RADIATE THALLUS 



103 



hyphae of the outer cortex are compactly fibrous; added toughness is 

 given by the presence of some longitudinal strands of hyphae in the central 

 pith. 



Another still more familiar grey lichen, Physcia ciliaris, has long flat 

 branching fronds which, though dorsiventral in structure, are partly upright 

 in habit. Strength is secured as in Teloschistes by the fibrous upper cortex. 

 Other species of Physciae are somewhat similar in habit and in structure. 



In Dendrographa leucophaea, a slender strap-shaped rock lichen, Darbi- 

 shire' has described the outer cortex as composed of closely compacted 

 parallel hyphae resembling the strengthening cortex of Alectoria and very 

 different from the fastigiate cortex of the Roccellae with which it is usually 

 classified. 



B. Special strengthening Structures 



a. Sclerotic strands. This form of strengthening tissue is charac- 

 teristic of Ramalina. With the exception of R. thrausta (more truly an 

 Alectoria) all the species have a rather weak cortical layer of branching 

 intricate thick-walled hyphae, regarded by Brandt^ as plectenchymatous, 

 but more correctly by Hue' as "decomposed" on account of the gelatinous 

 walls and diminishing lumen of the irregularly arranged cells. 



In R. evermoides, a plant with very wide flat almost decumbent fronds 

 of soft texture, in R. ceruchis and in R. homalea there is a somewhat compact 

 medulla which gives a slight stiffness to the thallus. The other species of 

 the genus are provided with strengthening mechanical tissue within the 

 cortex formed of closely united sclerotic hyphae that run parallel to the 

 surface (Fig. 62). In a transverse section of the thallus, this tissue appears 



A B 



Fig. 62. Ramalina minuscula Nyl. A, transverse section 

 of frond x 37 ; B, longitudinal strengthening hyphae of 

 inner cortex x 430 (after Brandt). 



' Darbishire 1895. 



^ Brandt 1906. 



•" Hue 1906. 



