156 SENSITIVENESS OF THE Chap. I ft 



inch square, or rather less) were attached in the same 

 manner to one side of the radicle at a distance of 3 or 

 4 mm. above the apex. In our first trial on 15 radicles 

 no effect was produced. In a second trial on the same 

 number, three became abruptly curved (but only one 

 strongly) towards the card within 24 h. From these 

 cases we may infer that the pressure from a bit of card 

 affixed with shellac to one side above the apex, is hardly 

 a sufficient irritant ; but that it occasionally causes the 

 radicle to bend like a tendril towards this side. 



We next tried the effect of rubbing several radicles 

 at a distance of 4 mm. from the apex for a few seconds 

 with lunar caustic (nitrate of silver) ; and although the 

 radicles had been wiped dry and the stick of caustic 

 was dry, yet the part rubbed was much injured and a 

 slight permanent depression was left. In such cases 

 the opposite side continues to grow, and the radicle 

 necessarily becomes bent towards the injured side. 

 But when a point 4 mm. from the apex was momen- 

 tarily touched with dry caustic, it was only faintly 

 discoloured, and no permanent injury was caused. This 

 was shown by several radicles thus treated straighten- 

 ing themselves after one or two days ; yet at first they 

 became curved towards the touched side, as if they had 

 been there subjected to slight continued pressure. 

 These cases deserve notice, because when one side of 

 the apex was just touched with caustic, the radicle, as 

 we have seen, curved itself in an opposite direction, that 

 is, away from the touched side. 



The radicle of the common pea at a point a little 

 above the apex is rather more sensitive to continued 

 pressure than that of the bean, and bends towards the 

 pressed side.* We experimented on a variety {York- 



* Sachs, ' Aibeiten Bot, lustitut., Wurzburg,' Heft iii. p. 438. 



