100 



MORPHOLOGY 



In the two former the habit is more or less fruticose as the plants are 

 affixed to the substratum at a basal point, but the fronds are decumbent and 

 the internal structure is of the dorsiventral type : there is an upper "fibrous"' 

 cortex of closely compacted parallel hyphae, a gonidial zone — the gonidia 

 lying partly in the cortex and partly among the loose hyphae of the 

 medulla^ — -and a lower cortex formed of a weft of hyphae which also run 

 somewhat parallel to the surface. Both species are distinguished by the 

 numerous marginal cilia, either pale or dark in colour. These two lichens 

 are greyish-coloured on the upper surface and greyish or whitish below. 



Everniafurfuracea-with. a basal attachment^ and with a partly horizontal 

 and partly upright growth, has a dorsiventral thallus, dg.rk greyish-green 

 above and black beneath, with occasional rhizinae towards the base. The 

 cortex of both surfaces belongs to the "decomposed" type; the gonidial 

 zone lies below the upper surface, and the medullary tissue is of loose hyphae. 

 In certain forms of the species isidia are abundant on the upper surface, 

 a character of foliose rather than of fruticose lichens. E. furfuracea grows 

 on trees and very frequently on palings. 



Fig. 5Q. Evernia prunastri Kz\i. (M. P., Photo.) 

 ' See p. io8. 



