148 SENSITIVENESS OF THE APEX Chap. 111. 



of size of an object whicli will act on the radicle of 

 the bean. But it is remarkable that when the bits of 

 bristle did act, that they should have acted so quickly 

 and efficiently. 



As the apex of a radicle in penetrating the ground 

 must be pressed on all sides, we wished to learn 

 whether it could distinguish between harder or more 

 resisting, and softer substances. A square of the sanded 

 paper, almost as stiff as card, and a square of extremely 

 thin paper (too thin for writing on), of exactly the 

 same size (about ^-th of an inch), were fixed Avitli 

 shellac on opposite sides of the apices of 12 suspended 

 radicles. The sanded card was between 0'15 and 

 • 20 mm. (or between • 0059 and • 0079 of an inch), 

 and the thin paper only 0"045 mm. (or 0' 00176 of an 

 inch) in thickness. In 8 out of the 12 cases tlicre 

 could be no doubt that the radicle was deflected from 

 the side to wliich the card-like paper was attached, and 

 towards the opposite side, bearing the very thin paper. 

 This occurred in some instances in 9 h., but in others 

 not until 24 h. had elapsed. Moreover, some of the 

 four failures can hardly be considered as really failures : 

 thus, in one of them, in which the radicle remained 

 quite straight, the square of thin pajjer was found, 

 wiien both were removed from the apex, to have been 

 so thickly coated with shellac that it was almost as 

 stiff as the card : in the second case, the radicle was 

 bent upwards into a semicircle, but the deflection 

 was not directly from the side bearing the card, and 

 this was explained by the two squares having become 

 cemenled laterally together, forming a sort of stiff 

 gable, from which the radicle was deflected : in the 

 third case, the square of card had been fixed by 

 mistake in front, and though there was deflection 

 from it, this might have been due to Sachs' curvature , 



