349 



Changes of Electrical Conductivity under Geotropic Stimulation. 

 Bv James Small, M.Sc. (Lond.), F.L.S. 

 (Communicated by Dr. A. D. ^Valler, F.E.S. Received May 11, 1917.) 



The very short perception period* for the epieotyl and hypocotyl or 

 various plants, which Fitting (3) has proved, and the presence of geotropic 

 response in the absence of starch grains in many fungi and higher plants 

 tend to indicate that the starch grain or statohth apparatus is not absolutely 

 necessary for the perception of gravity by plants. The differential effect of 

 gra\-ity on the permeabihty of the upper and under sides of the pulvinus 

 of Phaseolus indicates the possibility of a similar effect being produced in 

 the roots of the plants. 



Fitting's numerous experiments were examined in order to ascertain 

 whether the strength of the geotropic reaction bore any constant relation to 

 the geotropic stimulus. Putting the data given in Fitting's Table 10 [(3), 

 Teil I, p. 282] in the form of a graph, we get a curve (fig. 1) which is 



angle witm vertical 

 Fig. 1. 



* Perceptifm period is used here, not in the psychological sense, but in the sense 

 generally accepted among botanists, and defined as the smallest fraction of the presenta- 

 iion time which gives a cumulative efiect when the stimulus is repeated for a sufficient 

 number of times with less than the relaxation time between the successive stimulations. 



