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Show 356 PHYSIOGNOMY OF PLANTS. Zacaria Ebn el Awam, Libro de Agricultura, traducido por J. A. Banqueri, t. ii. Madr. 1802, p. 736.) The conditions of mild temperature and an atmosphere nearly saturated with vapor, together with great equability of climate in respect to both temperature and moisture, are fulfilled on the declivities of the mounta1ns, in the valleys of the Andes, and above all in the mild and humid atmosphere of the Southern Hemisphere, where arborescent ferns extend not only to New Zealand and Van Diemen Island (Tasmania), but even to the Straits of Magellan and to Campbell Island, or to a latitude almost corresponding to that of Berlin in the Northern Hemisphere. Of tree-ferns, Dicksonia squarrosa grows vigorously in 46° south latitude, in Dusky Bay (New Zealand) i D. antarctica of Labillardiere, in Tasmania i a Thyrsopteris in Juan Fernandez i an undescribed Dicksonia, with stems from 12 to 15 (nearly 13 to 16 English) feet, in the south of Chili, not far from Valdivia i and a Lomaria, of rather less height, in the Straits of Magellan. Campbell Island is still nearer to the South Pole, in 52!0 lat., and even there the stem of the Aspidium venustum rises to 4 feet (4 feet 3 inches, English) before the fronds branch off. The climatic relations under which Ferns in general flourish, are manifested in the numerical laws of their quotients of distribution, taken in the manner alluded to in an earlier part of the present volume. In the low plains of the great continents within the tropics, the quotient for ferns is, according to Robert Brown, and according to late researches, 1-20th of all the species of phrenogamous plants growing in the same region i in the mountainous parts of the great continents in the same latitudes it is from 1-8th to 1-6th. But a very different ratio is found in the small islands dispersed over the wide ocean. The proportion of ferns to the whole number of Phanerogamre increases there in such a manner that, in the groups of islands between the tropics in the Pacific, the ferns equal a fourthand in the solitary, far-detached islands in the Atlantic Ocean, St. Helena, and Ascension-almost equal the half of the entire phrenogamous vegetation. (See an excellent memoir of D'Urville, entitled Distribution geographique des Fougeres sur la su :face du Globe, in the Annales des Sciences Nat. t. vi. 1825, pp. 51, 66, and 73.) |
