978 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 iropical 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 (Hpipetreous) ; and, thirdly, those ` 
that grow upon trees (Epiphylal) Lastrea Filiz-mas, 
Athyrium Filiz-femina, 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 — e 
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, - Se 
for instance, there are many scandent species of Desmo- oe 
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 et z 
