The Power of Movement in Plants by Charles Darwin (book club books TXT) 📖

and the ten with their tips cauterised on the upper side were equally geotropic; and we believe that they became curved downwards before the controls. The ten which had been cauterised on the lower side presented a widely different appearance: No. 1, however, was perpendicularly geotropic, but this was no real exception, for on examination under the microscope, there was no vestige of a coloured mark on the tip, and it was evident that by a mistake it had not been touched with the caustic. No. 2 was plainly geotropic, being inclined at about 45o beneath the horizon; No. 3 was slightly, and No. 4 only just perceptibly geotropic; Nos. 5 and 6 were strictly horizontal; and the four remaining ones were bowed upwards, in opposition to geotropism. In these four cases the radius of the upward curvatures (according to Sachs’ cyclometer) was 5

mm., 10 mm., 30 mm., and 70 mm. This curvature was distinct long before the 24 h. had elapsed, namely, after 8 h. 45 m. from the time when the lower sides of the tips were touched with the caustic.

 

Phaseolus multiflorus.—Eight radicles, serving as controls, were extended horizontally, some in damp friable peat and some in damp air. They all became (temp 20o - 21o C.) plainly geotropic in 8 h. 30 m., for they then stood at an average angle of 63o beneath the horizon. A rather greater length of the radicle is bowed downwards by geotropism than in the case of Vicia faba,

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that is to say, rather more than 6 mm. as measured from the apex of the root-cap. Nine other radicles were similarly extended, three in damp peat and six in damp air, and dry caustic was held transversely to their tips during 4 or 5 seconds. Three of their tips were afterwards examined: in (1) a length of 0.68 mm. was discoloured, of which the basal 0.136 mm. was yellow, the apical part being black; in (2) the discoloration was 0.65 mm.

in length, of which the basal 0.04 mm. was yellow; in (3) the discoloration was 0.6 mm. in length, of which the basal 0.13 mm. was yellow. Therefore less than 1 mm. was affected by the caustic, but this sufficed almost wholly to prevent geotropic action; for after 24 h. one alone of the nine cauterised radicles became slightly geotropic, being now inclined at 10o beneath the horizon; the eight others remained horizontal, though one was curved a little laterally.

 

The terminal part (10 mm. in length) of the six cauterised radicles in the damp air, had more than doubled in length in the 24 h., for this part was now on an average 20.7 mm. long. The increase in length within the same time was greater in the control specimens, for the terminal part had grown on an average from 10 mm. to 26.6 mm. But as the cauterised radicles had more than doubled their length in the 24 h., it is manifest that they had not been seriously injured by the caustic. We may here add that when experimenting on the effects of touching one side of the tip with caustic, too much was applied at first, and the whole tip (but we believe not more than 1 mm. in length) of six horizontally extended radicles was killed, and these continued for two or three days to grow out horizontally.

 

Many trials were made, by coating the tips of horizontally extended radicles with the before described thick grease. The geotropic curvature of 12 radicles, which were thus coated for a length of 2 mm., was delayed during the first 8 or 9 h., but after 24 h. was nearly as great as that of the control specimens. The tips of nine radicles were coated for a length of 3 mm., and after 7 h. 10 m. these stood at an average angle of 30o beneath the horizon, whilst the controls stood at an average of 54o. After 24 h. the two lots differed but little in their degree of curvature. In some other trials, however, there was a fairly well-marked difference after 24 h. between those with greased tips and the controls. The terminal part of eight control specimens increased in 24 h. from 10 mm. to a mean length of

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24.3 mm., whilst the mean increase of those with greased tips was 20.7 mm.

The grease, therefore, slightly checked the growth of the terminal part, but this part was not much injured; for several radicles which had been greased for a length of 2 mm. continued to grow during seven days, and were then only a little shorter than the controls. The appearance presented by these radicles after the seven days was very curious, for the black grease had been drawn out into the finest longitudinal striae, with dots and reticulations, which covered their surfaces for a length of from 26 to 44

mm., or of 1 to 1.7 inch. We may therefore conclude that grease on the tips of the radicles of this Phaseolus somewhat delays and lessens the geotropic curvature of the part which ought to bend most.

 

Gossypium herbaceum.—The radicles of this plant bend, through the action of geotropism, for a length of about 6 mm. Five radicles, placed horizontally in damp air, had their tips touched with caustic, and the discoloration extended for a length of from 2/3 to 1 mm. They showed, after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet the terminal portion, 9 mm. in length, had increased on an average to 15.9 mm.

Six control radicles, after 7 h. 45 m., were all plainly geotropic, two of them being vertically dependent, and after 23 h. all were vertical, or nearly so.

 

Cucurbita ovifera.—A large number of trials proved almost useless, from the three following causes: Firstly, the tips of radicles which have grown somewhat old are only feebly geotropic if kept in damp air; nor did we succeed well in our experiments, until the germinating seeds were placed in peat and kept at a rather high temperature. Secondly, the hypocotyls of the seeds which were pinned to the lids of the jars gradually became arched; and, as the cotyledons were fixed, the movement of the hypocotyl affected the position of the radicle, and caused confusion. Thirdly, the point of the radicle is so fine that it is difficult not to cauterise it either too much or too little. But we managed generally to overcome this latter difficulty, as the following experiments show, which are given to prove that a touch with caustic on one side of the tip does not prevent the upper part of the radicle from bending. Ten radicles were laid horizontally beneath and on damp friable peat, and their tips were touched with caustic on the upper side. After 8 h. all were plainly geotropic, three of them rectangularly; after 19 h.

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all were strongly geotropic, most of them pointing perpendicularly downwards. Ten other radicles, similarly placed, had their tips touched with caustic on the lower side; after 8 h. three were slightly geotropic, but not nearly so much so as the least geotropic of the foregoing specimens; four remained horizontal; and three were curved upwards in opposition to geotropism. After 19 h. the three which were slightly geotropic had become strongly so. Of the four horizontal radicles, one alone showed a trace of geotropism; of the three up-curved radicles, one retained this curvature, and the other two had become horizontal.

 

The radicles of this plant, as already remarked, do not succeed well in damp air, but the result of one trial may be briefly given. Nine young radicles between .3 and .5 inch in length, with their tips cauterised and blackened for a length never exceeding � mm., together with eight control specimens, were extended horizontally in damp air. After an interval of only 4 h. 10 m. all the controls were slightly geotropic, whilst not one of the cauterised specimens exhibited a trace of this action. After 8 h. 35

m., there was the same difference between the two sets, but rather more strongly marked. By this time both sets had increased greatly in length.

The controls, however, never became much more curved downwards; and after 24 h. there was no great difference between the two sets in their degree of curvature.

 

Eight young radicles of nearly equal length (average .36 inch) were placed beneath and on peat-earth, and were exposed to a temp. of 75o - 76o F.

Their tips had been touched transversely with caustic, and five of them were blackened for a length of about 0.5 mm., whilst the other three were only just visibly discoloured. In the same box there were 15 control radicles, mostly about .36 inch in length, but some rather longer and older, and therefore less sensitive. After 5 h., the 15 control radicles were all more or less geotropic: after 9 h., eight of them were bent down beneath the horizon at various angles between 45o and 90o, the remaining seven being only slightly geotropic: after 25 h. all were rectangularly geotropic. The state of the eight cauterised radicles after the same intervals of time was as follows: after 5 h. one alone was slightly geotropic, and this was one with the tip only a very little discoloured: after 9 h. the one just mentioned was rectangularly geotropic, and two others were slightly so, and these were the three which had been scarcely [page 539]

affected by the caustic; the other five were still strictly horizontal.

After 24 h. 40 m. the three with only slightly discoloured tips were bent down rectangularly; the other five were not in the least affected, but several of them had grown rather tortuously, though still in a horizontal plane. The eight cauterised radicles which had at first a mean length of .36 inch, after 9 h. had increased to a mean length of .79 inch; and after 24 h. 40 m. to the extraordinary mean length of 2 inches. There was no plain difference in length between the five well cauterised radicles which remained horizontal, and the three with slightly cauterised tips which had become abruptly bent down. A few of the control radicles were measured after 25 h., and they were on an average only a little longer than the cauterised, viz., 2.19 inches. We thus see that killing the extreme tip of the radicle of this plant for a length of about 0.5 mm., though it stops the geotropic bending of the upper part, hardly interferes with the growth of the whole radicle.

 

In the same box with the 15 control specimens, the rapid geotropic bending and growth of which have just been described, there were six radicles, about .6 inch in length, extended horizontally, from which the tips had been cut off in a transverse direction for a length of barely 1 mm. These radicles were examined after 9 h. and again after 24 h. 40 m., and they all remained horizontal. They had not become nearly so tortuous as those above described which had been cauterised. The radicles with their tips cut off had grown in the 24 h. 40 m. as much, judging by the eye, as the cauterised specimens.

 

Zea mays.—The tips of several radicles, extended horizontally in damp air, were dried with blotting-paper and then touched in the first trial during 2

or 3 seconds with dry caustic; but this was too long a contact, for the tips were blackened for a length of rather above 1 mm. They showed no signs of geotropism after an interval of 9 h., and were then thrown away. In a second trial the tips of three radicles were touched for a shorter time, and were blackened for a length of from 0.5 to 0.75 mm.: they all remained horizontal for 4 h.,