CHAI-. II. ACTION OF THE RADICLE. 73 



of the upper surface continues at the normal rate, 

 or may be even somewhat increased. He has further 

 shown by attaching a thread, running over a pulley, 

 to a horizontal radicle of large size, namely, that 

 of the common bean, that it was able to pull up a 

 weight of only one gramme, or 15*4 grains. We may 

 therefore conclude that geotropism does not give a 

 radicle force sufficient to penetrate the ground, but 

 merely tells it (if such an expression may be used) 

 which course to pursue. Before we knew of Sachs' 

 more precise observations we covered a flat surface of 

 damp sand with the thinnest tin-foil which we could 

 procure ('02 to '03 mm., or -00012 to -00079 of an inch 

 in thickness), and placed a radicle close above, in such 

 a position that it grew almost perpendicularly down- 

 wards. When the apex came into contact with the 

 polished level surface it turned at right angles and 

 glided over it without leaving any impression; yet 

 the tin-foil was so flexible, that a little stick of soft 

 wood, pointed to the same degree as the end of the 

 radicle and gently loaded with a weight of only a 

 quarter of an ounce (120 grains) plainly indented the 

 tin-foil. 



Eadicles are able to penetrate the ground by the 

 force due to their longitudinal and transverse growth ; 

 the seeds themselves being held down by the weight 

 of the superincumbent soil. In the case of the bean 

 the apex, protected by the root-cap, is sharp, and 

 the growing part, from 8 ,to 10 mm. in length, is 

 much more rigid, as Sachs has proved, than the part 

 immediately above, which has ceased to increase in 

 length. We endeavoured to ascertain the downward 

 pressure of the growing part, by placing germinating 

 beans between two small metal plates, the upper one 

 of which was loaded with a known weight; and the 



