378 GEOGRAPHICAL DISTRIBUTION. 



circumstances, such as the relation of land and water, 

 mountains and valleys, cause great deviation from this; 

 thus, species of the cooler temperate regions are also found 

 in warmer regions, and those considered tropical often 

 extend to the warm valleys of the temperate zones ; the 

 favourite place of growth of many being groves and 

 ravines, and even in caves, while others make themselves 

 at home on rocks exposed to the extremes of heat and 

 cold. 



With regard to their nature, as indicated by their places 

 of growth, they may be classed under three heads first, 

 those that grow upon the ground (Epigeous) ; secondly, 

 those that grow in the crevices, or adhere to rocks, or in 

 caves open to the light (Epipetreous) and, thirdly, those 

 that grow upon trees (Epiphytal). Lastrea Filix-mas, 

 Athyrium Filix-fcemina, Pteris Aquilina, and all tree Ferns, 

 are examples of the first class, while Asplenium septen- 

 trionale, A. Ruta-muraria, A. trichomanes, A. marinum, 

 Ceterach officinarum, and Scolopendrium vulgare are our 

 native representatives of the rock and stone lovers of 

 temperate regions, the different species of Nothochlena and 

 Cheilanthes taking their places in warmer regions. 



It may be taken as a rule that the greater number of 

 the division Desmobrya belong to the two first, while the 

 Epiphytal class belong chiefly to the division Eremobrya. 

 There are, however, many deviations from these rules, as, 

 for instance, there are many scandent species of Desmo- 

 brya ; and some Eremobrya, such as Polypodium vulgare, 

 are often found Epipetreous as well as Epiphytal, and even 

 on the ground, indeed many species conform to climatic 

 influences of very different natures. Some grow in very 

 wet places, as Osmunda regalis, which attains a very large 

 size in bogs of this country, and even grows in caves facing 



