510 SENSITIVENESS TO GRAVITATION. Chap. XL 



tlie 8 h. 30 m. to a mean length of 13 mm. ; and in the controls 

 to 14-3 mm. 



In a third trial the tips of five radicles (exposed to a temp, 

 of 70°-71°) were touched with the caustic only once and very 

 slightly ; they were afterwards examined under the microscope, 

 and the part which was in any way discoloured was on an 

 average -76 mm. in length. After 4 h. 10 m. none were bent ; 

 after 5 h. 45 m., and again after 23 h. 30 m., they still remained 

 horizontal, excepting one which was now inclined 20° beneath 

 the horizon. The terminal part, 10 mm. in length, had in- 

 creased greatly in length during the 23 h. 30 m., viz., to an 

 average of 26 mm. Pour control radicles became slightly geo- 

 tropic after the 4 h. 10 m., and plainly so after the 5 h. 45 m. 

 Their mean length after the 23 h. 30 m. had increased from 

 10 mm. to 31 mm. Therefore a slight cauterisation of the tip 

 checks slightly the growth of the whole radicle, and manifestly 

 stops the bending of that part which ought to bend most under 

 the influence of geotropism and which still continues to 

 increase greatly in length. 



Concluding Remarks. — Abundant evidence has now 

 been given, showing that' with various plants the tip 

 of the radicle is alone sensitive to geotropism ; and 

 that when thus excited, it causes the adjoining parts 

 to bend. The exact length of the sensitive part seems 

 to be somewhat variable, depending in part on the age 

 of the radicle ; but the destruction of a length of from 

 less than 1 to 1"5 mm. (about -io^h. of an inch), in tht, 

 several species observed, generally sufficed to prevent 

 any part of the radicle from bending within 24 h., or 

 even for a longer period. The fact of the tip alone 

 being sensitive is so remarkable a fact, that we will 

 here give a brief summary of the foregoing experiments. 

 The tips were cut off 29 horizontally extended radicles 

 of Vicia faha, and with a few exceptions they did not 

 become geotropic in 22 or 23 h., whilst unmutilated 

 radicles were always bowed downwards in 8 or 9 h. It 

 should be borne in mind that the mere act of cutting 



