STRATOSE THALLUS . 85 



gelatinous lichens in which the tips of the hyphae are cut off at the surface 

 by one or more septa. The resulting cells are wider than the hyphae and 

 they cohere together to form, in some species, disconnected patches of cells; 

 in others, a continuous cortical covering one or more cells thick, while in 

 the margin of the apothecium they form a deep cellular layer. The cellular 

 type of cortex is found also, as already stated, in some crustaceous Pertu- 

 sariae, and in a few squamulose genera or species. It forms the uppermost 

 layer of the Peltigera thallus and both cortices of many of the larger foliose 

 lichens such as Sticta, Parmelia, etc. 



5. The "fibrous" cortex must be added to this series, as was pointed 

 out by Heber Howe 1 who gave the less appropriate designation of "simple" 

 to the type. It consists of long rather sparingly branched slender hyphae 

 that grow in a direction parallel with the surface of the thallus (Fig. 48). 

 It is characteristic of several fruticose and foliose lichens with more or less 

 upright growth, such as we find in several of the Physciae, and in the allied 

 genus Teloschistes y in Alectoria, several genera of Roccellaceae, in Usnea 

 longissima and in Parmelia pubescens, etc. Zukal would have included all 

 the Usneae as the tips are fibrous. 



Fig. 48. Physcia ciliaris DC. Vertical section of thallus. a, cortex ; 

 b, gonidial zone; c, medulla, x 100. 



More than one type of cortex, as already stated, may appear in a genus; 

 a striking instance of variability occurs in Solorina where, as Hue 2 has 

 pointed out, the cortex of 5". octospora is fastigiate, that of all the other 

 species being plectenchymatous. Cortical development is a specific rather 

 than a generic characteristic. 



b. ORIGIN OF VARIATION IN CORTICAL STRUCTURE. The immediate 

 causes making for differentiation in cortical development are: the prevailing 

 direction of growth of the hyphae as they rise from the gonidial zone; the 

 amount of branching and the crowding of the filaments; the frequency of 

 septation; and the thickening or degeneration of the cell-walls which may 



i Heber Howe 1912. Hue 191 1. 



