170 SENSITIVENESS OF THE APEX Chap. IH 



of card, and 13 were not acted on. Eather large 

 squares, though difficult to affix, seemed more efficient 

 than very small ones. 



We were much more successful with caustic ; but in 

 our first trial, 15 radicles were too much cauterised, 

 and only two became curved from the blackened side ; 

 the others being either killed on one side, or blackened 

 equally all round. In our next trial the dried tips 

 of 11 radicles were touched momentarily with dry 

 caustic, and after a few minutes were immersed iu 

 water. The elongated marks thus caused were never 

 black, only brown, and aboiit J mm. in length, or 

 even less. In 4 h. 3U m. after the cauterisation, 6 of 

 them were plainly curved from the side with the 

 brown mark, 4 slightly, and 1 not at all. The latter 

 proved unhealthy, and never grew ; and the marks on 

 2 of the 4 slightly curved radicles were excessively 

 minute, one being distinguishable only with the aid 

 of a lens. Of 10 control specimens tried in the same 

 jars at the same time, not one was in the least curved. 

 In 8 h. 40 m. after the cauterisation, 5 of the radicles 

 out of the 10 (the one unhealthy one being omitted) 

 were deflected at about 90°, and 3 at about 45° from 

 the perpendicular and from the side bearing the 

 brown mark. After 24 h. all 10 radicles had in- 

 creased immensely in length ; in 5 of them the curva- 

 ture was nearly the same, in 2 it had increased, and 

 in 3 it had decreased. The contrast presented by the 

 10 controls, after both the 8 h. 40 m. and the 24 h. 

 intervals, was very great ; for they had continued to 

 grow vertically downwards, excepting two which, from 

 some unknown cause, had become somewhat tortuous. 



In the chapter on Geotropism we shall see that 

 10 radicles of this plant were extended horizontally on 

 and beneath damp friable peat, under which conditions 



