STRATOSE THALLUS 73 



hyphae ; but in those in which a distinct crust or granules are formed, three 

 different strata of tissue are discernible: 



1st. An upper cortical tissue of interlaced hyphae with frequent septa- 

 tion and with swollen gelatinous walls, closely compacted and with the 

 lumen of the cells almost obliterated, not unfrequently a layer of mucilage 

 serving as an outer cuticle. This type of cortex has been called by Hue 1 

 "decomposed." It is subject to constant surface weathering, thin layers 

 being continually peeled off, but it is as continually being renewed endo- 

 genously by the upward growth of hyphae from the active gonidial zone. 

 Exceptions to this type of cortex in crustaceous lichens are found in some 

 Pertusariae where a secondary plectenchymatous cortex is formed, and in 

 Dirina where it is fastigiate 2 as in Roccella. 



2nd. The gonidial zone a somewhat irregular layer of algae and 

 hyphae below the cortex which varies in thickness according to the species. 



3rd. The medullary tissue of somewhat loosely intermingled branching 

 hyphae, with generally rather swollen walls and narrow lumen. It rests 

 directly on the substratum and follows every inequality and crack so 

 closely, even where it does not penetrate, that the thallus cannot be 

 detached without breaking it away. 



In Verrucaria mucosa, a smooth brown maritime lichen found on rocks 

 between tide-levels, the thallus is composed of tightly packed vertical rows 

 of hyphae, slender, rather thin-walled, and divided into short cells. The 

 gonidia are chiefly massed towards the upper surface, but they also occur in 

 vertical rows in the medulla. One or two of the upper cells are brown and 

 form an even cortex. The same formation occurs in some other sea-washed 

 species; the arrangement of the tissue elements recalls that of crustaceous 

 Florideae such as Hildenbrandtia, Cruoria, etc. 



cc. Formation of areolae. An "areolate" thallus is seamed and scored 

 by cracks of varying width and depth which divide it 

 into minute compartments. These cracks or -fissures or 

 chinks originate in two ways depending on the presence 

 or absence of hypothallic hyphae. Where the hypothallus 

 'is active, new areolae arise when the filaments encounter 

 new groups of algae. More vigorous growth starts at once 

 and proceeds on all sides from these algal centres, until 



Fig. 40. Young 



similarly formed areolae are met, a more or less pro- thallus of Rhizo- 

 nounced fissure marking the limits of each. This primary S^DcT^wtth 

 areolation, termed rimose or rimulose, is well seen in the primary and sub- 



,. T-, , . , . /T -.. x sequent (dotted 



thin smooth thallus of Rhizocarpon geograpfncum (rig. 40); lines ) areo iation 

 but the first-formed areolae are also very frequently slightly x 5- 

 1 Hue 1906. 2 See p. 83. 



