THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 315 



hyphae become more closely aggregated together and give off branches 

 which turn and run down at right angles to the surface, to form the tissue 

 of the tube walls (Fig. 309). At the lower surface these hyphae turn again 



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Fig. 307. — Polyporus betulinus. Fruiting body on the trunk of a Birch. 



and form the tissue over the pores, which in a young fruiting body consists 

 of a continuous layer. As the tubes mature their openings are formed by 

 the dissolution of these hyphae, so that in a section through a mature pore 

 the margin is found to consist of an almost solid ring of disorganized cells 

 which are slightly bent outwards to form a shallow collar or rim around 

 the pore. 



The hymenium which lines the tubes is formed from short branches 



