72] 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.  Nevertheless, when young radicles of Phaseolus multiflorus 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 softened 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 downward 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* has shown that whilst thus bending, the growth of the lower surface is greatly retarded, whilst that

* See, also, Prof.  Hensen’s statements (’Zeitschrift für Wissen, Zool.,’ B. xxviii. p. 354, 1877) to the same effect.  He goes so far as to believe that roots are able to penetrate the ground to a great depth only by means of the burrows made by worms.

* ‘Arbeiten des bot.  Inst.  Würzburg,’ vol. i. 1873, p. 461.  See also p. 397 for the length of the growing part, and p. 451 on the force of geotropism. [page 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 downwards.  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.