Every country has many plants not now, if ever, made use of by man, and therefore not designedly propagated by him, but which cluster around his dwelling, and continue to grow luxuriantly on the ruins of his rural habitation after he has abandoned it. The site of a cottage, the very foundation stones of which have been carried off, may often be recognized, years afterwards, by the rank weeds which cover it, though no others of the same species are found for miles.
"Mediaeval Catholicism," says Vaupell, "brought us the red horsehoof—whose reddish-brown flower buds shoot up from the ground when the snow melts, and are followed by the large leaves—comfrey and snake-root, which grow only where there were convents and other dwellings in the Middle Ages."—Bogens Indvandring & de Daneke Skove, pp. 1, 2. ]
Introduction of Foreign Plants.
"A negro slave of the great Cortez," says Humboldt, "was the first who sowed wheat in New Spain. He found three grains of it among the rice which had been brought from Spain as food for the soldiers."
About twenty years ago, a Japanese forage plant, the Lesperadeza striata, whose seeds had been brought to the United States by some unknown accident made its appearance in one of the Southern States. It spread spontaneously in various directions, and in a few years was widely diffused. It grows upon poor and exhausted soils, where the formation of a turf or sward by the ordinary grasses would be impossible, and where consequently no regular pastures or meadows can exist. It makes excellent fodder for stock, and though its value is contested, it is nevertheless generally thought a very important addition to the agricultural resources of the South. [Footnote: Accidents sometimes limit, as well as promote the propagation of foreign vegetables in countries new to them. The Lombardy poplar is a deciduous tree, and is very easily grown from cuttings. In most of the countries into which it has been introduced the cuttings hare been taken from the male, and as, consequently, males only have grown from them, the poplar does not produce seed in those regions. This is a fortunate circumstance, for otherwise this most worthless and least ornamental of trees would spread with a rapidity that would make it an annoyance to the agriculturist.]
In most of the Southern countries of Europe, the sheep and horned cattle winter on the plains, but in the summer are driven, sometimes many days' journey, to mountain pastures. Their coats and fleeces transport seeds in both directions. Hence we see Alpine plants in champaign districts, the plants of the plains on the borders of the glaciers, though in neither case do these vegetables ripen their seeds and propagate themselves. This explains the occurrence of tufts of common red clover with pallid and sickly flowers, on the flanks of the Alps at heights exceeding seven thousand feet.
The hortus siccus of a botanist may accidentally sow seeds from the foot of the Himalayas on the plains that skirt the Alps; and it is a fact of very familiar observation, that exotics, transplanted to foreign climates suited to their growth, often escape from the flower garden and naturalize themselves among the spontaneous vegetation of the pastures. When the cases containing the artistic treasures of Thorvaldsen wore opened in the court of the museum where they are deposited, the straw and grass employed in packing them were scattered upon the ground, and the next season there sprang up from the seeds no less than twenty-five species of plants belonging to the Roman campagna, some of which were preserved and cultivated as a new tribute to the memory of the great Scandinavian sculptor, and at least four are said to have spontaneously naturalized themselves about Copenhagen. [Footnote: Vaupell, Bogens Indvandring i de Danske Skove, p. 2.]
The Turkish armies, in their incursions into Europe, brought Eastern vegetables in their train, and left the seeds of Oriental wall plants to grow upon the ramparts of Buda and Vienna. [Footnote: I believe it is certain that the Turks introduced tobacco into Hungary, and probable that they in some measure compensated, the injury by introducing maize also, which, as well as tobacco, has been claimed as Hungarian by patriotic Magyars.]
In the campaign of 1814, the Russian troops brought, in the stuffing of their saddles and by other accidental means, seeds from the banks of the Dnieper to the valley of the Rhine, and even introduced the plants of the steppes into the environs of Paris.
The forage imported for the French army in the war of 1870-1871 has introduced numerous plants from Northern Africa and other countries into France, and this vegetable emigration is so extensive and so varied in character, that it will probably have an important botanical, and even economical, effect on the flora of that country. [Footnote: In a communication lately made to the French Academy, M. Vibraye gives numerous interesting details on this subject, and says the appearance of the many new plants observed in France in 1871, "results from forage supplied from abroad, the seeds of which had fallen upon the ground. At the present time, several Mediterranean plants, chiefly Algerian, having braved the cold of an exceptionally severe winter, are being largely propagated, forming extensive meadows, and changing soil that was formerly arid and produced no vegetable of importance into veritable oases." See Nature, Aug. 1, 1872, p. 263. We shall see on a following page that canals are efficient agencies in the unintentional interchange of organic life, vegetable as well as animal, between regions connected by such channels.]