ANNOTATIONS AND ADDITIONS. 191 



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 phseriogamous 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 dis- 

 persed over the wide ocean. The proportion of ferns to the 

 whole number of Phanerogamse 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 phsenogamous 

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

 Distribution geographique des Fougeres 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 conti- 

 nents 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 ^ for France ^ f r Germany -^ 

 for the dry parts of the south of Italy -yV, and for Greece T 1 4 . 

 Towards the colder regions of the north we see the relative 



