298 LICHENS. PARTII. 



SECTION IV. 



LICHENS. 



LICHENS are essentially air plants, being nourished, like 

 the Algse, by the medium in which they grow. They vary 

 from a pulverulent or dry papillose crust, to a leathery 

 or horny expansion, and even acquire an erect stem. 

 They are independent of the matrix to which they are 

 attached. Hence they spread their coloured frond, or 

 thallus, in circular or indefinite patches on old walls, 

 the tiles of houses, stones, and rocks. They appear in 

 large expansions of red, golden yellow, grey or white, 

 on barren heaths, under plantations, and on the stems of 

 aged trees ; while others of them often hang from the 

 branches like long shaggy grey hair, and many form 

 forests of miniature bushes on the northern plains. The 

 lichen is the last trace of vegetation on the tops of the 

 mountains, and on the arctic deserts. Some lichens are 

 patient of severe cold, yet in general they prefer heat 

 and moderate warmth, and they love bright light so 

 much, that they are usually barren, or else yield little 

 fruit under shade. Though differing greatly from fungi 

 in slowness of growth, length of life, and the power 

 of forming chlorophyll, they resemble them in having 

 a mycelium in their youth, and in their ascigerous 

 fructification. A perfect lichen without an ascus would 

 be an anomaly, for the asci contain the true fruit, 

 associated with vertical threads or elongated cells called 

 paraphyses, which sometimes bear secondary spores 



