20 LIGHT AND THE BEHAVIOR OF ORGANISMS 



a new tip was reformed; and this was then acted on by 

 geotropism, and the radicle became curved perpendicu- 

 larly downwards." ^ (pp. 572-573), " We believe that there 

 is no structure in plants more wonderful, as far as its 

 functions are concerned, than the tip of the radicle. If 

 the tip be lightly pressed or burnt or cut, it transmits an 

 influence to the upper adjoining part causing it to bend 

 away from the affected side; and, what is more surprising, 

 the tip can distinguish between a slightly harder and softer 

 object, by which it is simultaneously pressed on opposite 

 sides. If, however, the radicle is pressed by a similar 

 object a little above the tip, the pressed part does not 

 transmit any influence to the more distant parts, but bends 

 abruptly towards the object. If the tip perceives the air 

 to be moister on one side than on the other, it likewise 

 transmits an influence to the upper adjoining part, which 

 bends towards the source of moisture. When the tip is 

 excited by light (though in the case of radicles this was 

 ascertained in only a single instance), the adjoining part 

 bends from the light; but when excited by gravitation the 

 same part bends towards the center of gravity. In almost 

 every case we can clearly perceive the final purpose or 

 advantage of the several movements. Two, or perhaps 

 more, of the exciting causes often act simultaneously on 

 the tip, and one conquers the other, no doubt in accordance 

 with its importance for the life of the plant. The course 

 pursued by the radicle in penetrating the ground must be 

 determined by the tip; hence it has acquired such diverse 

 kinds of sensitiveness. It is hardly an exaggeration to say 

 that the tip of the radicle thus endowed, and having the 

 power of directing the movements of the adjoining parts, 

 acts like the brain of one of the lower animals; the brain 

 being seated within the anterior end of the body, receiving 



^ This experiment was first performed by Ciesielski (1875). Darwin's 

 interpretation of the results has been questioned. See Francis Darwin's 

 interesting presentation of the controversy concerning this and related sub- 

 jects (1907, PP- 35-42; 69-76). 



