THE SEEDLING AND YOUNG PLANT 



35 



pressed that they form 

 mere flattened sheets 

 many times broader than 

 they are thick (fig. 8). 



It has already been 

 mentioned that the tip of 

 the young primary root 

 circumnutates, and Dar- 

 win also found that the 

 tip of the radicle is ex- 

 tremely sensitive to the 

 irritation of small bodies 

 in contact with it. It is 

 also positively geotropic, 

 directing itself vertically 

 downwards if the partially 

 grown radicle is laid hori- 

 zontally ; and it may be 

 assumed from the be- 

 haviour of other plants of 

 the same kind that the 

 tip of the radicle is nega- 

 tively heliotropic i.e. it 

 turns away from the 

 source of light. Whether 

 it is also sensitive to dif- 

 ferences in the degree of 

 moisture on different sides 

 (hydrotropic), or to differ- 



FIG. 8. Portion of an older root 

 of an oak, which had pene- 

 trated while young between 

 two pieces of hard rock, and 

 had to adapt its form accord- 

 ingly as it thickened. (After 

 Dobner.) 



D 2 



