GEO-ELECTRIC RESPONSE 



435 



contents, whether of the sap itself or of those heavy particles 

 like starch-grains, which are contained in it. The former, or 

 Theory of Hydrostatic Pressure, was suggested by Pfeffer 

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

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



In the case of a multicellular plant laid horizontally 

 (fig. 264), E and E' may be regarded as areas in which stimu- 

 lation is caused by the 

 weight of the particles. 

 It is obvious that the 

 effects produced on the 

 upper and lower sides of 

 the shoot are antago- 

 nistic; yet, in spite of 



FIG. 264. Diagrammatic Representation of 

 a Multicellular Organ laid Horizontally 

 and Exposed to Geotropic Stimulus. 



On the upper side the statoliths act on the 

 inner, and on the lower side on the 

 outer, tangential wall (after Francis 

 Darwin). 



this, we obtain a resultant 

 curvature upwards. This 

 shows that the excitation 

 of one side must be 

 greater than that of the 

 other, the particular direc- 

 tioned mechanical effect 

 being due to this fact. 



It is clear, then, that the induced curvature upwards of the 

 horizontally-laid shoot is due to the effective action of the 

 weight particles on either the upper or the lower side of the 

 shoot. We can see that if it is the upper which is the more 

 effective, then curvature must take place by excitatory con- 

 traction ; if, on the contrary, the lower be the more effective, 

 the curvature is then to be regarded as the result of respon- 

 sive expansion. There has been considerable uncertainty as 

 to which of these is actually the case, the prevailing view 

 being that it is the expansion of the lower surface which is 

 the active factor. 1 



That it is, however, the excitatory contraction of the 

 upper side which is the active factor in this curvature I have 

 already demonstrated by alternate unilateral applications of 



1 For more detailed account see Plant Response, pp. 495 to 511. 



F F 2 



