Fig. 67.  Pisum sativum:  a radicle extended horizontally in damp air with a little square of card affixed to the lower side of its tip, causing it to bend upwards in opposition to geotropism.  The deflection of the radicle after 21 hours is shown at A, and of the same radicle after 45 hours at B, now forming a loop.

to have been at all affected by the attached squares.  Four others were not acted on by geotropism within the first 6 or 8 h., but after 23 h. were much bowed down.  Two others remained almost horizontal for 23 h., but afterwards were acted on.  So that in these latter six cases the action of geotropism was much delayed.  The eleventh radicle was slightly curved down after 8 h., but when looked at again after 23 h. the terminal portion was curved upwards; if it had [page 162] been longer observed, the tip no doubt would have been found again curved down, and it would have formed a loop as in the following case.  The twelfth radicle after 6 h. was slightly curved downwards; but when looked at again after 21 h., this curvature had disappeared and the apex pointed upwards; after 30 h. the radicle formed a hook, as shown at A (Fig. 67); which hook after 45 h. was converted into a loop (B).  The thirteenth radicle after 6 h. was slightly curved downwards, but within 21 h. had curved considerably up, and then down again at an angle of 45o beneath the horizon, afterwards becoming perpendicular.  In these three last cases geotropism and the irritation caused by the attached squares alternately prevailed in a highly remarkable manner; geotropism being ultimately victorious.

Similar experiments were not always quite so successful as in the above cases.  Thus 6 radicles, horizontally extended with attached squares, were tried on June 8th at a proper temperature, and after 7 h. 30 m. none were in the least curved upwards and none were distinctly geotropic; whereas of 6 radicles without any attached squares, which served as standards of comparison or controls, 3 became slightly and 3 almost rectangularly geotropic within the 7 h. 30 m.; but after 23 h. the two lots were equally geotropic.  On July 10th another trial was made with 6 horizontally extended radicles, with squares attached in the same manner beneath their tips; and after 7 h. 30 m., 4 were slightly geotropic, 1 remained horizontal, and 1 was curved upwards in opposition to gravity or geotropism.  This latter radicle after 48 h. formed a loop, like that at B (Fig. 67).

An analogous trial was now made, but instead of attaching squares of card to the lower sides of the [page 163] tips, these were touched with dry caustic.  The details of the experiment will be given in the chapter on Geotropism, and it will suffice here to say that 10 peas, with radicles extended horizontally and not cauterised, were laid on and under damp friable peat; these, which served as standards or controls, as well as 10 others which had been touched on the upper side with the caustic, all became strongly geotropic in 24 h.  Nine radicles, similarly placed, had their tips touched on the lower side with the caustic; and after 24 h., 3 were slightly geotropic, 2 remained horizontal, and 4 were bowed upwards in opposition to gravity and to geotropism.  This upward curvature was distinctly visible in 8 h. 45m. after the lower sides of the tips had been cauterised.