532 SENSITIVENESS TO GRAVITATION. Ohap. XL 



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

 radicles of different ages grow at different r.ites, and the growth 

 of different individuals is likewise affected by unknown causes. 

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

 caiiterised for a rather longer time than usual, was as f-dlows ; 

 the blackened apex, or the part wliich 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 I'l mm. in length, and 1'4 mm. indiarrieterat tlie 

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

 7 mm., and the diameter 0'7 mm.; in C, the length was 0'8 

 mm., and the fliameter 1 '2 mm. 



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

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

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

 become geotropio 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. fiom the apex of 

 the root-cap. 



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

 control specimens were plainly geotropic in 8i 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 

 Si h., during which time no geotropio action had occurred 

 When the specimens were re-examined after an additional 

 interval of 15 i 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 2i 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 Rpecimens all bei-nme geotropic in 8 h. Of the cauterised 

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



