G YRA TION OF PLANTS. 155 



alluded, and that is the sensitiveness of the tip of the 

 radicle. 



The radicle of the bean was selected for the 

 majority of the experiments in this connexion, the 

 results of which appear to prove that " when one side 

 of the apex (of a radicle) is pressed by any object 

 the growing part bends away from the object ; and 

 this seems a beautiful adaptation for avoiding 

 obstacles in the soil and for following the lines of 

 least resistance. Many organs when touched bend 

 in one fixed direction, such as the stamens of 

 Berberis, the lobes of Dionoea, &c. ; and many organs, 

 such as tendrils, whether modified leaves or flower 

 peduncles, and some few stems, bend towards a 

 touching object ; but no case, we believe, is known of 

 an organ bending away from a touching object."^ 



That the radicle of many plants, indeed, of most, 

 if not all, are sensitive to pressure continuously 

 exerted upon them, or to injury, and are capable of 

 bending away from it, was shown by many experi- 

 ments, such as attaching small objects to the side of 

 the tip, touching it with caustic, or cutting off a slice 

 from it. All these interferences seemed to act in a 

 similar manner, causing the tip to diverge from its 

 direct downward course and turn in the direction 

 opposite to the obstruction or injury. This sensitive 



' Darwin, " Movements of Plants,"' p. 132. 



