X PREFACE 



tropic curvature has been discovered in the transverse 

 conduction of excitation across the organ, by which the 

 response becomes gradually transformed from the positive 

 to the negative. The irritabiUty of the root has been found 

 to be in no way different from that of the shoot. A new 

 method for the physiological discrimination of gradation of 

 excitability in different layers of a tissue has led to the dis- 

 covery that there are certain cells in the lower half of the 

 pulvinus of Mimosa pudica which are far more sensitive and 

 active than the rest. 



In the investigation of geotropism, the exact direction of 

 the incident stimulus has been determined, as also the funda- 

 mental reaction under this mode of stimulation. The device 

 of the Photo-Geotropic Balance has made possible the com- 

 parison of the effects of two modes of stimulation on an 

 identical organ. Investigation of the various characteristics 

 of the geotropic reaction has been greatly facilitated by the 

 discovery of geo-electric response, first described in my 

 ' Comparative Electro-Physiology ' (1907). The geo-per- 

 ceptive layer has been locahsed by the Electric Probe, and 

 a probable explanation had been suggested of the opposite 

 geotropic curvature of the root and the shoot in the ex- 

 perimentally established fact that the stimulation of the 

 responding region of the root is indirect, whereas it is 

 direct in the shoot. 



Another important result is the demonstration of the 

 torsional response of dorsiventral organs under different 

 modes of lateral stimulation, and the establishment of the 

 Law of Torsional Response. The extension of this particular 

 method of inquiry has led to the solution of various problems 

 connected with the torsion of twining stems. 



Finally, from the fully demonstrated facts that direct 

 stimulation induces contraction while indirect stimulation 

 causes expansion, a wide generalisation has been established, 

 which includes within its scope the diverse tropic movements 

 of plant-organs. 



I have endeavoured to link all the observed facts together 



