OCR Text |
Show 72 ACTION OF THE RADICLE. CHAl'. II. lJrotrude from the seed than at a rather later peri~d; but whether this is an accidental or an adaptive coincidence we do not pretend to decide. Nevertheless, when young radicles of Pha,seolus mul~ijlorus were fixed vertically close over damp sand, m the expectation that as soon as t~ey _reached it they would form circular furrows, this did not occur,-a fact which may be accounted for, as we believe, Ly 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 itso~f by circumnutating, this movemen~ _can hardly !all to be of high importance, by gmdmg 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 t has shown that whilst thus bending, the growth of the lower surface is greatly retarded, whilst that * See, also, Prof. Hensen's statements (' Zeitschrift fiir Wissen, Zool.,' B. xxviii. p. 354, 1877) to the snme effect. He goes so far as to belitwe that roots are able to penetrnte the ground to a great depth only by means of the bur-rows mnde by worms. t ' Arbeiten des bot. Inst. Wiirzburg,' vol. i. 1873, p. 461. See n,l:;;o p. H97 for the length of the growing pnrt, and. p. 451 on the force of geotropism. CnAP. 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 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. · Radicles 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 ~uch more rigid, as Sachs has proved, than the part lmmediately above, which has ceased to increase in length. vVe 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 |