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a. Primary and Secondary Thallus. 



The terms primary thallus and secondary thallus apply only to 

 the genera Pilophoron, Stereocaulon, Cladonia and Thamnolia; 

 they would not be applicable to Baeomyces any more than to the 

 lower Caliciaceae. 



The primary thallus is the thallus originally formed. It is 

 crustaceous or warty in Pilofhoron and Stereocaulon and foliose in 

 Cladonia ; in Thamnolia it is entirely wanting or at most rarely 

 present. There is considerable chance for speculation as to the reason 

 why the secondary thallus (podetium) should have supplanted the 

 primary thallus ; it is essentially a change from a horizontal thallus 

 to a fruticose thallus. The prime factor which brought about this 

 change was no doubt the presence of the apothecial stalk ; this 

 structure was suitable for the location of the assimilating algae, 

 thereby forming a vertical thallus with a marked radial structure. 

 Essentially the same conditions are met with in Sfhaerofhorus as al- 

 ready explained. 



b. Stipes and Podetia. 



According to definition stipes differ from podetia in being devoid 

 of any symbiotic algae ; both take their origin from the hyphal 

 tissue of the thallus at a point somewhat below the algal layer ; a 

 group of hyphal branches form an apical area which extends upward 

 pushing its way through the superimposed structures without inducing 

 any increased cell-proliferation in them ; the hyphae continue to 

 grow upward parallel to each other, and very early in its development 

 the apothecium begins to form at the apex. This is the mode of 

 development of the stipes in the Caliciaceae as well as in Baeomyces. 

 The so-called podetia of PiJophoron and Stereocaulon take their 

 origin in a similar manner, but the algal zone takes part in the cell- 

 proliferation covering the apothecium-bearing stalk ; the algal layer 

 of the stalk may not be entire, often being present only in patches. 



The podetia reach their highest development in Cladonia and 

 Thamnolia. In these genera we find that the podetium which was 

 originally an apothecial stalk developed from the hyphal symbiont 

 has become converted into a true lichen-thallus, replacing the primary 

 thallus which has entirely disappeared in Thamnolia and some of 

 the higher Cladoniae. Its primary function, which was that of form- 

 ing a suitable support for the apothecia and aiding the distribution of 



