INQUIRY INTO POSITIVE GEOTROPISM 



541 



stimulus, would accelerate growth ; while a section made in 

 a better-conducting tissue, or nearer the growing region, with 

 its stronger stimulation, would transmit the direct effect, with 

 concomitant retardation of growth. But a stimulation of 

 intermediate intensity would initiate no change. 



I shall, however, describe an investigation by which the 

 question of the variation of responsive excitability in the 

 growing region by amputation is put to the test of direct 

 experiment. For this purpose I mounted a straight water- 

 root of Bindweed in my Crescograph, and took a record of 

 its normal rate of growth. It should be stated that the 

 root was attached to the Optic 

 Lever at a distance of 3 mm. 

 below the tip, which was thus 

 left free for amputation. The 

 normal rate of growth was 

 considerable, being "0105 mm. 

 per minute. I next took a 

 record of three successive con- 

 tractile responses to thermal 

 shocks (fig. 227). After this, 

 without disturbing the ar- 

 rangement, the root-tip was 

 amputated, to a distance of 

 2 mm., and a record of the 

 growth was again taken after an interval of fifteen minutes. 

 It was now found to be practically the same as before amputa- 

 tion, the rate being -oioo mm. per minute. The contractile 

 responses to thermal shocks of the same intensity as before 

 were also, to all intents and purposes, the same. With another 

 specimen the rate of growth before section was -0066 mm. per 

 minute ; after amputation of 3 mm. of tip, the rate was 

 •0055 mm. The mechanical responses before and after section 

 were almost the same. These experiments, then, show that 

 amputation of the root-tip does not abolish the general ex- 

 citability of the responding region. The fact that the gravita- 

 tional response of the root, however, is abolished by the removal 



Fic. 227. Curves allowing Effect of 

 Amputation on Rate of Growth 

 and Response in Root of Biiuhved 



{a) continuous line indicates normal 

 rate of growth. Dotted curves 

 represent three responses to three 

 successive uniform stimuli ; {b) 

 exhibits the rate of growth and 

 responses after amputation. It 

 will l)e seen that there is practically 

 very little variation. 



