INTRODUCTION. 



The Lichenes are a Natural Order of aerial cellular plants in- 

 termediate between the Algse and Fungi, approaching the former 

 principally, but not exclusively, through the Collemacei — and the 

 Ascomycetes of the latter, by having the spores in 'asci or thecse 

 and their hymenium often included in a closed receptacle or peri- 

 thecium. Their own peculiar characteristic is that they have 

 gonidia or minute spherical cellules containing chlorophyll or some 

 green granular matter, generated by the action of light, immediately 

 underneath the cortical layer of the thallus. 



They may be found everywhere, clothing rocks, trees, old pal- 

 'ings, stones, and earth, and giving to these localities those beautiful 

 patches of colour which the artist delights to touch in, and which 

 give such brilliance and finish to his landscape sketches. 



The living strains which Nature's hand alone, 

 Profuse of life, pours forth upon the stone, 

 For ever growing ; where the common eye 

 Can but the bare and rocky bed descry. — Crabbe. 



Nevertheless, Lichens are not indifferent to either their sub- 

 stratum or locality ; some prefer the harder rocks, as granite, 

 micaoeous-schist, gneiss, quartz, or sandstone, others calcareous 

 rocks, others the bark of trees and old partially decayed or decay- 

 ing wood, others the earth, whether calcareous, siliceous, or pro- 

 duced from, the decay of vegetable matter ; others grow on decayed 

 mosses or jiingermanniffi, others on the leaves of evergreens, others 

 again on various substances, as iron, bone, leather, flints, and glass; 

 others again, reduced to apothecia alone, become parasitical on 

 the thalli or apothecia of other Lichens, and others affect both 

 rocks and trees indifferently. 



Certain Lichens are peculiar to mountains, others to subalpine 

 regions, others to plains, others to moist places, others to shady 

 places, some like the neighbourhood of water, or even contact with 

 water, others are constantly or partially submerged either in the 

 rocky beds of rivers or on the sea shore, and others again on the 

 under-surface of stones in stone walls or in caves, or on the under - 

 surface of overhanging masses of rock. 



They obtain no nutrition by absorption through the prehensile 

 rhizina? or the hypothallus by which they are attached to the sub- 

 strata or surfaces on which they grow. All their nutrition is 

 derived from the atmosphere, the water or humidity of which act- 

 ing chemically on the elements of the different surfaces to which 



