The Power of Movement in Plants, Charles Darwin [the reading list book TXT] 📗

(8.) After one day deflection distinct; on the third day it amounted to 44o, and on the fourth day to 72o from the perpendicular and the cauterised side.

(9.) After two days deflection slight, yet distinct; on the third day the tip was again touched on the same side with caustic and thus killed.

(10.) After one day slight deflection, which after six days increased to 50o from the perpendicular and the cauterised side.

(11.) After one day decided deflection, which after six days increased to 62o from the perpendicular and from the cauterised side.

(12.) After one day slight deflection, which on the second day amounted to 35o, on the fourth day to 50o, and the sixth day to 63o from the perpendicular and the cauterised side.

(13.) Whole tip blackened, but more on one side than the other; on the fourth day slightly, and on the sixth day greatly deflected from the more blackened side; the deflection on the ninth day amounted to 90o from the perpendicular.

(14.) Whole tip blackened in the same manner as in the last case: on the second day decided deflection from the more blackened side, which increased on the seventh day to nearly 90o; on the following day the radicle appeared unhealthy.

(15.) Here we had the anomalous case of a radicle bending [page 174]

slightly towards the cauterised side on the first day, and continuing to do so for the next three days, when the deflection amounted to about 90o from the perpendicular. The cause appeared to lie in the tendril-like sensitiveness of the upper part of the radicle, against which the point of a large triangular flap of the seed-coats pressed with considerable force; and this irritation apparently conquered that from the cauterised apex.]

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.

No. 3. Square attached on the 19th; on the 21st a trace of curvature from the square, which amounted on the 22nd to about 40o, and on the 23rd to 53o from the perpendicular.

No. 4. Square attached on the 21st; on the 22nd trace of curvature from the square; on the 23rd completely hooked with the point turned up to the zenith. Three days afterwards (i.e. 26th) the curvature had wholly disappeared and the apex pointed perpendicularly downwards.

No. 5. Square attached on the 21st; on the 22nd decided [page 176]

though slight curvature from the square; on the 23rd the tip had curved up above the horizon, and on the 24th was hooked with the apex pointing almost to the zenith, as in Fig. 68.

No. 6. Square attached on the 21st; on the 22nd slightly curved from the square; 23rd more curved; 25th considerably curved; 27th all curvature lost, and the radicle was now directed perpendicularly downwards.

No. 7. Square attached on the 21st; on the 22nd a trace of curvature from the square, which increased next day, and on the 24th amounted to a right angle.

It is, therefore, manifest that the apex of the radicle of the oak is highly sensitive to contact, and retains its sensitiveness during several days. The movement thus induced was, however, slower than in any of the previous cases, with the exception of that of Aesculus. As with the bean, the terminal growing part, after bending, sometimes straightened itself through the action of geotropism, although the object still remained attached to the tip.

The same remarkable experiment was next tried, as in the case of the bean; namely, little squares of exactly the same size of the card-like sanded paper and of very thin paper (the thicknesses of which have been given under Vicia faba) were attached with shellac on opposite sides (as accurately as could be done) of the tips of 13 radicles, suspended in damp air, at a temperature of 65o - 66o F. The result was striking, for 9 out of these 13 radicles became plainly, and 1 very slightly, curved from the thick paper towards the side bearing the thin paper. In two of these cases the apex became completely hooked after two days; in four cases the deflection from the perpendicular and from the side bearing the thick paper, amounted in from two to four days to angles of 90o, 72o, 60o, and 49o, but in two other cases to only 18o and 15o. It should, however, be stated that in the

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case in which the deflection was 49o, the two squares had accidentally come into contact on one side of the apex, and thus formed a lateral gable; and the deflection was directed in part from this gable and in part from the thick paper. In three cases alone the radicles were not affected by the difference in thickness of the squares of paper attached to their tips, and consequently did not bend away from the side bearing the stiffer paper.

Zea mays: Sensitiveness of the apex of the Radicle to contact.—A large number of trials were made on this plant, as it was the only monocotyledon on which we experimented. An abstract of the results will suffice. In the first place, 22 germinating seeds were pinned to cork-lids without any object being attached to their radicles, some being exposed to a temperature of 65o - 66o F., and others to between 74o and 79o; and none of them became curved, though some were a little inclined to one side. A few were selected, which from having germinated on sand were crooked, but when suspended in damp air the terminal part grew straight downwards. This fact having been ascertained, little squares of the card-like paper were affixed with shellac, on several occasions, to the tips of 68 radicles. Of these the terminal growing part of 39 became within 24 h. conspicuously curved away from the attached squares and from the perpendicular; 13 out of the 39

forming hooks with their points directed towards the zenith, and 8 forming loops. Moreover, 7 other radicles out of the 68, were slightly and two doubtfully deflected from the cards. There remain 20 which were not affected; but 10 of these ought not to be counted; for one was diseased, two had their tips quite surrounded by shellac, and the squares on 7 had slipped so as to stand parallel to the apex, instead of obliquely [page 178]

on it. There were therefore only 10 out of the 68 which certainly were not acted on. Some of the radicles which were experimented on were young and short, most of them of moderate length, and two or three exceeded three inches in length. The curvature in the above cases occurred within 24 h., but it was often conspicuous within a much shorter period. For instance, the terminal growing part of one radicle was bent upwards into a rectangle in 8 h. 15 m., and of another in 9 h. On one occasion a hook was formed in 9 h. Six of the radicles in a jar containing nine seeds, which stood on a sand-bath, raised to a temperature varying from 76o to 82o F., became hooked, and a seventh formed a complete loop, when first looked at after 15

hours.

The accompanying figures of four germinating seeds (Fig. 69) show, firstly, a radicle (A) the apex of which has become so much bent away from the attached square as to form a hook. Secondly (B), a hook converted through the continued irritation of the card, aided perhaps by geotropism, into an almost complete circle or loop. The tip in the act of forming a loop generally rubs against the upper part of the radicle, and pushes off the attached square; the loop then contracts or closes, but never disappears; and the apex afterwards grows vertically downwards, being no longer irritated by any attached object. This frequently occurred, and is represented at C. The jar above mentioned with the six hooked radicles and another jar were kept for two additional days, for the sake of observing how the hooks would be modified. Most of them became converted into simple loops, like that figured at C; but in one case the apex did not rub against the upper part of the radicle and thus remove the card; and it consequently made, owing

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to the continued irritation from the card, two complete loops, that is, a helix of two spires; which afterwards became pressed closely together. Then geotropism prevailed and caused the apex to grow perpendicularly downwards.

In another case, shown at (D), the apex

Fig. 69. Zea mays: radicles excited to bend