72 ACTION OF THE EADICLE. Chap. II 



protrude from the seed than at a rather later period ; 

 but whether this is an accidental or an adaptive 

 coincidence we do not pretend to decide. Never- 

 theless, when young radicles of Phaseolus mtdtiflorus 

 were fixed vertically close over damp sand, in the 

 expectation that as soon as they reached it they 

 would form circular furrows, this did not occur, — a 

 fact which may be accounted for, as we believe, by 

 the furrow being filled up as soon as formed by the 

 rapid increase of thickness in the apex of the radicle. 

 Whether or not a radicle, when surrounded by soft- 

 ened earth, is aided in forming a passage for itself 

 by circumnutating, this movement can hardly fail 

 to be of high importance, by guiding the radicle 

 along a line of least resistance, as will be seen in the 

 next chapter when we treat of the sensibility of the 

 tip to contact. If, however, a radicle in its down- 

 ward growth breaks obliquely into any crevice, or a 

 hole left by a decayed root, or one made by the 

 larva of an insect, and more especially by worms, the 

 circumnutating movement of the tip will materially 

 aid it in following such open passage ; and we have 

 observed that roots commonly run down the old 

 burrows of worms.* 



When a radicle is placed in a horizontal or inclined 

 position, the terminal growing part, as is well known, 

 bends down towards the centre of the earth; and 

 Sachs t has shown that whilst thus bending, the growth 

 of the lower surface is greatly retarded, whilst that 



* Sec, also, Prof. Hensen's state- rovrs made by worms, 



ments ('Zeitschrift fiir Wissen, t 'Arbeiten des bot. Inst. 



Zool.,' B. xxviii. p. 354, 1S77) to Wurzburg,' vol. i. 1873, p. 461. 



the same eifeet. He goes so far See also p. 397 for the length of 



as to believe that roots are able the growing part, and p. 451 on 



to penetrate the ground to a great the force of geotropism. 

 depth only by means of the bur- 



