FEENS OP GREAT BRITAIN. 13 



tree-ferns were known, but a large number have been 

 described by later botanists ; and more than three thou- 

 sand species of ferns, comprising the arborescent and 

 herbaceous forms, have been enumerated. The tree-ferns 

 greatly resemble palm-trees in appearance, and the stems 

 of both are so much alike, that fossil specimens have 

 frequently been described as ferns, but which on further 

 investigation have proved to belong to the Palm tribe. 



The conditions under which ferns flourish difier some- 

 what in difierent genera ; but heat, moisture, and shade 

 are necessary for the luxuriant development of the 

 greater number. They are more numerous in islands 

 than on continents, the arborescent species being almost 

 confined to the torrid zone : the shrubby species gene- 

 rally also preferring a climate of intense heat, and the 

 herbaceous species grow in temperate climes, and are 

 found more rarely in the colder countries, while the 

 northern part of the globe seems quite destitute of any 

 species of this elegant family of plants. As regards the 

 ferns of this kingdom, some grow in almost every 

 county; while some, peculiar to mountainous districts, 

 delighting in limestone soils, or thriving only on the 

 basaltic trap, are necessarily local or rare. Very few of 

 our native species wiU bear the sea air, yet this is needed 

 for the luxuriance of that beautiful plant of the sea-caves 

 and cliffs, the Sea Spleejiwort; while the WaU Rue 

 and Black Spleenwort grace the ruined building or 

 barren rock. The Northern Hard Fern is unhurt by its 

 exposure to the sun and wind of the heath; and the 

 Lastrea Thelypteris is our only Marsh Fern. Most of 

 our ferns luxuriate in a shady spot, on a vegetable 



