By the fourth morning new tips had been re-formed, and now geotropism was able to act on them again, and they became bent perpendicularly downwards, exactly as in the case of the five radicles described in the last paragraph and as is shown in (Fig. 195) here given.
Fig. 195. Vicia faba: radicle, rectangularly bent at A, after the amputation of the tip, due to the previous influence of geotropism. L, side of bean which lay on the peat, whilst geotropism acted on the radicle. A, point of chief curvature of the radicle, whilst standing vertically downwards. B, point of chief curvature after the regeneration of the tip, when geotropism again acted. C, regenerated tip.
Lastly, five other radicles were similarly treated, but were exposed to geotropism during only 45 m. After 8 h. 30 m. only one was doubtfully affected; after 24 h. two were just perceptibly curved towards the side which had been acted on by geotropism; after 48 h. the one first mentioned had a radius of curvature of 60 mm. That this curvature was due to the action of geotropism during the horizontal position of the radicle, was shown after 4 days, when a new tip had been re-formed, for it then grew perpendicularly downwards. We learn from this [page 528] case that when the tips are amputated after an exposure to geotropism of only 45 m., though a slight influence is sometimes transmitted to the adjoining part of the radicle, yet this seldom suffices, and then only slowly, to induce even moderately well-pronounced curvature.
In the previously given experiments on 29 horizontally extended radicles with their tips amputated, only one grew irregularly in any marked manner, and this became bowed upwards at an angle of 65o. In Ciesielski's experiments the radicles could not have grown very irregularly, for if they had done so, he could not have spoken confidently of the obliteration of all geotropic action. It is therefore remarkable that Sachs, who experimented on many radicles with their tips amputated, found extremely disordered growth to be the usual result. As horizontally extended radicles with amputated tips are sometimes acted on slightly by geotropism within a short time, and are often acted on plainly after one or two days, we thought that this influence might possibly prevent disordered growth, though it was not able to induce immediate curvature. Therefore 13 radicles, of which 6 had their tips amputated transversely for a length of 1.5 mm., and the other 7 for a length of only 0.5 mm., were suspended vertically in damp air, in which position they would not be affected by geotropism; but they exhibited no great irregularity of growth, whilst observed during 4 to 6 days. We next thought that if care were not taken in cutting off the tips transversely, one side of the stump might be irritated more than the other, either at first or subsequently during the regeneration of the tip, and that this might cause the radicle to bend to one side. It has also been shown in Chapter III. that if a thin slice be cut off one side of the tip of the radicle, this causes the radicle to bend from the sliced side. Accordingly, 30 radicles, with tips amputated for a length of 1.5 mm., were allowed to grow perpendicularly downwards into water. Twenty of them were amputated at an angle of 20o with a line transverse to their longitudinal axes; and such stumps appeared only moderately oblique. The remaining ten radicles were amputated at an angle of about 45o. Under these circumstances no less than 19 out of the 30 became much distorted in the course of 2 or 3 days. Eleven other radicles were similarly treated, excepting that only 1 mm. (including in this and all other cases the root-cap) was amputated; and of these only one grew much, and two others slightly [page 529] distorted; so that this amount of oblique amputation was not sufficient. Out of the above 30 radicles, only one or two showed in the first 24 h. any distortion, but this became plain in the 19 cases on the second day, and still more conspicuous at the close of the third day, by which time new tips had been partially or completely regenerated.