These several cases show beyond doubt that the irritation of one side of the apex, excites the upper part of the radicle to bend slowly towards the opposite side.  This fact was well exhibited in one lot of five seeds pinned to the cork-lid of a jar; for when after 6 days the lid was turned upside down and viewed from directly above, the little black marks made by the caustic were now all distinctly visible on the upper sides of the tips of the laterally bowed radicles.  A thin slice was shaved off with a razor from one side of the tips of 22 radicles, in the manner described under the common bean; but this kind of irritation did not prove very effective.  Only 7 out of the 22 radicles became moderately deflected in from 3 to 5 days from the sliced surface, and several of the others grew irregularly.  The evidence, therefore, is far from conclusive.

Quercus robur:  Sensitiveness of the apex of the Radicle.—­The tips of the radicles of the common oak are fully as sensitive to slight contact as are those of any plant examined by us.  They remained healthy in damp air for 10 days, but grew slowly.  Squares of the card-like paper were fixed with shellac to the tips of 15 radicles, and ten of these became conspicuously bowed from the perpendicular and from the squares; two slightly, and three not at all.  But two of the latter were not real exceptions, as they were at first very short, and hardly grew afterwards.  Some of the more [page 175] remarkable cases are worth describing.  The radicles were examined on each successive morning, at nearly the same hour, that is, after intervals of 24 h.

[No. 1.  This radicle suffered from a series of accidents, and acted in an anomalous manner, for the apex appeared at first insensible and afterwards sensitive to contact.  The first square was attached on Oct 19th; on the 21st the radicle was not at all curved, and the square was accidentally knocked off; it was refixed on the 22nd, and the radicle became slightly curved from the square, but the curvature disappeared on the 23rd, when the square was removed and refixed.  No curvature ensued, and the square was again accidentally knocked off, and refixed.  On the morning of the 27th it was washed off by having reached the water in the bottom of the jar.  The square was refixed, and on the 29th, that is, ten days after the first square had been attached, and two days after the attachment of the last square, the radicle had grown to the great length of 3.2 inches, and now the terminal growing part had become bent away from the square into a hook (see Fig. 68).

Fig. 68.  Quercus robur:  radicle with square of card attached to one side of apex, causing it to become hooked.  Drawing one-half natural scale.

No. 2.  Square attached on the 19th; on the 20th radicle slightly deflected from it and from the perpendicular; on the 21st deflected at nearly right angles; it remained during the next two days in this position, but on the 25th the upward curvature was lessened through the action of geotropism, and still more so on the 26th.