38 



THE POPULAR SCIENCE MONTHLY. 



It lias already been mentioned that the root-tip, when coming 

 against an obstruction, turns aside and thus avoids being pushed 

 against it. This has been taken to indicate that the tip is sensi- 

 tive to contact as well as to moisture and gravity. To test this 

 supposition, Darwin tried the experiment of affecting one side of 

 the root-tip with a slight but constant mechanical irritant. In 



Fig. 0. (1) Centrifugal Water-shed in Caladium, and (2) Centripetal Water-shed in 

 Khdbarb showing corresponding distribution of rootlets. (Kerner.) 



some cases the irritation was produced by a tiny bit of card at- 

 tached obliquely to the tip by shellac or gum ; shellac by itself 

 was sometimes used, and in other instances the sensitive region 

 was touched with caustic. In nearly every case the tip be- 

 came bent away from the side irritated (Fig. 7). Occasionally 

 it happened that the region just above the tip became irritated 

 (by displacement of the card or otherwise), and in such cases 

 the end of the root was bent strongly toward the source of irrita- 

 tion. These results seem to warrant the conclusion that the end 

 of the root is not only sensitive to contact, but responds in oppo- 

 site ways according as the side of the tip or the region just above is 

 affected, and we get an explanation both of the way the tip bends 

 when meeting an obstructing surface, and of the abrupt curve it 

 makes when the edge of the obstruction is reached. It has been 

 urged, however, that these experiments do not really prove that 



