CHAP. XII. CONCLUDING REMARKS. 551 



with caustic, yet if the side be much cauterised the 

 injury is too great, and the power of transmitting some 

 influence to the adjoining parts causing them to bend, 

 is lost. Other analogous cases are known to occur. 



After a radicle has been deflected by some obstacle, 

 geotropisin directs the tip again to grow perpendicu- 

 larly downwards ; but geotropism is a feeble power, 

 and here, as Sachs has shown, another interesting 

 adaptive movement comes into play ; for radicles at. 

 a distance of a few millimeters from the tip are 

 sensitive to prolonged contact in such a manner that 

 they bend towards the touching object, instead of from 

 it as occurs when an object touches one side of the 

 tip. Moreover, the curvature thus caused is abrupt ; 

 the pressed part alone bending. Even slight pressure 

 suffices, such as a bit of card cemented to one side. 

 Therefore a radicle, as it passes over the edge of any 

 obstacle in the ground, will through the action of geo- 

 tropisin press against it ; and this pressure will cause 

 the radicle to endeavour to bend abruptly over the 

 edge. It will thus recover as quickly as possible its 

 normal downward course. 



Eadicles are also sensitive to air which contains 

 more moisture on one side than the other, and they 

 bend towards its source. It is therefore probable that 

 they are in like manner sensitive to dampness in the 

 soil. It was ascertained in several cases that this 

 sensitiveness resides in the tip, which transmits an 

 influence causing the adjoining upper part to bend 

 in opposition to geotropism towards the moist object. 

 We may therefore infer that roots will be deflected 

 from their downward course towards any source oi 

 moisture in the soil. 



Again, most or all radicles are slightly sensitive to 

 light, and, according to Wiesner, generally bend a little 

 36 



