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Fig. 54.—Caltrops, or fruits of Tribulus terrestris.
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Fig. 55.—Fruits of Pedalium murex.
More efficient still are the recurved hooks with which some of the spines of fruits are terminated. To a limited extent this may be seen in the small fruits of the carrot, and a few other of the Umbelliferae, but much more strongly developed in the "burrs" of the burdock (Lappa). We are familiar enough with the tenacity with which these "burrs" will adhere to the clothing of any one passing amongst the plants, but their entanglement in the woolly coat of animals is much more complete.
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Fig. 56.--Burdock (Lappa minor)
Similar burrs are produced by Xanthium strumarium, which is not a native plant, but has been introduced by the pertinacious adhesion of its fruits to the coats of animals. An allied common tropical species (Xanthium spinosum) has by a similar means spread itself over a wide area, becoming thereby a nuisance in some of its adopted homes. In South Africa it has established itself, by means of the merino sheep, and "extended itself through the sheep-walks of the colony to such a degree, and so endangered the character of the wool through its achenes, that special legislative enactments have been made in regard to its extirpation, and rigid enforcement of penalties alone has kept it from being a sweeping curse to the wool-producers. In the Orange River Republic, where only until last year (1872) this weed was allowed to revel undisturbed, save where some stray Dutch boer was given less to coffee-drinking and sleep, and more to an intelligent regard for the future of his pasturage, it had so affected the wool of some parts of the country as to make it nearly unremunerative as a staple product. Tardy legislation on the obnoxious introduction had to be adopted there also."
The "cleavers," or "goose-grass" (Galium aparine), found in every hedge and thicket has small fruits which are densely covered with minute hooks, and transportation is rendered easy. It is in warmer climates, where hooked fruits attain a larger dimension, that they present a formidable appearance. In one of these (Martynia diandra) the pair of hooks are very sharp and rigid, the points entering the flesh like a needle; but even these are exceeded by another species (Proboscidea jussieui), which the late Frank Buckland was wont to declare must have been created for the express purpose of sticking to the tails of the wild horses that roam the plains of South America. The horns in this species are often five or six inches in length, and the aspect may be readily imagined from our reduced figure (fig. 58). The same family contains the Grapnel plants, of which one species (Harpagophytum leptocarpum) is found in Madagascar, and another in Africa (Harpagophytum procumbens). The latter, and most effective of the two, although least formidable in appearance, has the capsule armed with a number of rigid woody thorns, standing out in all directions, their tips furnished with two or three recurved hooks like miniature grapnels. It is easy to imagine how such a fruit may be transported, the difficulty being rather to believe in the possibility of its remaining at rest (fig. 59).
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Fig. 57.—Hooked fruits of Martynia diandra.
<!-- image -->Fig. 58.—Fruit of Proboscidea jussieui reduced.
There could scarcely be a more conclusive evidence of the utility of a large national collection of such objects as those which are the subject of this chapter than a visit to the museums at Kew Gardens would afford. Instead of a hasty glance at mere curiosities, we would suppose the visitor to foster some such design as to go in search of specialised forms of fruits which would be serviceable in dispersion; or to trace, in passing from case to case and from order to order, the recurrence of similar forms in diverse families, so like as to suggest mimicry. With such an object, only to be gratified by such an institution, we would venture to affirm that not only would a visit afford far greater satisfaction, and would excite more intense pleasure, but would also considerably increase the visitor's own appreciation of the educational value of an exhibition too often looked upon, we fear, without so much as leaving a trace on the memory.
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fig. 59.—Fruit of Grapnel plant, natural size (Harpagophytum leptocarpum).
What interpretation is to be assigned to the peculiar form of the fruits in the different species of Trapa, or "water-chestnut," is not so clear. The plants float in the water, and the fruit of one species (Trapa bicornis), commonly cultivated in China, resembles the head and horns of a bull. Another species (Trapa bispinosa) is largely cultivated in Cashmere. In this fruit the bull's horns are replaced by acute spines. The latter is cultivated to such an extent that it is said to constitute a large portion of the food of the inhabitants, and yields about £12,000 a year in revenue. Moorcroft asserts that from 96,000 to 128,000 ass loads are yielded by the lake of Ooller. There is still another species, with rather smaller fruits, in which four long rigid spines are placed nearly in the same plane, at right angles to each other (Trapa quadrispinosa). Had they been produced on land instead of water, it might fairly have been assumed that these appendages would have assisted in dispersion, but under the existing circumstances their utility is not so evident.
<!-- image -->Fig. 60.—Fruit of Trapa bicornis.
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Fig. 61.—Fruit of Trapa bispinosa.
The suspension or retention of seeds in a favourable position, until in a fit condition to germinate, has been observed in some cases in a manner so marked as to suggest a special contrivance for the perpetuation of the species. A provision of this nature has been recorded in a plant of the sedge family, native of New Zealand (Gahnia xanthophylla), which consists in the filaments of the stamens, which are at first short, and afterwards greatly lengthen themselves. When the ovary is ripened so as to form the nut containing the seed, it is detached from the investing scales, and would fall to the ground if it were not caught and retained by the long filaments. It is probable that the object is to obtain a more perfect maturity of the seeds before they drop to the ground.
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Fig. 62.—Fruit of Gahnia xanthophylla suspended.
There is probably some relation in principle between the suspension of the seeds in Gahnia to what takes place amongst the members of one or two families of trees. For instance, the mangroves, which inhabit the swamps on the margins of great rivers, generally retain their seeds suspended to the branch until after germination has commenced, and when they drop it is into the soft mud, where they immediately take root. In like manner some of the Magnolias have the seeds suspended at the end of the umbilical cords from the margins of the carpels, presumably in order that they may reach a proper degree of maturity before they fall. Examples of this kind are not numerous, but sufficient to afford some explanation of the phenomenon first alluded to. Sir James Smith says, in allusion to the Egyptian bean (Nelumbium speciosum), "in process of time the receptacle separates from the stalk, and, laden with ripe oval nuts, floats down the water. The nuts vegetating, it becomes a cornucopia of young sprouting plants, which at length break loose from their confinement and take root in the mud."
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Fig. 63. Receptacle of the Egyptian Bean (Nelumbium speciosum).
The most remarkable of tropical fruits, in their structural aspect, are some of the myrtle family, the seeds of which are enclosed in a large woody urn, or capsule, like a drinking-vessel with a movable lid. In some of them the fruit is no larger than a small walnut, in others as large as a man's head. In some the form is elegant and urn-like, in others it resembles a vase (fig. 64). They are produced on large forest-trees, and are common throughout South America, but especially in the forests of Brazil. The monkeys are exceedingly fond of the delicious "sapucaia" nuts which are produced within these capsules. As Kingsley writes:—"The great urn-shaped fruits, big enough to serve for drinking-vessels, each kindly provided with a round wooden cover, which becomes loose, and lets out the savoury sapucaia nuts inside, to the comfort of all our 'poor relations.' Ah, when will there arise a tropic Landseer to draw for us some of the strange fashions of the strange birds and beasts of these lands?—to draw, for instance, the cunning, selfish, greedy grin of delight on the face of some burly, hairy, goitred old red howler, as he lifts off a 'monkey-cacao' cover, and looks defiance out of the corners of his winking eyes at his wives and children, cousins and grandchildren, who sit round jabbering and screeching, and, monkey fashion, twisting their heads upside down as they put their arms round each other's waists, to peer over each other's shoulders at the great bully, who must feed himself first as his fee for having roared to them for an hour at sunrise on a tree top while they sat on the lower branches and looked up, trembling and delighted, at the sound and fury of the idiot sermon."
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Fig. 64.—Monkey pots (Lecythis sp.).
The name of monkey pot, as applied to these fruits, is said to have arisen thus: "When the cup of a Lecythis falls, its lid drops off, the seeds roll out, and it then becomes a hard pot, with a narrow mouth. These pots are used for catching monkeys. Filled with sugar they are placed on the ground which such animals frequent. The sugar attracts the latter, who pick it out leisurely till they are disturbed, when they insert the paw, grasp as much sugar as it will hold, and endeavour to escape with their prize. But their doubled fist being larger than the mouth of the pot cannot be withdrawn, and the monkeys tenaciously holding the sugar, run off with a pot firmly enclosing one paw. This renders it impossible for them to escape from their pursuers by climbing, and they are easily run down." To the credit of the monkeys, it may be added, that it is the young and inexperienced that are caught in this manner, and not the old and wary patriarchs, as intimated by the proverb, common in South America, "He is too old a monkey to be caught with a Cabomba."
The nearest resemblance we have in this country to the structure of the fruits of the Lecythis are the comparatively minute and insignificant little capsules of the Pimpernel (Anagallis) and the Henbane. In these instances the seeds are enclosed in a capsule, which opens with a deciduous lid. When the capsule is mature the lid falls off, and the seeds are dispersed. In a remote manner, if such a comparison can be legitimately made, the hood, or cap, of the theca, or capsule, in mosses falls off, and the spores escape, except that they are subject to further retention, until a suitable season, by the incurved teeth of the peristome.