MOVEMENTS OF SEEDLINGS. 43 



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. More- 

 over, 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 geotropism, press against it, and this pressure 

 will cause the radicle to endeavor to bend abruptly 

 over the edge. It will thus recover as quickly as 

 possible its normal downward course. 



" Radicles 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 

 toward any source of moisture in the soil. 



" Again, most or all radicles are slightly sensitive 

 to light, and, according to Wiesner, generally bend 

 a little from it. Whether this can be of any ser- 



