THE ROOTS 43 



found on aquatics covering the tips of roots which 

 hang down in the water and do not touch any hard 

 object. In such examples the sheath is generally 

 very long, extending back over the younger portion 

 of the root, and as it is filled with bitter substances 

 it prevents swimming animals from eating or injur- 

 ing the tips of the roots. It may be seen with 

 difficulty on land plants, but on aquatics, such as 

 the water hyacinth, it is a fourth of an inch long, 

 and may be pulled off and examined with the eye, 

 or better by the aid of a hand lens. A sketch of 

 the appearance of the root should be made, and a 

 number of species should be examined in search 

 for this protecting cap. 



48. The sensitiveness of roots to touch or contact 

 with solid objects. The root-cap would not be suf- 

 ficient to protect the tip of the root from all danger 

 of injury if it were pushed forward in a straight 

 line and did not turn aside for an obstruction. As 

 a means of avoiding injury from this cause the 

 root has become sensitive to the touch or contact 

 of hard objects, in such manner that it bends away 

 from them. This may be seen by the repetition 

 of one of Darwin's classical experiments. Fix a 



