechanical interpretation of Darwin's, 

 curvatures with the altered tension of the root-cap. In fact the 

 root-cap was, to his mind, alone responsible for the phenomenon. 



