I02 



MORPHOLOGY 



thallus. It is also present in Ramalina {Alectoria) thrausta, a species with 

 slender fronds (Fig. 60). 







Fig. 60. Alectoria thrausta Ach. A, transverse section of frond; 

 a, cortex; b, gonidia; c, arachnoid medulla x 37. B, fibrous 

 hyphae from longitudinal section of cortex, x 430 (after Brandt). 



In Usnea longissima the cortex both of the fibrillose branchlets and of 

 the main axis is fibrous, and is composed of narrow thick-walled hyphae 



which grow in a long spiral round 

 the central strand. The hyphae 

 become more frequently septate 

 further back from the apex (Fig. 61). 

 Such a type of cortex provides an 

 exceedingly elastic and efficient pro- 

 tection for the long slender thallus. 



The same type of cortex forms 

 the strengthening element in the 

 fruticose or partly fruticose members 

 of the family Physciaceae. One of 

 these, Teloschistes flavicans, is a bright 

 yellow filamentous lichen with a 

 somewhat straggling habit. The 

 fronds are very slender and are either 

 cylindrical or slightly flattened. The 



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Fig. 61. Usnea longissima Ach. Longitudinal 

 sections of outer cortex. A, near the apc-c; B, 

 the middle portion of a fibril, x 525 (after 

 Schulte). 



