532 SENSITIVENESS TO GRAVITATION. CHAP. XL 



than the cauterised; but this no doubt was accidental, for 

 radicles of different ages grow at different rates, and the growth 

 of different individuals is likewise affected by unknown causes. 

 The state of the tips of these three radicles, which had been 

 cauterised for a rather longer time than usual, was as follows : 

 the blackened apex, or the part which had been actually touched 

 by the caustic, was succeeded by a yellowish zone, due probably 

 to the absorption of some of the caustic; in A, both zones 

 together were 1*1 mm. in length, and 1'4 mm. in diameter at the 

 base of the yellowish zone ; in B, the length of both was only 

 ' 7 mm., and the diameter ' 7 mm. ; in C, the length was 8 

 mm., and the diameter 1 '2 mm. 



Three other radicles, the tips of which had been touched with 

 caustic during 2 or 3 seconds, remained (temp. 58-59 F.) 

 horizontal for 23 h. ; the control radicles having, of course, 

 become geotropic within this time. The terminal growing part, 

 10 mm. in length, of the cauterised radicles had increased in 

 this interval to a mean length of 24 '5 mm., and of the controls 

 to a mean of 26 mm. A section of one of the cauterised tips 

 showed that the blackened part was 5 mm. in length, of which 

 0'2mm. extended into the vegetative point; and a faint dis- 

 coloration could be detected even to 1 6 mm. from the apex of 

 the root-cap. 



In another lot of six radicles (temp. 55-57 F.) the three 

 control specimens were plainly geotropic in 82 h. ; and after 24 h. 

 the mean length of their terminal part had increased from 

 10 mm. to 21 mm. When the caustic was applied to the three 

 cauterised specimens, it was held quite motionless during 



5 seconds, and the result was that the black marks were ex- 

 tremely minute. Therefore, caustic was again applied, after 

 82 h., during which time no geotropic action had occurred. 

 When the specimens were re-examined after an additional 

 interval of 15 h., one was horizontal and the other two showed, 

 to our surprise, a trace of geotropism which in one of them 

 soon afterwards became strongly marked; but in this latter 

 specimen the discoloured tip was only f mm. in length. The 

 growing part of these three radicles increased in 24 h. from 

 10 mm. to an average of 16 '5 mm. 



It would be superfluous to describe in detail the behaviour 

 of the 10 remaining cauterised radicles. The corresponding 

 control specimens all became geotropic in 8 h. Of the cauterised, 



6 were first looked at after 8 h., and one alone showed a trace 



