Page:Aspects of nature in different lands and different climates; with scientific elucidations (IA b29329668 0002).pdf/199

 *[Footnote: 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 phænogamous plants growing in the same region; 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 Phanerogamæ increases there in such a manner that in the groups of islands between the tropics in the Pacific the ferns equal a fourth,—and in the solitary far detached islands in the Atlantic Ocean, St. Helena, and Ascension,—almost equal the half of the entire phænogamous vegetation. (See an excellent memoir of D'Urville entitled Distribution géographique des Fougères sur la surface du Globe, in the Annales des Sciences Nat. T. vi. 1825, p. 51, 66, and 73). From the tropics (where in the great continents D'Urville estimates the ratio generally at 1:20) we see the relative frequency of ferns decrease rapidly in the temperate zone. The quotients are: for North America and for the British Islands 1/33, for France 1/58, for Germany 1/52, for the dry parts of the south of Italy 1/74, and for Greece 1/84. Towards the colder regions of the north we see the relative]*