Chap. lU. OF THE EADICLE OF PHASEOLUS. 163 



tips, tliese were touched with dry caustic. The, det.ails 

 of the experiment will be given in the chapter on 

 Geotropism, and it will suffice here to say that 10 

 peas, with radicles extended horizontally and not cau- 

 terised, were laid on and under damp friable peat ; 

 these, which served as standards or controls, as well as 

 10 others which had been touched on the upper side 

 with the caustic, all became strongly geotropic in 24 h. 

 Nine radicles, similarly placed, had their tips touched 

 on the lower side with the caustic ; and after 24 h., 



3 were slightly geotropic, 2 remained horizontal, and 



4 were bowed upwards in opposition to gravity and to 

 geotropism. This upward curvature was distinctly 

 visible in 8 h. 45 m. after the lower sides of the tips 

 had been cauterised. 



Little squares of card were affixed with shellac on 

 two occasions to the tips of 22 young and short 

 secondarij radicles, which had been emitted from the 

 primary radicle whilst growing in water, but were now 

 suspended in damp air. Besides tlie difficulty ol' 

 attaching the squares to such finely pointed objects 

 as were these radicles, the temperature was too high, 

 — varying on the first occasion from 72° to 77° F., and 

 on the second being almost steadily 78° F. ; and this 

 probably lessened the sensitiveness of the tips. The 

 result was that after an interval of 8 h. 30 m,, 6 of the 

 22 radicles were bowed upwards (one of them greatly) 

 in opposition to gravity, and 2 laterally ; the remain- 

 ing 14 were not affected. Considering the unfavour- 

 able circumstances, and bearing in mind the case of 

 the bean, the evidence appears sufficient to show that 

 the tips of the secondary radicles of the pea are 

 sensitive to slight contact. 



PJiaseolus multiflorus : Sensitiveness of the apex of the 

 Radicle. — Fifty-nine radicles were tried with squares 



