The Power of Movement in Plants by Charles Darwin (best fiction books to read TXT) 📕

After 24 h. it had become nearly vertical. Whether the exciting cause of the downward movement is geotropism or apheliotropism was not ascertained; but probably it is not apheliotropism, as all the gynophores grew straight down towards the ground, whilst the light in the hot-house entered from one side as well as from above. Another and older gynophore, the apex of which had nearly reached the ground, was observed during 3 days in the same manner as the first-mentioned short one; and it was found to be always circumnutating. During the first 34 h. it described a figure which * ‘Gard. Chronicle,’ 1857, p. 566.

[page 519]

represented four ellipses. Lastly, a long gynophore, the apex of which had buried itself to the depth of about half an inch, was Fig. 193 Arachis hypogoea: circumnutation of vertically dependent young gynophore, traced on a vertical glass from 10 A.M. July 31st to 8 A.M. Aug.

2nd.

Fig. 194. Arachis hypogoea: downward movement of same young gynophore, after being extended horizontally; traced on a vertical glass from 8.30

A.M. to 8.30 P.M. Aug. 2nd.

pulled up and extended horizontally: it quickly began to curve downwards in a zigzag line; but on the following day the ter-

[page 520]

minal bleached portion was a little shrivelled. As the gynophores are rigid and arise from stiff branches, and as they terminate in sharp smooth points, it is probable that they could penetrate the ground by the mere force of growth. But this action must be aided by the circumnutating movement, for fine sand, kept moist, was pressed close round the apex of a gynophore which had reached the ground, and after a few hours it was surrounded by a narrow open crack. After three weeks this gynophore was uncovered, and the apex was found at a depth of rather above half an inch developed into a small, white, oval pod.

Amphicarpoea monoica.—This plant produces long thin shoots, which twine round a support and of course circumnutate. Early in the summer shorter shoots are produced from the lower parts of the plant, which grow perpendicularly downwards and penetrate the ground. One of these, terminating in a minute bud, was observed to bury itself in sand to a depth of 0.2 inch in 24 h. It was lifted up and fixed in an inclined position about 25o beneath the horizon, being feebly illuminated from above. In this position it described two vertical ellipses in 24 h.; but on the following day, when brought into the house, it circumnutated only a very little round the same spot. Other branches were seen to penetrate the ground, and were afterwards found running like roots beneath the surface for a length of nearly two inches, and they had grown thick. One of these, after thus running, had emerged into the air. How far circumnutation aids these delicate branches in entering the ground we do not know; but the reflexed hairs with which they are clothed will assist in the work. This plant produces pods in the air, and others beneath the ground; which differ greatly in appearance. Asa Gray says* that it is the imperfect flowers on the creeping branches near the base of the plant which produce the subterranean pods; these flowers, therefore, must bury themselves like those of Arachis. But it may be suspected that the branches which were seen by us to penetrate the ground also produce subterranean flowers and pods.]

DIAGEOTROPISM.

Besides geotropism and apogeotropism, there is, according to Frank, an allied form of movement,

* ‘Manual of the Botany of the Northern United States,’ 1856, p. 106.

[page 521]

namely, “transverse-geotropism,” or diageotropism, as we may call it for the sake of matching our other terms. Under the influence of gravitation certain parts are excited to place themselves more or less transversely to the line of its action.* We made no observations on this subject, and will here only remark that the position of the secondary radicles of various plants, which extend horizontally or are a little inclined downwards, would probably be considered by Frank as due to transverse-geotropism. As it has been shown in Chap. I. that the secondary radicles of Cucurbita made serpentine tracks on a smoked glass-plate, they clearly circumnutated, and there can hardly be a doubt that this holds good with other secondary radicles. It seems therefore highly probable that they place themselves in their diageotropic position by means of modified circumnutation.

Finally, we may conclude that the three kinds of movement which have now been described and which are excited by gravitation, consist of modified circumnutation. Different parts or organs on the same plant, and the same part in different species, are thus excited to act in a widely different manner. We can see no reason why the attraction of gravity should directly modify the state of turgescence and subsequent growth of one part on the upper side and of another part on the lower side. We are therefore led to infer that both geotropic, apogeotropic, and diageotropic movements, the purpose of which we can generally understand, * Elfving has lately described (‘Arbeiten des Bot. Instituts in W�rzburg,’

B. ii. 1880, p. 489) an excellent instance of such movements in the rhizomes of certain plants.

[page 522]

have been acquired for the advantage of the plant by the modification of the ever-present movement of circumnutation. This, however, implies that gravitation produces some effect on the young tissues sufficient to serve as a guide to the plant.

[page 523]

CHAPTER XI.

LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.

General considerations—Vicia faba, effects of amputating the tips of the radicles—Regeneration of the tips—Effects of a short exposure of the tips to geotropic action and their subsequent amputation—Effects of amputating the tips obliquely—Effects of cauterising the tips—Effects of grease on the tips—Pisum sativum, tips of radicles cauterised transversely, and on their upper and lower sides—Phaseolus, cauterisation and grease on the tips—Gossypium—Cucurbita, tips cauterised transversely, and on their upper and lower sides—Zea, tips cauterised—Concluding remarks and summary of chapter—Advantages of the sensibility to geotropism being localised in the tips of the radicles.

CIESIELSKI states* that when the roots of Pisum, Lens and Vicia were extended horizontally with their tips cut off, they were not acted on by geotropism; but some days afterwards, when a new root-cap and vegetative point had been formed, they bent themselves perpendicularly downwards. He further states that if the tips are cut off, after the roots have been left extended horizontally for some little time, but before they have begun to bend downwards, they may be placed in any position, and yet will bend as if still acted on by geotropism; and this shows that some influence had been already transmitted to the bending part from the tip before it was amputated. Sachs repeated these experiments; he cut off a length of between .05 and 1 mm. (measured from the apex of the * ‘Abwartskr�mmung der Wurzel,’ Inaug. Dissert. Breslau, 1871, p. 29.

[page 524]

vegetative point) of the tips of the radicles of the bean (Vicia faba), and placed them horizontally or vertically in damp air, earth, and water, with the result that they became bowed in all sorts of directions.* He therefore disbelieved in Ciesielski’s conclusions. But as we have seen with several plants that the tip of the radicle is sensitive to contact and to other irritants, and that it transmits some influence to the upper growing part causing it to bend, there seemed to us to be no a priori improbability in Ciesielski’s statements. We therefore determined to repeat his experiments, and to try others on several species by different methods.

Vicia faba.—Radicles of this plant were extended horizontally either over water or with their lower surfaces just touching it. Their tips had previously been cut off, in a direction as accurately transverse as could be done, to different lengths, measured from the apex of the root-cap, and which will be specified in each case. Light was always excluded. We had previously tried hundreds of unmutilated radicles under similar circumstances, and found that every one that was healthy became plainly geotropic in under 12 h. In the case of four radicles which had their tips cut off for a length of 1.5 mm., new root caps and new vegetative points were reformed after an interval of 3 days 20 h.; and these when placed horizontally were acted on by geotropism. On some other occasions this regeneration of the tips and reacquired sensitiveness occurred within a somewhat shorter time. Therefore, radicles having their tips amputated should be observed in from 12 to 48 h. after the operation.

Four radicles were extended horizontally with their lower surfaces touching the water, and with their tips cut off for a length of only 0.5 mm.: after 23 h. three of them were still horizontal; after 47 h. one of the three became fairly geotropic; and after 70 h. the other two showed a trace of this action. The fourth radicle was vertically geotropic after 23 h.; but by an

* ‘Arbeiten des Bot. Instituts in W�rzburg,’ Heft. iii. 1873, p. 432.

[page 525]

accident the root-cap alone and not the vegetative point was found to have been amputated; so that this case formed no real exception and might have been excluded.

Five radicles were extended horizontally like the last, and had their tips cut off for a length of 1 mm.; after 22-23 h., four of them were still horizontal, and one was slightly geotropic; after 48 h. the latter had become vertical; a second was also somewhat geotropic; two remained approximately horizontal; and the last or fifth had grown in a disordered manner, for it was inclined upwards at an angle of 65o above the horizon.

Fourteen radicles were extended horizontally at a little height over the water with their tips cut off for a length of 1.5 mm.; after 12 h. all were horizontal, whilst five control or standard specimens in the same jar were all bent greatly downwards. After 24 h. several of the amputated radicles remained horizontal, but some showed a trace of geotropism, and one was plainly geotropic, for it was inclined at 40o beneath the horizon.

Seven horizontally extended radicles from which the tips had been cut off for the unusual length of 2 mm. unfortunately were not looked at until 35

h. had elapsed; three were still horizontal, but to our surprise, four were more or less plainly geotropic.

The radicles in the foregoing cases were measured before their tips were amputated, and in the course of 24 h. they had all increased greatly in length; but the measurements are not worth giving. It is of more importance that Sachs found that the rate of growth of the different parts of radicles with amputated tips was the same as with unmutilated ones. Altogether twenty-nine radicles were operated on in the manner above described, and of these only a few showed any geotropic curvature within 24 h.; whereas radicles with unmutilated tips always became, as already stated, much bent down in less than half of this time. The part of the radicle which bends most lies at the distance of from 3 to 6 mm. from the tip, and as the bending part continues to grow after the operation, there does not seem any reason why it should not have been acted on by geotropism, unless its curvature depended on some influence transmitted from the tip. And we have clear evidence of such transmission in Ciesielski’s experiments, which we repeated and extended in the following manner.

Beans were embedded in friable peat with the hilum downwards, and after their radicles had grown perpendicularly down for a length of from � to 1

inch, sixteen were selected which

[page 526]

were perfectly straight,