Chap. Ill SENSITIVENESS OF THE EADICLK 155 



curvature. The curvature amounted in some cases to 

 about 45° above the horizon, in others to 90°, and then 

 the tip pointed to the zenith. In one instance a 

 distinct upward curvature was observed in 8 h. 15 m., 

 but usually not until 24 h. had elapsed. Although 

 only 9 out of 39 radicles were affected, yet the curva- 

 ture was so distinct in several of them, that there could 

 be no doubt that the tip is sensitive to slight contact, 

 and that the growing part bends away from the touch- 

 ing object. It is possible that some secondary radicles 

 are more sensitive than others ; for Sachs has proved * 

 the interesting fact that each individual secondary 

 radicle possesses its own peculiar constitution. 



Sensitiveness to contact of the Primary Radicle, a little 

 above the afex, in the Bean ( Vioia faba) and Pea {Pisuni 

 sativum). — The sensitiveness of the apex of the radicle 

 in the previously described cases, and the consequent 

 curvature of the upper part from the touching object 

 or other source of irritation, is the more remarkable, 

 because Sachs j has shown that pressure at the distance 

 of a few millimeters above the apex causes the radicle 

 to bend, like a tendril, towards the touching object. 

 By fixing pins so that they pressed against the radicles 

 of beans suspended vertically in damp air, we saw this 

 kind of curvature ; but rubbing the part with a twig 

 or needle for a few minutes produced no effect. Haber- 

 landt remarks,^ that these radicles in breaking through 

 the seed-coats often rub and press against the ruptured 

 edges, and consequently bend round them.- As little 

 squares of the card-like paper affixed with shellac to 

 the tips were highly efficient in causing the radicles 

 to bend away from them, similar pieces (of about -j^th 



* ' Arbeilen Bnt. Instit., Wiirz- J ' Die Solmtzeinriohtungen dei 



lurg,' Heft. iv. 1874, p. 620. Keimpflanze,' 1877, p. 25. 



t Ibid. Heft iii. 1873, p. 437. 



