CHAPTER XXIX 



THE GEO-ELECTRIC RESPONSE OF THE SHOOT 



Having described the geotropic response and its modifica- 

 tion under variation of external conditions, the question 

 arises as to the underlying mechanism by which stimulation 

 is effected. The only conceivable way in which gravity 

 can produce stimulation in the higher plants is by the 

 pressure of weight acting on the sensitive ectoplasm of the 

 cells. The pressure of weight can be exerted by the cell- 

 contents, whether the sap, or the heavy particles, crystals or 

 starch-grains, contained in the cell. The former, the 

 Theory of Hydrostatic Pressure, was suggested by Pfeffer 

 and supported by Czapek ; the other, the Theory of Stato- 

 liths, has been advocated by Noll, Haberlandt, and Nemec. 



In the case of a multicellular stem laid horizontally, 

 E and E' as indicated in the diagram (fig. 189) may be 

 regarded as the tissue in the cells of which stimulation is 

 caused by the pressure of particles. It is to be noted 

 that in the sensitive cells of the upper half the pressure is 

 exerted on the inner tangential protoplasmic layer, while in 

 the lower half the pressure is exerted on the outer ecto- 

 plasmic layer. Facts will be described which indicate that 

 the protoplasmic layer is not equally excitable on all sides of 

 the cell, which may possibly offer an explanation of the 

 opposite reactions on the two sides of the organ, the upper 

 exhibiting excitatory contraction, while the lower exhibits 

 the opposite reaction of expansion. 



Having discovered that all excitatory contractions in 

 plant-tissues can be detected by the induced galvanometric 



