1908-1909.] The Sense-Organs of Plants. 167 



1900, 34. p. 507) and Newcombe ('Ann. of Bot./ 1902, 16. 

 p. 429), who have shown that the rheo tropic irritability is not 

 confined to the root- tip, but is possessed by the growing region 

 behind it as well. 



In Plate IX., fig. 2, are illustrated the rheo tropic curvatures 

 executed by some Pea roots in response to the stimulus of a 

 water current flowing at the rate of '3 mm. to '4 mm. per 

 second, — root 4 unstimulated growing straight downward, 

 while 1, 2, and 3 show positive rheotropic curvatures of vary- 

 ing extent. The necessary apparatus is described in Pfeffer's 

 * Plant Physiology,' iii. 185. 



Although considered together here for convenience, it is 

 not implied that there is any sort of relationship between the 

 phenomena underlying hydrotropism and rheotropism, for the 

 latter may yet, as F. Darwin has suggested (* Lectures on the 

 Physiology of Movement in Plants,' 1907, p. 8), prove to be 

 a variety of pressure irritability. 



Geotropism. — The pioneer experiments on the localisation 

 of geotropic sensitiveness in the root were those of Darwin 

 (' Movements of Plants,' 1880, p. 523). He found that decap- 

 itated roots did or did not give a curvature according as the 

 decapitation was performed after or before they had been sub- 

 jected to the gravity stimulus — that is, after or before they 

 were laid horizontally. From the fact that geotropic curvature 

 invariably occurred in the former but not in the latter case, 

 he concluded that the root-tip was the percipient organ. The 

 fact that the shock effects of the operation might complicate 

 the result led to some doubt as to the validity of the con- 

 clusion. Darwin's results were fully confirmed in 1894 by 

 the work of Czapek ('Annals of Botany,' 1894). Czapek's ex- 

 periment consisted in inducing the root to grow into a small 

 glass boot in such a manner that the root-tip — presumably 

 the sense-organ — was held at right angles to the motor area 

 — the sub-apical region. When the seedling was supported 

 so that the tip was vertical, there was no curvature; when 

 supported so that the tip was horizontal, curvature took place 

 in the motor area, bringing the apex — the percipient organ — 

 back to the vertical, the position of geotropic equilibrium. A 

 successful repetition of Czapek's experiment is illustrated 

 in Plate X., figs. 1, 2. The seedlings in fig. 1 show the 



