THE FUNGI 241 



group is a true Discomycetous Fungus as regards its 

 fructification, but it belongs to a set of plants which 

 are so different in habit and mode of life from all other 

 Fungi, that they used until recently to be treated as a 

 distinct class of the vegetable kingdom. These are the 

 Lichens, plants with a definite and often conspicuous 

 thallus, freely exposed to the air and light, very different 

 from the merely filamentous mycelium of ordinary 

 Fungi, which is usually immersed in the substratum. 

 Some Lichens grow on the bark of trees, some on rocks, 

 walls or roofs, and others on the ground. 



1. STRUCTURE AND MODE OF LIFE. 



Our example, Physcia parietina, is extremely common 

 (especially near the sea) on rocks, old walls and roofs, 

 where it forms a conspicuous and most beautiful object, 

 owing to its brilliant orange colour. We see at once, 

 from the habitat of the plant, that its mode of nutrition 

 must be totally different from that of a typical Fungus. 

 So far from requiring any organic matter, living or dead, 

 on which to feed, Physcia grows on the most barren and 

 unpromising substratum conceivable. Many Lichens, in 

 fact, thrive for years and even centuries under conditions 

 of drought and apparent starvation, which would be 

 absolutely intolerable to any other plants whatsoever. 

 A Lichen, considered as a whole, is neither a parasite 

 nor a saprophyte ; it requires nothing but a little mineral 

 food, and can provide itself with carbon from the Carbon 

 Dioxide of the air, like an ordinary green plant. Lichens 

 therefore can only live in the light, which is not the 

 case with Fungi. We will now proceed to describe the 

 structure of Physcia, and find out the explanation of its 

 remarkable mode of life. 

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