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

but after 8 h. 30 m. one of them, in which the blackened tip was only 0.5 mm. in length, was inclined at 21o beneath the horizon. Six control radicles all became slightly geotropic in 4 h., and strongly so after 8 h. 30 m., with the chief seat of curvature generally between 6 or 7 mm. from the apex. In the cauterised specimens, the terminal growing part, 10 mm. in length, increased during [page 540]

the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3 mm.

 

In a third trial the tips of five radicles (exposed to a temp. of 70o -

71o) were touched with the caustic only once and very slightly; they were afterwards examined under the microscope, and the part which was in any way discoloured was on an average .76 mm. in length. After 4 h. 10 m. none were bent; after 5 h. 45 m., and again after 23 h. 30 m., they still remained horizontal, excepting one which was now inclined 20o beneath the horizon.

The terminal part, 10 mm. in length, had increased greatly in length during the 23 h. 30 m., viz., to an average of 26 mm. Four control radicles became slightly geotropic after the 4 h. 10 m., and plainly so after the 5 h. 45

m. Their mean length after the 23 h. 30 m. had increased from 10 mm. to 31

mm. Therefore a slight cauterisation of the tip checks slightly the growth of the whole radicle, and manifestly stops the bending of that part which ought to bend most under the influence of geotropism, and which still continues to increase greatly in length.]

 

Concluding Remarks.—Abundant evidence has now been given, showing that with various plants the tip of the radicle is alone sensitive to geotropism; and that when thus excited, it causes the adjoining parts to bend. The exact length of the sensitive part seems to be somewhat variable, depending in part on the age of the radicle; but the destruction of a length of from less than 1 to 1.5 mm. (about 1/20th of an inch), in the several species observed, generally sufficed to prevent any part of the radicle from bending within 24 h., or even for a longer period. The fact of the tip alone being sensitive is so remarkable a fact, that we will here give a brief summary of the foregoing experiments. The tips were cut off 29

horizontally extended radicles of Vicia faba, and with a few exceptions they did not become geotropic in 22 or 23 h., whilst unmutilated radicles were always bowed downwards in 8 or 9 h. It should be borne in mind that the mere act of cutting

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off the tip of a horizontally extended radicle does not prevent the adjoining parts from bending, if the tip has been previously exposed for an hour or two to the influence of geotropism. The tip after amputation is sometimes completely regenerated in three days; and it is possible that it may be able to transmit an impulse to the adjoining parts before its complete regeneration. The tips of six radicles of Cucurbita ovifera were amputated like those of Vicia faba; and these radicles showed no signs of geotropism in 24 h.; whereas the control specimens were slightly affected in 5 h., and strongly in 9 h.

 

With plants belonging to six genera, the tips of the radicles were touched transversely with dry caustic; and the injury thus caused rarely extended for a greater length than 1 mm., and sometimes to a less distance, as judged by even the faintest discoloration. We thought that this would be a better method of destroying the vegetative point than cutting it off; for we knew, from many previous experiments and from some given in the present chapter, that a touch with caustic on one side of the apex, far from preventing the adjoining part from bending, caused it to bend. In all the following cases, radicles with uncauterised tips were observed at the same time and under similar circumstances, and they became, in almost every instance, plainly bowed downwards in one-half or one-third of the time during which the cauterised specimens were observed. With Vicia faba 19

radicles were cauterised; 12 remained horizontal during 23-24 h.; 6 became slightly and 1 strongly geotropic. Eight of these radicles were afterwards reversed, and again touched with caustic, and none of them became geotropic in 24 h., whilst the reversed control specimens became strongly bowed downwards within this time.

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With Pisum sativum, five radicles had their tips touched with caustic, and after 32 h. four were still horizontal. The control specimens were slightly geotropic in 7 h. 20 m., and strongly so in 24 h. The tips of 9 other radicles of this plant were touched only on the lower side, and 6 of them remained horizontal for 24 h., or were upturned in opposition to geotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolus multiflorus, 15 radicles were cauterised, and 8 remained horizontal for 24

h.; whereas all the controls were plainly geotropic in 8 h. 30 m. Of 5

cauterised radicles of Gossypium herbaceum, 4 remained horizontal for 23 h.

and 1 became slightly geotropic; 6 control radicles were distinctly geotropic in 7 h. 45 m. Five radicles of Cucurbita ovifera remained horizontal in peat-earth during 25 h., and 9 remained so in damp air during 8 � h.; whilst the controls became slightly geotropic in 4 h. 10 m. The tips of 10 radicals of this plant were touched on their lower sides, and 6

of them remained horizontal or were upturned after 19 h., 1 being slightly and 3 strongly geotropic.

 

Lastly, the tips of several radicles of Vicia faba and Phaseolus multiflorus were thickly coated with grease for a length of 3 mm. This matter, which is highly injurious to most plants, did not kill or stop the growth of the tips, and only slightly lessened the rate of growth of the whole radicle; but it generally delayed a little the geotropic bending of the upper part.

 

The several foregoing cases would tell us nothing, if the tip itself was the part which became most bent; but we know that it is a part distant from the tip by some millimeters which grows quickest, and which, under the influence of geotropism, bends most. We have no reason to suppose that this part is injured by the death or injury of the tip; and it is certain [page 543]

that after the tip has been destroyed this part goes on growing at such a rate, that its length was often doubled in a day. We have also seen that the destruction of the tip does not prevent the adjoining part from bending, if this part has already received some influence from the tip. As with horizontally extended radicles, of which the tip has been cut off or destroyed, the part which ought to bend most remains motionless for many hours or days, although exposed at right angles to the full influence of geotropism, we must conclude that the tip alone is sensitive to this power, and transmits some influence or stimulus to the adjoining parts, causing them to bend. We have direct evidence of such transmission; for when a radicle was left extended horizontally for an hour or an hour and a half, by which time the supposed influence will have travelled a little distance from the tip, and the tip was then cut off, the radicle afterwards became bent, although placed perpendicularly. The terminal portions of several radicles thus treated continued for some time to grow in the direction of their newly-acquired curvature; for as they were destitute of tips, they were no longer acted on by geotropism. But after three or four days when new vegetative points were formed, the radicles were again acted on by geotropism, and now they curved themselves perpendicularly downwards. To see anything of the above kind in the animal kingdom, we should have to suppose than an animal whilst lying down determined to rise up in some particular direction; and that after its head had been cut off, an impulse continued to travel very slowly along the nerves to the proper muscles; so that after several hours the headless animal rose up in the predetermined direction.

 

As the tip of the radicle has been found to be the [page 544]

part which is sensitive to geotropism in the members of such distinct families as the Leguminosae, Malvaceae, Cucurbitaceae and Gramineae, we may infer that this character is common to the roots of most seedling plants.

Whilst a root is penetrating the ground, the tip must travel first; and we can see the advantage of its being sensitive to geotropism, as it has to determine the course of the whole root. Whenever the tip is deflected by any subterranean obstacle, it will also be an advantage that a considerable length of the root should be able to bend, more especially as the tip itself grows slowly and bends but little, so that the proper downward course may be soon recovered. But it appears at first sight immaterial whether this were effected by the whole growing part being sensitive to geotropism, or by an influence transmitted exclusively from the tip. We should, however, remember that it is the tip which is sensitive to the contact of hard objects, causing the radicle to bend away from them, thus guiding it along the lines of least resistance in the soil. It is again the tip which is alone sensitive, at least in some cases, to moisture, causing the radicle to bend towards its source. These two kinds of sensitiveness conquer for a time the sensitiveness to geotropism, which, however, ultimately prevails. Therefore, the three kinds of sensitiveness must often come into antagonism; first one prevailing, and then another; and it would be an advantage, perhaps a necessity, for the interweighing and reconciling of these three kinds of sensitiveness, that they should be all localised in the same group of cells which have to transmit the command to the adjoining parts of the radicle, causing it to bend to or from the source of irritation.

 

Finally, the fact of the tip alone being sensitive to [page 545]

the attraction of gravity has an important bearing on the theory of geotropism. Authors seem generally to look at the bending of a radicle towards the centre of the earth, as the direct result of gravitation, which is believed to modify the growth of the upper or lower surfaces, in such a manner as to induce curvature in the proper direction. But we now know that it is the tip alone which is acted on, and that this part transmits some influence to the adjoining parts, causing them to curve downwards. Gravity does not appear to act in a more direct manner on a radicle, than it does on any lowly organised animal, which moves away when it feels some weight or pressure.

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CHAPTER XII.

 

SUMMARY AND CONCLUDING REMARKS.

 

Nature of the circumnutating movement—History of a germinating seed—The radicle first protrudes and circumnutates—Its tip highly sensitive—

Emergence of the hypocotyl or of the epicotyl from the ground under the form of an arch - Its circumnutation and that of the cotyledons—The seedling throws up a leaf-bearing stem—The circumnutation of all the parts or organs—Modified circumnutation—Epinasty and hyponasty—Movements of climbing plants—Nyctitropic movements—Movements excited by light and gravitation—Localised sensitiveness—Resemblance between the movements of plants and animals—The tip of the radicle acts like a brain.

 

IT may be useful to the reader if we briefly sum up the chief conclusions, which, as far as we can judge, have been fairly well established by the observations given in this volume. All the parts or organs in every plant whilst they