PERCEPTION AND RESPONSE 221 



intensities of centrifugal action. Sachs is evidently incorrect in supposing 

 the lateral roots to be only very feebly geotropic, since when bent down- 

 wards they soon curve back to their proper plagio-geotropic position x . 



Our knowledge of the human eye or ear affords a good instance of 

 how the most intimate familiarity with the structure and localization of the 

 organs of perception fails to reveal the processes of sensation and perception. 

 Even if the electrical vibrations which we call light excited syntonic 

 electrical surgings in the rods and cones of the retina with whose length 

 their wave-length harmonizes, and even if the fibres of Corti's organ 

 resonated to the sound-waves travelling in the lymph of the inner ear, 

 we should still have advanced no further than when we found that the 

 curvature of a tendril was induced by the pressure of discrete particles upon 

 the sensitive epidermis. Hence, to speak of the heliotropic organs as 

 forming a field of heliotropic sense, and the geotropic ones as forming one 

 of geotropic sense, is simply to clothe facts already known in a new dress, 

 which does not conceal our ignorance concerning their intimate causation. 

 Noll's 2 attempts to elaborate stimulatory fields in cells or tissues, which 

 would theoretically produce the results actually observed, are devoid of 

 scientific value, and are in the first instance based upon the untenable 

 assumption that the orientation of the organ is directly dependent upon 

 the position of the supposed stimulatory fields in regard to the direction 

 of the orienting agency. Discussions of this kind, based on supposed 

 physical analogies, are usually highly misleading. It is possible to make 

 mechanical arrangements which will assume definite positions of equilibrium 

 according to the direction of incidence of light, gravity, or of contact - 

 stimuli, and which will return to the same position when disturbed ; but 

 no direct conclusions can be made upon a basis of this kind as to the 

 mode of orientation in the living organism. In other words, mechanical 

 models may serve to direct attention to vital phenomena, but afford no 

 explanation of them in the absence of any proof of a similarity of 

 mechanism. 



SECTION 51. Instances of Specific Tropic Irritability. 



THIGMOTROPISM affords a very good instance of the localization of 

 irritability, since a gentle touch which is insufficient to produce any 

 perceptible deformation in the epidermal cells stimulates the peripheral 

 layer of protoplasm and creates an excitation which spreads to the opposite 

 side. Diffuse contact on all sides does not excite the transitory accelera- 



1 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 248. 



a Noll, Heterogene Induction, 1892, p. 18 ; cf. also Fitting, Jahrb. f. wiss. Bot., 1903, 

 Bd. xxxvin, p. 619. 



