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protecting, aerating or conducting tissue (cortical layers of foliose 

 thalli: cyphellae : longitudinal bundles of hyphae). For conveni- 

 ence of description, the mechanical adaptations met with in lichens 

 are divided into five types. Type I. Hollow Cylinder. Type II. 

 Central Strand. Type III. Direct Guys. Type IV. Lateral Guys- 

 Type V. Cap with Basal Ring. 



Type I. Hollow Cylinder : Podetia of Cladonia. This form of 

 mechanical tissue occurs also in Alectoria, Bryofogon, as well as 

 in Pilophoron. Here are included all flattened thalli whether hori- 

 zontal or vertical, which have both upper and lower cortical layers. 

 Such a structure may be considered as a compressed cylinder, whose 

 function is the same as that of the normal cylinder, namely, to resist 

 lateral tensions. It is very evident that such mechanical tissues re- 

 quire filling material to prevent the collapse of the broad surfaces. 

 This filling material is supplied by the medullary and algal tissues. 

 The hollow cylinder consists of much elongated, sparingly branch- 

 ing parallel hyphae, which in cross-section appear as a continuous 

 internal ring.. The cells are very long and closely united. The 

 typical mechanical hollow cylinder is without filling material. But 

 if the cylinder is of considerable diameter and the tissue thin, special 

 support is necessary to prevent collapse due to excessive' lateral pres- 

 sure. In Cladonia and other groups this is prevented by transverse 

 bundles of hyphal tissue which extend at right angles to the lateral 

 compressing force. These bundles are, therefore, subject to a pull- 

 ing tension ; they are also able to resist vertical (longitudinal com- 

 pression) tensions, but to a much lesser degree. 



The cylinder composed of longitudinal hyphal tissue may also be 

 external, that is, outside of the algal layer as in Tkelosckistes, Bryo- 

 ■pogon and Alectoria. This evidently makes it a more perfect me- 

 chanical structure but doubtless interferes somewhat with the func- 

 tion of assimilation. 



Type II. Central Strand: Usnea barbata. This mechanical 

 structure reaches its most highly specialized development in Usnea 

 barbata. It also occurs in Stereocaulon and Baeomyces as well as 

 in the stipes of Caliciaceae, but in these cases it is apparently not 

 as highly specialized as a mechanical tissue. In Usnea the central 

 strand serves to resist pulling tensions due to the weight of the enor- 

 mously elongated branches. This structure, like the hollow cylin- 

 der, consists of much elongated, sparingly branched hyphal filaments 



