CHAPTER XXX 



LOCALISATION OF GEO-PERCEPTIVE LAYER BY ELECTRIC 



PROBE 



Having found that plant-organs are sensitive to geotropic 

 stimulation, the question arises as to the distribution of the 

 cells which perceive the stimulus, the impulse from which 

 causes neighbouring cells to carry out a movement of orienta- 

 tion in a definite direction. Are the perceptive cells diffusely 

 distributed in the plant or do they form a definite layer ? 

 Would it be at all possible to localise the sensitive cells 

 in the interior of the organ ? 



It is true that post-mortem examination of sectioned 

 tissues under the microscope suggested the statolith-theory, 

 according to which the contents of certain cells cause 

 geotropic excitation. But for the clear understanding of 

 the physiological reaction which induces the orientating 

 movement, it is necessary to get hold, as it were, of the 

 sensory cells in a condition of full vital activity ; to detect 

 and follow the changes induced in the perceptive organ and 

 the irradiation of excitation to neighbouring cells, through 

 the entire cycle of reaction from the onset of geotropic 

 stimulation to its cessation. 



The idea of obtaining access to the hypothetical geo- 

 perceptive cells in the interior of the organ would appear 

 at first sight to be almost hopeless ; the problem has been, 

 however, solved by the invention of the Electric Probe, by 

 which it has been possible to locate the excitatory electric 

 conditions in the interior of the plant. 



