60 PLANT LIFE ON THE FARM. 



pursue from the first the most advantageous direction ; 

 and we can thus understand why sensitiveness to gravita- 

 tion, to contact, and to moisture, all reside in the tip, and 

 why the tip determines the upper growing part to bend 

 either to or from the exciting cause. A radicle may be 

 compared with a burrowing animal, such as a mole, which 

 wishes to penetrate perpendicularly down into the ground. 

 By continually moving his head from side to side, or cir- 

 cumnutating, he will feel any stone or other obstacle, as 

 well as any difference in the hardness of the soil, and he 

 will turn from that side. If the earth is damper on one 

 than on the other side, he will turn thither as to better 

 hunting ground. Nevertheless, after each interruption, 

 guided by the sense of gravity, he will be able to recover 

 his downward course and to burrow to a greater depth." 



Elsewhere Darwin sums up the root movements as 

 follows : " We believe that there is no structure in plants 

 more wonderful, so 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 adjoin- 

 ing part, causing it to bend away from the affected side ; 

 and, what is more surprising, the tip can distinguish be- 

 tween 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 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, the adjoining part bends 

 from the light ; but when excited by gravitation, the same 



