348 AKBORESCENT FERNS. 



3. Those which grow within 30 or 35 degrees on each 

 side of the Equator, 1200 species. 



If we compare the amount of Ferns with the united num- 

 bers of other tribes of plants, we may form some idea of the 

 relative importance of this family in the vegetation of the 

 district, or period to which we apply such comparison. 

 Thus, in the entire number of known species of plants now 

 existing on the globe, we have 1500 Ferns and 45,000 Pha- 

 nerogamisB, being in the proportion of 1 to 30. In Europe 

 this proportion varies from 1 : 35 to 1 : 80, and may average 

 1 : 60. Between the Tropics, Humboldt estimates the num- 

 ber in Equinoxial America at 1 : 36, and Mr. Brown gives 

 1 : 20 as the proportion in those parts of intertropical Conti- 

 nents which are most favourable* to Ferns. 



Mr. Brown (Appendix to Tuckey's Congo Expedition) 

 states that the circumstances most favourable to the growth 

 of Ferns are humidity, shade, and heat. These circum- 

 stances are most frequently combined in the highest degree 

 in small and lofty tropical islands, where the air is charged 

 with humidity, which it is continually depositing on the 

 mountains, and thereby imparting freshness to the soil. 

 Thus in Jamaica Ferns are to the Phanerogamiae nearly in 

 the proportion of 1 to 10; in New Zealand as 1 to 6; in 

 Taiti as 1 to 4 ; in Norfolk Island as 1 to 3 ; in St. Helena 

 as 1 to 2 ; in Tristan d'Acunha (extratropical) as 2 to 3. 

 Ferns also are the most abundant Plants in the Islands of 

 the Indian Archipelago. 



It appears still farther, that not only are certain Genera 

 and tribes of Ferns peculiar to certain cHmates, but that 

 the enlarged size of the arborescent species depends in a 

 great degree on Temperature, since Arberescent Ferns are 

 now found chiefly within, or near the hmit of the Tropics.f 



* Botony of Congo, p. 42. 



+ The few exceptions to this rule appear to be confined to the southern 



