510 PLANT RESPONSE 



of growth alternately, the total growth during the whole of 

 the time being the same as it would have been without alter- 

 nation ; and (2) that in which, the original rate of growth 

 having been feeble or absent, equal alternations of pressure 

 have the net effect of causing a positive increase of rectilinear 

 growth. Now, in an ordinary growing plant, rotating on the 

 klinostat, we have an instance of the first of these two cases ; 

 for if at any given moment the side A be below, and the 

 side B above, then B under stimulus of gravity will undergo 

 contraction ; hence there will be a diminution of local tur- 

 gidity, and the expelled water will produce an increase of 

 turgidity on the opposite side A. At the end of a semi- 

 revolution of the klinostat, however, this state of things will 

 be exactly reversed, and the alternating effects being thus 

 equal and opposite, there will be no resultant increase of 

 rectilinear growth ; but in the second case— that is to say, of 

 stationary or feebly growing grass haulms — the resultant 

 effect is not nil, but a positive increase of rectilinear growth. 



Summary 



It has been shown that the effect of gravitational stimulus 

 on an apogeotropic organ is fundamentally the same as 

 that of any other form of stimulus, namely, a responsive 

 contraction. 



A rational explanation of the mode in which geotropic 

 stimulus acts, is afforded by the radial-pressure theory of 

 Pfeffer and Czapek, or by the statolithic theory of Noll, 

 Haberlandt, and Nemec — the essential element of both lying 

 in the hypothesis that stimulus is caused by means of the 

 weight of the cell-contents acting differentially on the inner 

 wall of the horizontally placed cell. 



That the unilateral pressure of particles is competent to 

 induce responsive curvature of the organ, has been shown 

 experimentally by pressure resulting from the magnetic 

 attraction of iron particles. 



