It has 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 sensitive 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. 6.—(1) Centrifugal Water-shed in Caladium, and (2) Centripetal Water-shed in Rhubarb—showing corresponding distribution of rootlets. (Kerner.)
some cases the irritation was produced by a tiny bit of card attached 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 became 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 irritation. These results seem to warrant the conclusion that the end of the root is not only sensitive to contact, but responds in opposite 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
