1 86 LIFE AND WORK OF SIR JAGADIS C. BOSE 



petioles, and provokes their contraction on the further 

 side, thus neutralising their former bending (Fig. 22, d). 

 The organ now places itself at right angles to the light, 

 and this particular reaction has been termed dia-helio- 

 tropism. In certain cases the transverse conductivity of 

 the organ is considerable ; the result of this is an enhanced 

 excitation and contraction of the further side, while the 

 contraction of the near side is reduced on account of fatigue 

 caused by over-excitation. The organ thus bends away 

 from light or exhibits the so-called negative heliotropism 

 (Fig. 22, e}. These effects are accentuated when one side of 

 the organ is more excitable than the other. But in every 

 one of these cases the tracings obtained by Bose's self- 

 recording apparatus show first a movement towards light, 

 then neutralisation, and finally a movement away from 

 light. In this way a continuity of reaction is demonstrated, 

 proving that the assumption of specific positive and 

 negative heliotropic sensibility is unjustified. 



With this comprehension of the dual effects of light- 

 stimulus, the adjustment of leaves to receive light and 

 also in certain cases, as above noticed in the garden, to 

 escape excess of it may alike be unravelled : since we now 

 see that the more or less sensitive surface of the pulvinus on 

 which the leaf -adjustment usually depends may be variously 

 affected, even to definite twistings, as when a leaf-organ 

 is placed edgewise to the light. 



So far, then, for these common phenomena we have 

 now got a simple and uniform dynamic explanation behind 

 the familiar utilitarian one. But every botanist knows 

 cases of further difficulty. The common Indian cress 

 (Tropaeolum) turns towards light in winter, but away 

 from it in summer. Bose shows that the conduction of 

 ' nervous ' excitation in the plant is exalted, as in the 

 animal, by the rise, and lowered by the fall, of temperature. 

 The transverse conduction of excitation is thus enhanced 

 by higher temperature in summer ; the excitation in 

 this season more easily percolates across the stem, 



