TROPISMS 191 



underwent rapid decline and abolition. The electric indi- 

 cation at the perceptive layer itself became abolished as 

 soon as geotropic stimulus was removed by the restoration 

 of the organ to the vertical position. Bose is thus able 

 to map out the contour lines of physiological excitation 

 inside a living organ. 



After localising by means of his electric explorer the 

 perceptive layer, Bose made section of the organ and found 

 that the particular cells contained large-sized starch-grains, 

 which were instrumental in causing gravi-perception by 

 their weight. 



If the fall of the heavy particles on the sensitive ecto- 

 plasmic layer of the lower side of the cells be the cause of 

 geotropic excitation, then the geotropic response should 

 take place after an interval necessary for the heavy particles 

 to fall from the base to the side of the cell. This period 

 could not exceed more than a few seconds, but the geo- 

 tropic reaction, as hitherto observed, seemed to be initiated 

 much later after periods varying from several minutes 

 to an hour or more. Bose, however, with his magnifying 

 recorder, was able to detect the commencement of geotropic 

 curvature in less than a minute ; his electric method also 

 showed the latent period not to exceed a few seconds. 



In geotropic response the only anomaly that remained 

 was in regard to the response of the root being opposite to 

 that of the shoot. Bose showed that every cut portion of 

 the growing region of the shoot responds to the stimulus 

 of gravity by bending upwards. The growing region of the 

 shoot is therefore both sensitive to stimulus and responsive 

 to it. Hence geotropic stimulation of the shoot is direct. 

 But this is not the case with the root. Here it is the tip 

 of the root which perceives the stimulus, for Darwin showed 

 that when the root-tip is amputated the root loses its 

 orientation. The actual geotropic bending takes place 

 in the growing region at some distance from the tip. 

 The stimulus is received at the tip and transmitted to 

 the distant responding region of growth. Hence geotropic 



