THE SEEDLING AND YOUNG PLANT. 35 



It has already been men- 

 tioned that the tip of the 

 young primary root circum- 

 nutates, and Darwin also 

 found that the tip of the 

 radicle is extremely sen- 

 sitive to the irritation of 

 small bodies in contact with 

 it. It is also positively geo- 

 tropic, directing itself ver- 

 tically downward if the par- 

 tially grown radicle is laid 

 horizontally ; and it may be 

 assumed from the behavior 

 of other plants of the same 

 kind that the tip of the 

 radicle is negatively helio- 

 tropic i. e., it turns away 

 from the source of light. 

 Whether it is also sensitive 

 to differences in the degree 

 of moisture on different 

 sides (hydrotropic), or to 

 differences of temperature 



FIG. 8. Portion of an older root of 



( thermotropic ) IS not an oak) w hich had penetrated 



known, but it may be in- while young between two pieces 



of hard rock, and had to adapt 



f erred that such is the its form accordingly as it thick- 

 case ; nor do we know ene<L < After D6bner - } 

 whether it is affected by electric currents in the earth. 



