DIV. i THALLOPHYTA 469 



mediate gelatinous layer. Within the PERIDIUM (also termed volva) the hyphal 

 tissue becomes differentiated into the axial stalk and the bell-shaped pileus, carry- 

 ing the gleba in the form of a mass of hyphal tissue, which contains the chambers 

 and basidial hymenium. At maturity the stalk becomes enormously elongated, 

 and pushing through the ruptured peridium raises the pileus with the adhering 

 gleba high above it. The gleba then deliquesces into a dropping, slimy mass, 

 which emits a carrion-like stench serving to attract carrion-flies, by whose agency 

 the spores embedded in it are disseminated. 



CLASS XV 

 Liehenes (Lichens) ( l > 51 > 87 91 ) 



The Lichens are symbiotic organisms; they consist of higher 

 Fungi, chiefly the Ascomycetes, more rarely Basidiomycetes, and uni- 

 cellular or filamentous Algae (Cyanophyceae or Chlorophyceae), living 

 in intimate connection, and together forming a compound thallus or 

 CONSORTIUM. Strictly speaking, both Fungi and Algae should be 

 classified in their respective orders ; but the Lichens exhibit among 

 themselves such an agreement in their structure and mode of life, 

 and have been so evolved as consortia, that it is more convenient to 

 treat them as a separate class. 



In the formation of the thallus the algal cells become enveloped by the mycelium 

 of the fungus in a felted tissue of hyphae (Fig. 422). The fungus derives its 



nourishment saprophytically from __^_^ -. 



the organic matter produced by the n\''o n 'D l?S??ff *$*$ j '/'/(? "l 

 assimilating alga ; it can also send MW/^ 

 haustoria into the algal cells, and 



so exhaust their contents. The 

 alga, on the contrary, derives a 

 definite advantage from its con- 

 sortism with the fungus, receiving 

 from it inorganic substances and 

 water, and probably organic sub- 

 stances also. 



The main advantage in this 

 mutualistic symbiosis is probably 

 on the side of the fungus. This is 

 especially the case in those Lichens Flf; - 42 ' 2 - Cetraria islandica. Transverse section 



which grow on bare rock, while in throu h the thallus; "> cortical la >" er of upper 

 ,, . , . n surface ; ur, of the lower surface ; m, medullary 



those growing on humus soil or on layer containing the green cells of the Alga> chloro . 



the bark of trees the fungus can, coccum humicola. (x 272.) 

 in part at least, derive its food sapro- 

 phytically from the substratum. The Alga, however, exhibits active multiplica- 

 tion, and both it and the -fungus can, as a result of the symbiosis, succeed in 

 situations where neither could live alone. 



The numerous Lichen acids, which are wanting only in the gelatinous Lichens, 

 are products of metabolism peculiar to the group. Their production is due to the 

 mutual chemical influence of the alga and fungus. They are deposited on the 



2H2 



