On the other hand, all roots which penetrate the ground (including the modified root-like petioles of Megarrhiza and Ipomœa leptophylla) are guided in their downward course by geotropism; and so are many aërial 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 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 Æsculus (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.
The Burying of Seed-capsules: Trifolium subterraneum.—The flower-heads of this plant are remarkable from producing only 3 or 4 perfect flowers, which are situated exteriorly. All the other many flowers abort, and are modified into rigid points, with a bundle of vessels running up their centres. After a time 5 long, elastic, claw-like projections, which represent the divisions of the calyx, are developed on their summits. As soon as the perfect flowers wither they bend downwards, supposing the peduncle to stand upright, and they then closely surround its upper part. This movement is due to epinasty, as is likewise the case with the flowers of T. repens. The imperfect central flowers ultimately follow, one after the other, the same course. Whilst the perfect flowers are thus bending down, the whole peduncle curves downwards and increases much in length, until the flower-head reaches the ground. Vaucher[[3]] says that when the plant is so placed that the heads cannot soon reach the ground, the peduncles grow to the extraordinary length of from 6 to 9 inches. In whatever position the branches may be placed, the upper part of the peduncle at first bends vertically upwards through heliotropism; but as soon as the flowers begin to wither the downward curvature of the whole peduncle commences. As this latter movement occurred in complete darkness, and with peduncles arising from upright and from dependent branches, it cannot be due to apheliotropism or to epinasty, but must be attributed to geotropism. Nineteen upright flower-heads, arising from branches in all sorts of positions, on plants growing in a warm greenhouse, were marked with thread, and after 24 h. six of them were vertically dependent; these therefore had travelled through 180° in this time. Ten were extended sub-horizontally, and these had moved through about 90°. Three very young peduncles had as yet moved only a little downwards, but after an additional 24 h. were greatly inclined.
[3] ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. 1841, p. 106.
At the time when the flower-heads reach the ground, the younger imperfect flowers in the centre are still pressed closely together, and form a conical projection; whereas the perfect and imperfect flowers on the outside are upturned and closely surround the peduncle. They are thus adapted to offer as little resistance, as the case admits of, in penetrating the ground, though the diameter of the flower-head is still considerable. The means by which this penetration is effected will presently be described. The flower-heads are able to bury themselves in common garden mould, and easily in sand or in fine sifted cinders packed rather closely. The depth to which they penetrated, measured from the surface to the base of the head, was between 1/4 and ½ inch, but in one case rather above 0.6 inch. With a plant kept in the house, a head partly buried itself in sand in 6 h.: after 3 days only the tips of the reflexed calyces were visible, and after 6 days the whole had disappeared. But with plants growing out of doors we believe, from casual observations, that they bury themselves in a much shorter time.
After the heads have buried themselves, the central aborted flowers increase considerably in length and rigidity, and become bleached. They gradually curve, one after the other, upwards or towards the peduncle, in the same manner as did the perfect flowers at first. In thus moving, the long claws on their summits carry with them some earth. Hence a flower-head which has been buried for a sufficient time, forms a rather large ball, consisting of the aborted flowers, separated from one another by earth, and surrounding the little pods (the product of the perfect flowers) which lie close round the upper part of the peduncle. The calyces of the perfect and imperfect flowers are clothed with simple and multicellular hairs, which have the power of absorption; for when placed in a weak solution of carbonate of ammonia (2 gr. to 1 oz. of water) their protoplasmic contents immediately became aggregated and afterwards displayed the usual slow movements. This clover generally grows in dry soil, but whether the power of absorption by the hairs on the buried flower-heads is of any importance to them we do not know. Only a few of the flower-heads, which from their position are not able to reach the ground and bury themselves, yield seeds; whereas the buried ones never failed, as far as we observed, to produce as many seeds as there had been perfect flowers.
We will now consider the movements of the peduncle whilst curving down to the ground. We have seen in Chap. IV., Fig. 92, p. 225, that an upright young flower-head circumnutated conspicuously; and that this movement continued after the peduncle had begun to bend downwards. The same peduncle was observed when inclined at an angle of 19° above the horizon, and it circumnutated during two days. Another which was already curved 36° beneath the horizon, was observed from 11 A.M. July 22nd to the 27th, by which latter date it had become vertically dependent. Its course during the first 12 h. is shown in Fig. 190, and its position on the three succeeding mornings until the 25th, when it was nearly vertical. During the first day the peduncle clearly circumnutated, for it moved 4 times down and 3 times up; and on each succeeding day, as it sank downwards, the same movement continued, but was only occasionally observed and was less strongly marked. It should be stated that these peduncles were observed under a double skylight in the house, and that they generally moved downwards very much more slowly than those on plants growing out of doors or in the greenhouse.
Fig. 190. Trifolium subterraneum: downward movement of peduncle from 19° beneath the horizon to a nearly vertically dependent position, traced from 11 A.M. July 22nd to the morning of 25th. Glass filament fixed transversely across peduncle, at base of flower-head.
Fig. 191. Trifolium subterraneum: circumnutating movement of peduncle, whilst the flower-head was burying itself in sand, with the reflexed tips of the calyx still visible; traced from 8 A.M. July 26th to 9 A.M. on 27th. Glass filament fixed transversely across peduncle, near flower-head.
Fig. 192. Trifolium subterraneum: movement of same peduncle, with flower-head completely buried beneath the sand; traced from 8 A.M. to 7.15 P.M. on July 29th.
The movement of another vertically dependent peduncle with the flower-head standing half an inch above the ground, was traced, and again when it first touched the ground; in both cases irregular ellipses were described every 4 or 5 h. A peduncle on a plant which had been brought into the house, moved from an upright into a vertically dependent position in a single day; and here the course during the first 12 h. was nearly straight, but with a few well-marked zigzags which betrayed the essential nature of the movement. Lastly the circumnutation of a peduncle was traced during 51 h. whilst in the act of burying itself obliquely in a little heap of sand. After it had buried itself to such a depth that the tips of the sepals were alone visible, the above figure (Fig 191) was traced during 25 h. When the flower-head had completely disappeared beneath the sand, another tracing was made during 11 h. 45 m. (Fig. 192); and here again we see that the peduncle was circumnutating.