500 THE POPULAR SCIENCE MONTHLY. 



the apex of the radicle, and if it could have remained there all the 

 time, the movement exhibited would have been much greater, for at 

 the close of our observations the tip, instead of standing upward, had 

 become bowed downward, through geotropism [gravitation], so as 

 almost to touch the zinc plate. As far as we could roughly ascertain 

 by measurements made with compasses on other seeds, the tip alone, 

 for a length of only -^-^ to y|^ of an inch, is acted on by geotropism. 

 But the tracing shows that the basal part of the radicle continued to 

 circumnutate irregularly during the whole time. The actual extreme 

 amount of movement of the bead at the end of the filament was nearly 

 05 inch, but to what extent the movement of the radicle was mag- 

 nified by the filament, which was nearly three fourths of an inch in 

 length, it was impossible to estimate. . . . 



" Another seed was treated and observed in the same manner, but 

 the radicle in this case protruded ! inch, and was not fastened so as 

 to project quite vertically upward. The filament was fixed close to 

 its base. The tracing (Fig. 2, reduced one half ) shows the movement 



'-"-t^' 



Fig. 2. Brassica Oleracea : circumnntating and geotropic movement of radicle, traced on hori- 

 zontal glass during forty-six hours. 



from 9 A. M., January 31st, to 7 a. m,, February 2d ; but it continued 

 to move during the whole of the day in the same general direction and 

 in a similar zigzag manner." The chapter contains fifty-four dia- 

 grams, giving the movements of all the parts of the seedlings of all 

 sorts of plants. 



Mr. Darwin thinks that these movements of the radicle are useful 

 at least in enabling it to take the line of least resistance, if they do not 

 directly aid it in forming a passage for itself ; and he adds : " If, how- 

 ever, a radicle in its downward growth breaks obliquely into any crev- 

 ice, 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 its descent ; and we have observed that 

 roots commonly run down the old burrows of worms." He says, fur- 

 ther, that the force due to longitudinal and transverse growth materi- 

 ally assists the radicle in penetrating the ground. He experimented 

 upon these points, and we give two of these experiments which relate 

 to the force exerted transversely by the radicles of beans. We may 

 say that these radicles have a sharp apex protected by a root-cap, and 

 their growing part is more rigid than the part just above. 



