TROPISMS 187 



reversing the normal positive curvature seen in winter. It 

 will thus be seen how diurnal, seasonal and climatic factors 

 may bring about modification in the response. 



Next we pass to ' Geotropism, Positive and Negative/ 

 the explanation of which offered difficulties almost un- 

 surmountable. From the youngest seedling to the lofty 

 tree, the shoot rises upwards, while the roots descend. 

 When laid flat, or inverted, the plant begins to right itself, 

 shoot and root turning in their respective directions. The 

 righting of the shoot is very manifest, and on the great scale, 

 in corn ' laid ' by the rain, of which the nodes soon renew 

 growth-activity and so raise the shoot anew. In itself, 

 the organism is thus as definitely bi-polar in its way as 

 is a magnet in its own. In and for this characteristic 

 behaviour gravity is evidently the external factor, to 

 which the organism has to adjust itself. Yet to understand 

 what may be this functional co-adjustment of organism 

 and environment has long been puzzling botanists. It 

 was at first thought that the descending root might be 

 merely sinking under its weight ; but with a basin of mercury 

 set below it, the root forces its way down against this potent 

 resistance, which would, of course, float it w r ere it passive. 

 Again, how can the shoot rise tens, even hundreds, of feet 

 against gravity ? And how can the same uniform stimulus 

 of gravity produce dual and contrary effects ? 



For the solution of these most difficult problems Bose 

 undertook investigation on the following subjects : 



1. What is the mechanism of the movement of response 

 under the stimulus of gravity ? 



2. Which is the particular layer of cells which perceives 

 the stimulus and acts as the sense-organ ? 



3. What is the reason of the opposite signs of response 

 in the shoot and in the root ? 



The research necessitated the invention of new methods 

 of investigation of extreme delicacy and reliability ; of 

 these may be specially mentioned the Method of Geo- 

 electric Response, and the Localisation of Geo-perceptive 



