The movement in the awn of Stipa spartea, an allied species in the Red River colony, cause the sharp, rigid points of the seed to enter and bury themselves in the wool of the sheep with which the grass comes into contact. Further than this, it is affirmed that the seeds penetrate the skin by their screw-like movement, and cause the death of the animals. At the same time that hygroscopism receives an illustration by these grasses, its utility is demonstrated by the dissemination of the seeds, and continuance of the species. In the Geranium family, after the ovules are fertilised, the centre of the receptacle continues to grow until it is prolonged into a long beak, with the seeds arranged around the base, and the elongated styles applied to the sides of the beak. The peculiar beak-like form which the fruit thus assumes has acquired for the plants the popular names of Crane's-bill, Stork's-bill, &c. The mode in which the carpels are loosened at the base, and curl upwards like a watch-spring to the top of the beak, is familiar to all who have observed the members of the family. That the twisting is the result of an hygrometric property in the awn has been demonstrated by Professor Asa Gray. He says: "The narrow carpel is pointed at the base; the long awn or style in drying bends at right angles with the carpel, and twists in many turns, depending on the amount of dryness, and untwists in a moister air or when wet. We had wondered that no one seemed to have given an account of the way in which this mechanism acts so as to bury the seed in the ground. Dispersed by the wind over the loose or sandy soil which these species prefer, the seed bearing end, being the heavier, lies next to the ground, and is the comparatively fixed point around which the long awn makes circular sweeps, whether in twisting or untwisting. This gives a rotary movement to the carpel, fixes the sharp end in the soil, and, whether twisting or untwisting, causes it to bore into and bury itself in the ground."
M. Roux says that in Erodium, when the seeds are thus interred, the moisture of the soil soon destroys the epidermis, and this allows the long beak to detach itself at its articulation with the style, leaving it planted in good condition quietly to germinate. Thus then, it may be seen that, by their own hygroscopy, the seeds become their own planters, and effectually secure themselves in a favourable position to ensure the continuance of the species.
The little cruciferous plant to which the name of "rose of Jericho" has been applied (Anastatica hierochuntina), has a divided claim to be included with hygroscopic, and also with mystic plants. It is a native of the dry wastes of Northern Africa and Palestine, and the sandy deserts of Arabia. It is a small bushy plant, not more than five or six inches high. After flowering the leaves fall off, and the branches become dry, shrivel, and curve inwards towards the centre, so as to form the plant into a kind of ball. In this condition it is easily uprooted from the soil, carried by the winds, blown and tossed across the desert into the sea. Upon coming into contact with water the plant again unfolds itself, the branches expand, the seed vessels open and disperse the seeds, which are carried by the tide and deposited on the shore. The property of expanding when in contact with moisture led to a superstitious regard for the plant which, it was believed, expanded on the anniversary of the birth of our Saviour. It was called also Rosa Mariae. The plant may be kept for years, if preserved in a dry place, but at any time when the root is placed in water, or the entire plant immersed it will expand and, it is said, in the course of a few hours the buds of the flowers will swell, and appear as if newly taken from the ground. Old Gerard calls it the "heath rose of Jericho," but he says that "the coiner spoiled the name in the mint, for of all plants that have bin written of, there is not any more unlike unto the rose." He gives a description similar to that recorded above, with figures of the plant in the dried and also in the expanded state.
Akin in its movements to the hierochuntina is the "wind witch" of the Russian steppes, so graphically described by Schleiden: "In autumn the stem of the thistle plant rots off and the globe of branches dries up into a ball, light as a feather, which is then driven through the air by the autumnal winds, over the steppe. Numbers of such balls often fly at once over the plain with such rapidity that no horseman can catch them, now hopping with short, quick springs along the ground, onward in a spirit-like dance over the turf, now caught by an eddy, rising suddenly a hundred feet into the air. Often one 'wind witch' hooks on to another, twenty more join company, and the whole gigantic, yet airy mass rolls away before the piping east wind."
The designation "rose of Jericho" has been applied to the capsular fruits of a species of fig marigold, from the Cape of Good Hope (Mesembryanthemum tripolium). On the approach of rain, or when placed in water, these seed vessels gradually open like a star, closing again as they become dry. Thunberg, remarking on this circumstance, says:—"In this we see the wisdom of an All-wise Creator, inasmuch as this plant, which is found in the most arid plains of South Africa, keeps its seeds closely locked up in time of drought; but when the rainy season comes, and the seeds can grow, it opens its capsules, and lets fall the seeds, that they may be dispersed abroad." These capsules have also been known by the name of "Flowers of Crete."
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Fig. 50.—Capsules of Mesembryanthemum tripolium closed.
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Fig. 51.—Capsule of Mesembryanthemum tripolium open.
The violent dehiscence of fruits is occasioned in many instances by the hygroscopism of some of the parts. Although this usually takes place in dead tissues, some of the instances are of interest in this connection. One of the best known is the fruit of the sand box-tree (Hura crepitans). The capsule is about the size of an orange, and consists of a number of carpels, packed together side by side. When dry the carpels separate and fly apart with a loud report. When once separated they cannot be compressed again into their original form. These capsules were formerly used as "sand-boxes," before the invention of blotting paper, but had to be bound together in order to prevent their sudden dehiscence. It was not unusual for them to fly in pieces after many years.
<!-- image -->Fig. 52.—Sand-box (Hura crepitans).
The pods of some plants of the pea and bean family (Leguminosae) have a tendency to separate at the valves, and become twisted or curl backwards with considerable force. The large pods of an African tree (Pentaclethra macrophylla) possess this property in an exaggerated degree. When fastened together by strong wires they break themselves in pieces in their efforts to become free! The length of these pods is from twenty-two to twenty-five inches, with a breadth of about three and a-half inches. From the observations of Professor Oliver, it has been ascertained that the increase and decrease of length between dryness and moisture is sixteen per cent., so that the contraction in one pod would not be less than three inches. Contractility of a similar character but to a less extent has been observed in a plant of the cucumber family (Echinocystis lobata).
An illustration of a different kind to those hitherto adduced is furnished by Dr. Darwin, in one of his works. It refers to a plant which bears a name in allusion to its hygrometric predilections (Porlieria hygrometrica). "In the Botanic Gardens at Wurzburg," he says, "there was a plant in a pot, out of doors, which was daily watered, and another in the open ground which was never watered. After some hot and dry weather there was a great difference in the state of the leaflets on these two plants; those on the unwatered plant, in the open ground, remaining half, or even quite, closed during the day. But twigs cut from this bush, with their ends standing in water, or wholly immersed in it, or kept in damp air under a bell glass, opened their leaves though exposed to a blazing sun, whilst those on the plant in the ground remained closed. The leaves on this same plant after some heavy rain, remained open for two days; they then became half closed during two days, and after an additional day were quite closed. This plant was now copiously watered, and on the following morning the leaflets were fully expanded. The other plant, growing in a pot, after having been exposed to heavy rain, was placed before a window, with its leaflets open, and they remained so during the daytime for forty-eight hours, but after an additional day were half closed. The plant was then watered, and the leaflets on the two following days remained open. On the third day they were again half closed, but on being again watered remained open during the two next days. From these facts we may conclude that the plant soon feels the want of water, and that as soon as this occurs, it partially or quite closes its leaflets, which in their then imbricated condition expose a small surface to evaporation. It is probable that this sleep-like movement, which occurs only when the ground is dry, is an adaptation against the loss of moisture."
The hygroscopic character of some of the species of Selaginella is familiar to horticulturists. These plants have somewhat the appearance of large mosses, and are not uncommon in greenhouses. In the classification there is one entire section devoted to the species which have the foliage curved inwards when dry, so that many of them roll up by contraction into the form of a ball. This is the habit of Selaginella convoluta, a species abundant in Bahia and Pernambuco, and also of Selaginella lepidophylla, which latter has been called the "Resurrection plant," from its habit of expanding under moisture. One of the earliest observers of this phenomenon in South America was the celebrated traveller Martius, who called the plant Lycopodium hygrometricum.
Sensibility to variations in humidity is also an important factor in the dispersion of the spores in many of the ferns. In these the sporangia are girt by an elastic ring, which assists in the rupture of the sporangium. "When the sporangia arrive at maturity, and are under certain favourable conditions as to dryness, the elasticity of the ring causes them to burst open with force and sound sufficient to be heard, and this takes place in a direction at, or very near to, a right angle with the direction of the ring." This serves to remind us that the bursting of the spathe in some palms, with a loud report, as sometimes recorded, will be due to a like cause. The dryness of the atmosphere inducing contraction in one series of cells, greater than in another, produces a violent rupture, as in the separation of the carpels of the "sand-box," accompanied by a sharp sound.
Microscopists hold in great favour an object which finds a place in almost every "cabinet," and consists of the peristome of the common hygrometric moss (Funaria hygrometrica). In this and many other species, especially Hypnum, the urn-shaped receptacles which contain the spores are at first covered with a lid, or hood, and when the latter falls away are seen to be fringed with a single or double row of teeth, called the peristome. These teeth converging inwards cover the spores, and prevent their escape; when expanded or recurved they permit of the free discharge of the contents of the urn. This fringe is exceedingly sensitive to moisture, opening and closing when breathed upon, or as the moisture of the breath evaporates. It is a very pretty and available illustration of a vegetable hygrometer. Some bryologists object to this as a legitimate inference. They assert that the movement is not vital, but is merely mechanical, resulting from the diverse character of the outer and inner layer of cells, of which the peristome is composed. Admitting the structure to be thus accurately described, it becomes a structural adaptation to secure a certain end, which is beneficial to the plant. Surely it must be too delicate a distinction to admit specialised structure in other instances, such as stigmatic surfaces, &c., and reject it in this. As the opening and closing of the peristome takes place whilst the plant is living, even whilst the urn of the plant is still attached and living; and as it is of a manifest utility in securing the dispersion of the spores at such a period as when the moisture of the atmosphere would best secure their germination, we are prepared to retain the peristome of mosses as a satisfactory illustration of hygroscopism.