416 MOVEMENTS. 



the soil, 1 and "as it is always endeavoring to bend to all sides, 

 it will press on all sides, and will thus be able to discriminate 

 between the harder and softer adjoining surfaces . . . conse- 

 quently it will tend to bend from the harder soil, and will thus 

 follow the lines of least resistance." 2 



1 Darwin : Power of Movement in Plants, p. 197. 



The two following passages should be carefully studied by the student, 

 since they embody in a few words Darwin's summary of most of the results of 

 his experiments upon radicles. Both passages are from the " Power of Move- 

 ment in Plants," 1880 :- 



" We see that the course followed by a root through the soil is governed by 

 extraordinarily complex and diversified agencies, - - by geotropism acting in a 

 different manner on the primary, secondary, and tertiary radicles, -- by sensi- 

 tiveness to contact, different in kind in the apex and in the part immediately 

 above the apex, and apparently by sensitiveness to the varying dampness of 

 different parts of the soil. These several stimuli to movement are all more 

 powerful than geotropism, when this acts obliquely on a radicle which has been 

 deflected from its perpendicular downward course. The roots, moreover, of 

 most plants are excited by light to bend either to or from it ; but as roots are 

 not naturally exposed to the light, it is doubtful whether this sensitiveness, 

 which is perhaps only the indirect result of the radicles being highly sensitive 

 to other stimuli, is of any service to the plant. The direction which the apex 

 takes at each successive period of the growth of a root ultimately determines 

 its whole course ; it is therefore highly important that the apex should pursue 

 from the first the most advantageous direction ; and we can thus understand 

 why sensitiveness to geotropism, to contact, and to moisture, all reside in the 

 tip, and why the tip determines the upper growing part to bend either from or 

 to 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 circumnutating, 

 he will feel any stone or other obstacle, as well as any difference in the hard- 

 ness 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 thitherward as a 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 " 

 (p. 199). 



"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 



