49 



tion into the substratum. The oil-bearing cells of calcivorous crus- 

 taceous lichens occur among the rhizoidal hyphae. 



Many of the crustaceous thalli develop below the surface of the 

 substratum. If below a rock surface they are said to be hypolithic ; 

 if below the surface of the bark they are said to be hypophloeodal. 

 Here also it must be remembered that there are all intermediate forms 

 between completely hypolithic and epilithic and between completely 

 hypophloeodal and epiphloeodal thalli. 



(b) The Foliose Type. 



In general it may be stated that under the foliose (foliaceous, 

 frondose) type are included all expanded distinctly dorsiventral 

 thalli having at least an upper cortical layer. This definition in- 

 cludes warty, isidioid and squamose thalli. In Collema, however, 

 we find an exception. Here we have a very marked thalloid expan- 

 sion but no cortical layers. 



It must be borne in mind that there is a great difference in the 

 morphological characters of the cortical tissue. It may be highly de- 

 veloped, or so deficient as to be scarcely recognizable. In the lower 

 lichens it usually consists of a thin layer of parallel or irregularly 

 interwoven agglutinate hyphae. In this structure the cells are 

 readily separated by various alkaline solvents. The cells are 

 comparatively long with gelatinous cell-walls (Lichen cellulose, 

 lichenin) . In the more highly developed foliose thalli the cortical 

 tissue consists of short cells with large cell lumina. The walls of the 

 adjacent cells are very closely united ; strong alkalies dissolve 

 the membranes with difficulty. There are all gradations between 

 this [form and the most rudimentary structure. It has been found 

 impracticable to recognize distinct forms of cortical tissues as some 

 authors have done (Zukal). By cortical tissue is, therefore, here 

 meant a hyphal structure of closely agglutinate cells with few inter- 

 cellular spaces. 



The foliose thallus is very variable in size ; it may be so small as 

 to require a lens for its detection, as in Dermatocarpon and in wartv 

 thalli. It reaches its maximum development in Gyrofhora and Um- 

 bilicaria, which may be a foot in diameter or more. It may be entire 

 or variously lobed and divided ; the lobes may be rounded, oblong 

 or strap-shaped. As a rule, it is held to the substratum by means 

 of the rhizoids. There are, however, also special mechanical struc- 



