198 SUMMARY OF CHAPTEE. Chap, ni 



to be doubled up into a contorted mass. But we have 

 seen -with radicles growing down inclined plates of 

 glass, that as soon as the tip merely touched a slip of 

 wood cemented across the plate, the whole terminal 

 growing part curved away, so that the tip soon stood 

 at right angles to its former direction ; and thus it 

 would be with an obstacle encountered in the ground, 

 as far as the pressure of the surrounding soil would 

 permit. We can also understand why thick and strong 

 radicles, like those of ^sculus, should be endowed 

 with less sensitiTcness than more delicate ones; for 

 the former would be able by the force of their growth 

 to overcome any slight obstacle. 



After a radicle, which has been deflected by some 

 stone or root from its natural downward course 

 reaches the edge of the obstacle, geotropism will direct 

 it to grow again straight downward ; but we know that 

 geotropism acts with very little force, and here another 

 excellent adaptation, as Sachs has remarked,* comes 

 into play. For the upper part of the radicle, a little 

 above the apex, is, as we have seen, likewise sensitive ; 

 and this sensitiveness causes the radicle to bend like a 

 tendril towards the touching object, so that as it rubs 

 over the edge of an obstacle, it will bend downwards ; 

 and the curvature thus induced is abrupt, in which 

 respect it differs from that caused by the irritation of 

 one side of the tip. This downward bending coincides 

 with that due to geotropism, and both will cause the 

 root to resume its original course. 



As radicles perceive an excess of moisture in the air 

 on one side and bend towards this side, we may infer 

 that they will act in the same manner with respect to 

 moisture In the earth. The sensitiveness to moisture 



' Arboitcn Dot. Inst- Wurzbuig,' Hoft iii. p. 45G. 



