70 MORPHOLOGY 



II. STRATOSE THALLUS 

 i. CRUSTACEOUS LICHENS 



A. GENERAL STRUCTURE 



In the series "Stratosae," the plant is dorsiventral, the tissues forming 

 the thallus being arranged more or less regularly in strata one above the 

 other (Fig. 37). On the upper surface there is a hyphal layer constituting 



Fig. 37. Vertical section of crustaceous lichen (Lecanora subfusca 

 var. chlarona Hue) on bark, a, lichen cortex; i>, gonidia; 

 f, cells of the periderm. x 100. 



a cortex, either rudimentary or highly elaborated; beneath the cortex is 

 situated the gonidial zone composed of algae and hyphae in close asso- 

 ciation ; and deeper down the medulla, generally a loose tissue of branching 

 hyphae. The lower cortex which abuts on the medulla may be as fully 

 developed as the upper or it may be absent. 



The growing tissue is chiefly marginal; the hyphae on the outer edge 

 remain "meristematic" 1 and provide for horizontal as well as vertical ex- 

 tension; and there is also continual increase of the algal cells. There is in 

 addition a certain amount of intercalary growth due to the activity of the 

 gonidial tissue, both algal and fungal, providing for the renewal of the 

 cortex, and even interposing new tissue. 



B. SAXICOLOUS LICHENS 



a. EPILITHIC LICHENS. The crustaceous lichens forming this group 

 spread over the rock surfaces. The support must be stable to allow the 

 necessary time for the slowly developing organism, and therefore rocks that 

 are friable or subject to continual weathering are bare of lichens. 



aa. Hypothallus or Prothallus. The first stage of growth in the lichen 

 thallus can be most easily traced in epilithic crustaceous species, especially 

 in those that inhabit a smooth rock surface. The spore, on germination, 

 produces a delicate branching septate mycelium which radiates on all sides, 

 as was so well observed and recorded by Tulasne 2 in Verrucaria muralis 

 (Fig. 14). Zukal 8 has called this first beginning the prothallus. In time the 



1 Wainio has adopted this term for growing hyphae 1897, p. 33. 

 8 Tulasne 1851. Zukal 1895. 



