The Power of Movement in Plants - Charles Darwin (top reads txt) 📗

the middle, so that the lower part answers to the main peduncle, [page 507]

and the upper part to one of the sub-peduncles of O. carnosa. The upper part bends downwards, after the flower has begun to wither, and the whole peduncle then forms a hook; that this bending is due to epinasty we may infer from the case of O. carnosa. When the pod is nearly ripe, the upper part straightens itself and becomes erect; and this is due to hyponasty or apogeotropism, or both combined, and not to heliotropism, for it occurred in darkness. The short, hooked part of the peduncle of a cleistogamic flower, bearing a pod nearly ripe, was observed in the dark during three days. The apex of the pod at first pointed perpendicularly down, but in the course of three days rose 90o, so that it now projected horizontally. The course during the two latter days is shown in Fig. 189; and it may be seen how greatly the peduncle, whilst rising, circumnutated. The lines of chief movement were at right angles to the plane of the originally hooked part.

The tracing was not continued any longer; but after two additional days, the peduncle with its capsule had become straight and stood upright.]

Concluding Remarks on Apogeotropism.—When apogeotropism is rendered by any means feeble, it acts, as shown in the several foregoing cases, by increasing the always present circumnutating movement in a direction opposed to gravity, and by diminishing that in the direction of gravity, as well as that to either side. The upward movement thus becomes unequal in rate, and is sometimes interrupted by stationary periods. Whenever irregular ellipses or loops are still formed, their longer axes are almost always directed in the line of gravity, in an analogous manner as occurred with heliotropic movements in reference to the light. As apogeotropism acts more and more energetically, ellipses or loops cease to be formed, and the course becomes at first strongly, and then less and less zigzag, and finally rectilinear. From this gradation in the nature of the movement, and more especially from all growing parts, which alone (except when pulvini are present) are acted on by apogeotropism, con-

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tinually circumnutating, we may conclude that even a rectilinear course is merely an extremely modified form of circumnutation. It is remarkable that a stem or other organ which is highly sensitive to apogeotropism, and which has bowed itself rapidly upwards in a straight line, is often carried beyond the vertical, as if by momentum. It then bends a little backwards to a point round which it finally circumnutates. Two instances of this were observed with the hypocotyls of Beta vulgaris, one of which is shown in Fig. 183, and two other instances with the hypocotyls of Brassica. This momentum-like movement probably results from the accumulated effects of apogeotropism. For the sake of observing how long such after-effects lasted, a pot with seedlings of Beta was laid on its side in the dark, and the hypocotyls in 3 h. 15 m. became highly inclined. The pot, still in the dark, was then placed upright, and the movements of the two hypocotyls were traced; one continued to bend in its former direction, now in opposition to apogeotropism, for about 37 m., perhaps for 48 m.; but after 61 m. it moved in an opposite direction. The other hypocotyl continued to move in its former course, after being placed upright, for at least 37 m.

Different species and different parts of the same species are acted on by apogeotropism in very different degrees. Young seedlings, most of which circumnutate quickly and largely, bend upwards and become vertical in much less time than do any older plants observed by us; but whether this is due to their greater sensitiveness to apogeotropism, or merely to their greater flexibility we do not know. A hypocotyl of Beta traversed an angle of 109o in 3 h. 8 m., and a cotyledon of Phalaris an angle of 130o in 4 h. 30 m. On the other hand, the stem of a herbaceous [page 509]

Verbena rose 90o in about 24 h.; that of Rubus 67o, in 70 h.; that of Cytisus 70o, in 72 h.; that of a young American Oak only 37o, in 72 h. The stem of a young Cyperus alternifolius rose only 11o in 96 h.; the bending being confined to near its base. Though the sheath-like cotyledons of Phalaris are so extremely sensitive to apogeotropism, the first true leaves which protrude from them exhibited only a trace of this action. Two fronds of a fern, Nephrodium molle, both of them young and one with the tip still inwardly curled, were kept in a horizontal position for 46 h., and during this time they rose so little that it was doubtful whether there was any true apogeotropic movement.

The most curious case known to us of a difference in sensitiveness to gravitation, and consequently of movement, in different parts of the same organ, is that offered by the petioles of the cotyledons of Ipomoea leptophylla. The basal part for a short length where united to the undeveloped hypocotyl and radicle is strongly geotropic, whilst the whole upper part is strongly apogeotropic. But a portion near the blades of the cotyledons is after a time acted on by epinasty and curves downwards, for the sake of emerging in the form of an arch from the ground; it subsequently straightens itself, and is then again acted on by apogeotropism.

A branch of Cucurbita ovifera, placed horizontally, moved upwards during 7

h. in a straight line, until it stood at 40o above the horizon; it then began to circumnutate, as if owing to its trailing nature it had no tendency to rise any higher. Another upright branch was secured to a stick, close to the base of a tendril, and the pot was then laid horizontally in the dark. In this position the tendril circumnutated and made [page 510]

several large ellipses during 14 h., as it likewise did on the following day; but during this whole time it was not in the least affected by apogeotropism. On the other hand, when branches of another Cucurbitaceous plant, Echinocytis lobata, were fixed in the dark so that the tendrils depended beneath the horizon, these began immediately to bend upwards, and whilst thus moving they ceased to circumnutate in any plain manner; but as soon as they had become horizontal they recommenced to revolve conspicuously.* The tendrils of Passiflora gracilis are likewise apogeotropic. Two branches were tied down so that their tendrils pointed many degrees beneath the horizon. One was observed for 8 h., during which time it rose, describing two circles, one above the other. The other tendril rose in a moderately straight line during the first 4 h., making however one small loop in its course; it then stood at about 45o above the horizon, where it circumnutated during the remaining 8 h. of observation.

A part or organ which whilst young is extremely sensitive to apogeotropism ceases to be so as it grows old; and it is remarkable, as showing the independence of this sensitiveness and of the circumnutating movement, that the latter sometimes continues for a time after all power of bending from the centre of the earth has been lost. Thus a seedling Orange bearing only 3 young leaves, with a rather stiff stem, did not curve in the least upwards during 24 h. whilst extended horizontally; yet it circumnutated all the time over a small space. The hypocotyl of a young seedling of Cassia tora, similarly placed, became vertical in 12 h.; that of an older seedling, 1 1/4 inch in height,

* For details see ‘The Movements and Habits of Climbing Plants,’ 1875, p.

131.

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became so in 28 h.; and that of another still older one, 1 � inch in height, remained horizontal during two days, but distinctly circumnutated during this whole time.

When the cotyledons of Phalaris or Avena are laid horizontally, the uppermost part first bends upwards, and then the lower part; consequently, after the lower part has become much curved upwards, the upper part is compelled to curve backwards in an opposite direction, in order to straighten itself and to stand vertically; and this subsequent straightening process is likewise due to apogeotropism. The upper part of 8

young cotyledons of Phalaris were made rigid by being cemented to thin glass rods, so that this part could not bend in the least; nevertheless, the basal part was not prevented from curving upward. A stem or other organ which bends upwards through apogeotropism exerts considerable force; its own weight, which has of course to be lifted, was sufficient in almost every instance to cause the part at first to bend a little downwards; but the downward course was often rendered oblique by the simultaneous circumnutating movement. The cotyledons of Avena placed horizontally, besides lifting their own weight, were able to furrow the soft sand above them, so as to leave little crescentic open spaces on the lower sides of their bases; and this is a remarkable proof of the force exerted.

As the tips of the cotyledons of Phalaris and Avena bend upwards through the action of apogeotropism before the basal part, and as these same tips when excited by a lateral light transmit some influence to the lower part, causing it to bend, we thought that the same rule might hold good with apogeotropism. Consequently, the tips of 7 cotyledons of Phalaris were [page 512]

cut off for a length in three cases of .2 inch and in the four other cases of .14, .12, .1, and .07 inch. But these cotyledons, after being extended horizontally, bowed themselves upwards as effectually as the unmutilated specimens in the same pots, showing that sensitiveness to gravitation is not confined to their tips.

GEOTROPISM.

This movement is directly the reverse of apogeotropism. Many organs bend downwards through epinasty or apheliotropism or from their own weight; but we have met with very few cases of a downward movement in sub-a�rial organs due to geotropism. We shall however, give one good instance in the following section, in the case of Trifolium subterraneum, and probably in that of Arachis hypogaea.

On the other hand, all roots which penetrate the ground (including the modified root-like petioles of Megarrhiza and Ipomoea leptophylla) are guided in their downward course by geotropism; and so are many a�rial roots, whilst others, as those of the Ivy, appear to be indifferent to its action. In our first chapter the movements of the radicles of several seedlings were described. We may there see (Fig. 1) how a radicle of the cabbage, when pointing vertically upwards so as to be very little acted on by geotropism, circumnutated; and how another (Fig. 2) which was at first placed in an inclined position bowed itself downwards in a zigzag line, sometimes remaining stationary for a time. Two other radicles of the cabbage travelled downwards in almost rectilinear courses. A radicle of the bean placed upright (Fig. 20) made a great sweep and zigzagged; but as it sank downwards and was more strongly acted on by geotropism, it moved in an [page 513]

almost straight course. A radicle of Cucurbita, directed upwards (Fig. 26), also zigzagged at first, and described small loops; it then moved in a straight line. Nearly the same result was observed with the radicles of Zea mays. But the best evidence of the intimate connection between circumnutation and geotropism was afforded by the radicles of Phaseolus, Vicia, and Quercus, and in a less degree by those of Zea and Aesculus (see Figs. 18, 19, 21, 41, and 52); for when these were compelled to grow and slide down highly inclined surfaces of smoked glass, they left distinctly serpentine tracks.

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