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 

 on 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 con- 

 ceivable. 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 C0 2 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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