536 SENSITIVENESS TO GRAVITATION. CHAP. XI 



that is to say, rather more than 6mm. as measured from the apex 

 of the root-cap. Nine other radicles were similarly extended, 

 three in damp peat and six in damp air, and dry caustic was 

 held transversely to their tips during 4 or 5 seconds. Three of 

 their tips were afterwards examined : in (1) a length of 0'68 mm. 

 was discoloured, of which the basal 0'136 mm. was yellow, the 

 apical part being black ; in (2) the discoloration was 65 mm. 

 in length, of which the basal 0'04mm. was yellow ; in (3) the dis- 

 coloration was ' 6 mm. in length, of which the basal O13 mm. 

 was yellow. Therefore less than 1 mm. was affected by the caustic, 

 but this sufficed almost wholly to prevent geotropic action ; for 

 after 24 h. one alone of the nine cauterised radicles became 

 slightly geotropic, being now inclined at 10 beneath the horizon ; 

 the eight others remained horizontal, though one was curved a 

 little laterally. 



The terminal part (10 mm. in length) of the six cauterised 

 radicles in the damp air, had more than doubled in length in 

 the 24 h., for this part was now on an average 20 '7 mm. long. 

 The increase in length within the same time was greater in 

 the control specimens, for the terminal part had grown on an 

 average from 10 mm. to 26 '6 mm. But as the cauterised 

 radicles had more than doubled their length in the 24 h., it is 

 manifest that they had not been seriously injured by the 

 caustic. We may here add that when experimenting on the 

 effects of touching one side of the tip with caustic, too much 

 was applied at first, and the whole tip (but we believe not more 

 than 1 mm. in length) of six horizontally extended radicles was 

 killed, and these continued for two or three days to grow out 

 horizontally. 



Many trials were made, by coating the tips of horizontally 

 extended radicles with the before described thick grease. The 

 geotropic curvature of 12 radicles, which were thus coated for 

 a length of 2 mm., was delayed during the first 8 or 9 h., but 

 after 24 h. was nearly as great as that of the control speci- 

 mens. The tips of nine radicles were coated for a length of 3 mm., 

 and after 7 h. 10 m. these stood at an average angle of 30 

 beneath the horizon, whilst the controls stood at an average of 

 54. After 24 h. the two lots differed but little in their degree 

 of curvature. In some other trials, however, there was a fairly 

 well-marked difference after 24 h. between those with greased 

 tips and the controls. The terminal part of eight control speci- 

 mens incveased in 24 h. from 10 mm. to a mean length of 



