GEOGRAPHICAL DISTRIBUTION. 379 



the sea. Many Hymenophyllece grow within the spray of 

 cascades or on dripping rocks, and on mossy trunks of 

 trees. Ceratopteris thalictroides grows in shallow waters, 

 its sterile fronds floating on the water, and may be said to 

 be the only truly aquatic Fern, and one of the very few 

 annuals ; like many other aquatic plants it has a wide 

 geographical range within or near the tropics of both 

 hemispheres. 



Many have a great predilection for the works of man, 

 whether elevated in the air or sunk below the earth, such 

 as old castles, walls, embankments, hedge and road-side 

 banks, quarries, or deep open pits or wells, this being 

 common to species of both tropical and temperate countries, 

 it is, therefore, not surprising to find the same species 

 assume different sizes and forms, as, for instance, the 

 common Hart's-tongue Fern, Scolopendrium vulgare, which 

 on walls and other dry places produces fronds only a few 

 inches in length, while in shady places or open pits or 

 wells they may be found lining the sides with fronds two 

 to three feet in length. 



Another example of a Fern making itself at home under 

 very opposite conditions is Pteris longifolia, which is com- 

 mon in or near the tropics of both hemispheres. On the 

 island of Ischia (in the Bay of Naples) it is found luxuriating 

 within the influence of the hot vapours rising from the 

 fissures of latent volcanic heat, growing in soft mud at a 

 temperature ranging from 140 to 160. In hothouses its 

 spores vegetate abundantly upon all moist surfaces, and in 

 crevices close to hot water tanks. It also establishes itself 

 in dry places, even in situations where the temperature is 

 often at or near freezing point. 



Although Ferns thus conform themselves to different 

 climates, many being wanderers, yet on taking a view of 



