CHAP. III. OF THE KADICLE OF THE BEAN. 149 



in the fourth case alone no reason could be assigned 

 why the radicle had not been at all deflected. These 

 experiments suffice to prove that the apex of the 

 radicle possesses the extraordinary power of discri- 

 minating between thin card and very thin paper, and 

 is deflected from the side pressed by the more re- 

 sisting or harder substance. 



Some trials were . next made by irritating the tips 

 without any object being left in contact with them. 

 Nine radicles, suspended over water, had their tips 

 rubbed, each six times with a needle, with sufficient 

 force to shake the whole bean ; the temperature was 

 favourable, viz. about 63 F. In 7 out of these cases 

 no effect whatever was produced ; in the eighth case 

 the radicle became slightly deflected from, and in the 

 ninth case slightly deflected towards, the rubbed side ; 

 but these two latter opposed curvatures were probably 

 accidental, as radicles do not always grow perfectly 

 straight downwards. The tips of two other radicles 

 were rubbed in the same manner for 15 seconds with 

 a little round twig, two others for 30 seconds, and two 

 others for 1 minute, but without any effect being pro- 

 duced. We may therefore conclude from these 15 

 trials that the radicles are not sensitive to temporary 

 contact, but are acted on only by prolonged, though 

 very slight, pressure. 



We then tried the effects of cutting off a very thin 

 slice parallel to one of the sloping sides of the apex, 

 as we thought that the wound would cause prolonged 

 irritation, which might induce bending towards the 

 opposite side, as in the case of an attached object. 

 Two preliminary trials were made : firstly, slices were 

 cut from the radicles of 6 beans suspended in damp 

 air, with a pair of scissors, which, though sharp, 

 probably caused considerable crushing, and no curva- 



