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 mountains, 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 Islands, or to a latitude almost corresponding to 

 that of Berlin in the Northern Hemisphere. Of tree-ferns, Dick- 

 sonia squarrosa grows vigorously in 46 south latitude, in Dusky 

 Bay (New Zealand); D. antarctica of Labillardiere, in Tasmania; a 

 Thyrsopteris in Juan Fernandez ; an undescribed Dicksonia, with 

 stems from 12 to 15 (nearly 13 to 16 English) feet, in the south of 

 Chili, not far from Valdivia ; and a Lomaria, of rather less height, 

 in the Straits of Magellan. Campbell Island is still nearer to the 

 South Pole, in 52 J 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, l-20th of all the species of phaenogamous plants 

 growing in the same region; in the mountainous parts of the great 

 continents in the same latitudes it is from l-8th to l-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 Pha- 

 nerogams 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 phaeno- 

 gamous vegetation. (See an excellent memoir of D'Urville, entitled 

 Distribution ge"ographique des Fougeres sur la surface du Globe, in 

 the Annales des Sciences Nat. t. vi. 1825, pp. 51, 66, and 73.) 



