i88 LIFE AND WORK OF SIR JAGADIS C. BOSE 



Layer by means of the Electric Probe. A description of 

 the methods and their applications will be given presently. 



As regards the mechanism of the up-curving of a hori- 

 zontally laid stem, it may be due either to the expansion of 

 the lower or contraction of the upper surface ; and no 

 experimental test had been devised to decide between the 

 two alternatives the prevalent opinion, however, being 

 that the movement was due to expansion. Here then is 

 an apparent exception to Bose's demonstration that all 

 forms of stimulus induce contraction as their direct effect, 

 and expansion as the indirect effect. 



In order to subject the question to a crucial test, Bose 

 devised his extremely delicate electric method to find 

 whether the upper side of the horizontally laid stem remains 

 passive or exhibits an active state of excitation. He had 

 in his previous work on ' Comparative Electro- physiology ' 

 demonstrated that the state of excitation in a vegetable 

 tissue is exhibited by two simultaneous reactions of con- 

 traction and of an electric change of negative sign. Thus 

 the state of active excitation of any point of the tissue can 

 be detected with the greatest certainty by means of a 

 galvanometer. Bose connected two sides of a stem with the 

 galvanometer, and the displacement of the stem from the 

 vertical to the horizontal position was immediately followed 

 by the clearest indication that the upper was the excited 

 side. The electrical response w r as found to increase as the 

 angle of inclination to the vertical was increased from zero 

 to 90 degrees. This direct stimulus of the upper surface 

 involves its contraction and results in the geotropic 

 curvature of the stem upwards. 



The next puzzling question is in regard to the sense-organ 

 which enables the plant to perceive the vertical direction and 

 move accordingly. We get our idea of direction of force 

 of gravity by means of plumb-lines, and our own orienta- 

 tion in space is so far understood as dependent on the 

 semicircular canals associated with the internal ear ; and 



