Any one who will observe a flower-head burying itself, will be convinced that the rocking movement, due to the continued circumnutation of the peduncle, plays an important part in the act. Considering that the flower-heads are very light, that the peduncles are long, thin, and flexible, and that they arise from flexible branches, it is incredible that an object as blunt as one of these flower-heads could penetrate the ground by means of the growing force of the peduncle, unless it were aided by the rocking movement. After a flower-head has penetrated the ground to a small depth, another and efficient agency comes into play; the central rigid aborted flowers, each terminating in five long claws, curve up towards the peduncle; and in doing so can hardly fail to drag the head down to a greater depth, aided as this action is by the circumnutating movement, which continues after the flower-head has completely buried itself. The aborted flowers thus act something like the hands of the mole, which force the earth backwards and the body forwards.
It is well known that the seed-capsules of various widely distinct plants either bury themselves in the ground, or are produced from imperfect flowers developed beneath the surface. Besides the present case, two other well-marked instances will be immediately given. It is probable that one chief good thus gained is the protection of the seeds from animals which prey on them. In the case of T. subterraneum, the seeds are not only concealed by being buried, but are likewise protected by being closely surrounded by the rigid, aborted flowers. We may the more confidently infer that protection is here aimed at, because the seeds of several species in this same genus are protected in other ways;[[4]] namely, by the swelling and closure of the calyx, or by the persistence and bending down of the standard-petal, etc. But the most curious instance is that of T. globosum, in which the upper flowers are sterile, as in T. subterraneum, but are here developed into large brushes of hairs which envelop and protect the seed-bearing flowers. Nevertheless, in all these cases the capsules, with their seeds, may profit, as Mr. T. Thiselton Dyer has remarked,[[5]] by their being kept somewhat damp; and the advantage of such dampness perhaps throws light on the presence of the absorbent hairs on the buried flower-heads of T. subterraneum. According to Mr. Bentham, as quoted by Mr. Dyer, the prostrate habit of Helianthemum prostratum “brings the capsules in contact with the surface of the ground, postpones their maturity, and so favours the seeds attaining a larger size.” The capsules of Cyclamen and of Oxalis acetosella are only occasionally buried, and this only beneath dead leaves or moss. If it be an advantage to a plant that its capsules should be kept damp and cool by being laid on the ground, we have in these latter cases the first step, from which the power of penetrating the ground, with the aid of the always present movement of circumnutation, might afterwards have been gained.
[4] Vaucher, ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. p. 110.
[5] See his interesting article in ‘Nature,’ April 4th, 1878, p. 446.
Arachis hypogoea.—The flowers which bury themselves, rise from stiff branches a few inches above the ground, and stand upright. After they have fallen off, the gynophore, that is the part which supports the ovarium, grows to a great length, even to 3 or 4 inches, and bends perpendicularly downwards. It resembles closely a peduncle, but has a smooth and pointed apex, which contains the ovules, and is at first not in the least enlarged. The apex after reaching the ground penetrates it, in one case observed by us to a depth of 1 inch, and in another to 0.7 inch. It there becomes developed into a large pod. Flowers which are seated too high on the plant for the gynophore to reach the ground are said[[6]] never to produce pods.
[6] ‘Gard. Chronicle,’ 1857, p. 566.
The movement of a young gynophore, rather under an inch in length and vertically dependent, was traced during 46 H. by means of a glass filament (with sights) fixed transversely a little above the apex. It plainly circumnutated (Fig. 193) whilst increasing in length and growing downwards. It was then raised up, so as to be extended almost horizontally, and the terminal part curved itself downwards, following a nearly straight course during 12 h., but with one attempt to circumnutate, as shown in Fig. 194. 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 represented four ellipses. Lastly, a long gynophore, the apex of which had buried itself to the depth of about half an inch, was pulled up and extended horizontally: it quickly began to curve downwards in a zigzag line; but on the following day the terminal 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.
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.
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 25° 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[[7]] 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.