CHAP. III. OF THE RADICLE OF THE BEAN. 151 



to bend over towards the injured side.* But in the 

 following experiments we endeavoured, generally with 

 success, to irritate the tips on one side, without badly 

 injuring them. This was effected by first drying the 

 tip as far as possible with blotting-paper, though it still 

 remained somewhat damp, and then touching it once 

 with quite dry caustic. Seventeen radicles were thus 

 treated, and were suspended in moist air over water at 

 a temperature of 58 J F. They were examined after 

 an interval of 21 h. or 24 h. The tips of two were 

 found blackened equally all round, so that they could 

 tell nothing and were rejected, 15 being left. Of 

 these, 10 were curved from the side which had been 

 touched, where there was a minute brown or blackish 

 mark. Five of these radicles, three of which were 

 already slightly deflected, were allowed to enter the 

 water in the jar, and were re-examined after an addi- 

 tional interval of 27 h. (i.e. in 48 h. after the appli- 

 cation of the caustic), and now four of them had 

 become hooked, being bent from the discoloured side 

 with their points directed to the zenith ; the fifth 

 remained unaffected and straight. Thus 11 radicles 

 out of the 15 were acted on. But the curvature of 

 the four just described was so plain, that they alone 

 would have sufficed to show that the radicles of the 

 bean bend away from that side of the apex which has 

 been slightly irritated by caustic. 



The power of an Irritant on the apex of the Radicle 



* Ciesielski found this to be the pended over water, with a thick 



case (' Untersuchungen tiber die layer of grease, which is very 



Abwartskriimmung der Wurzel,' injurious or even fatal to grow- 



1871, p. 28) after burning with ing parts; for after 48 hours 



heated platinum one side of a five of these radicles were curved 



radicle. So did we when we towards the greased side, two 



painted longitudinally half of the remaining straight, 

 whole length of 7 radicles, sus- 



