TROPISMS 189 



these are believed to function through the effect of gravity 

 on their contained fluid in our varying positions, and its 

 changing flow and pressure with our movements. In 

 water animals, whose specific gravity is little different 

 from that of the water they inhabit, heavy solid bodies 

 come into service : the large ' otoliths ' of the fish's ear, 

 and the sand-grains, mingled with tactile hairs, in the 

 lobster's. So if it be by such stimulus of solid particles, 

 with their always vertical fall, that animals are oriented, 

 must not the solid granules of various composition, albu- 

 minoid, starchy and other, which are found free in many 

 vegetable cells, have a similar action on their protoplasm 

 and practically serve as otoliths, giving the needed signal 

 and stimulus for proper orientation? Definite layers of 

 starch grains have been found in microscopic sections of 

 the plant, and from anatomical considerations of their dis- 

 tribution the theory of statoliths has been ably advocated 

 by Noll, Haberlandt, Nemec and others. 



The direct test needed for the localisation of geo- 

 perceptive layer is, however, the physiological reaction 

 of the living plant, giving unmistakable signal of its 

 perception of geotropic stimulus as it is disturbed from 

 its normal vertical position. Bose now worked out the 

 highly original device of his Electric Probe, by means of 

 which he is able to explore the interior of the plant and 

 detect the state of excitation in its different layers . Suppose 

 G and G 1 to be the layers of cells in a stem concerned in 

 the perception of the stimulus of gravity, G G 1 being the 

 longitudinal section of an annular ring (Fig. 23, p. 158). As 

 long as the stem remains vertical, geotropic stimulation will 

 be absent, but inclination to the vertical will cause irritation. 

 Bose's Electric Probe consists of an exceedingly fine platinum 

 wire, enclosed in a capillary glass tube, the probe being 

 electrically insulated except at the extreme tip. When 

 the probe, suitably connected with a galvanometer, is 

 slowly thrust into the stem so that it enters one side and 

 comes out at the other, the galvanometer will by its 



