282 PHYLOGENY 



crustaceous: at first an accretion of separate granules which may finally be 

 united into a continuous crust with a protective covering of thick- walled, 

 amorphous hyphae forming a " decomposed " cortex. The extension of 

 a granule by growth in one direction upwards and outwards gives jjletach- 

 ment from the substratum, and originates (2) the squamule which is, how- 

 ever, often of primitive structure and attached to the support, like the granule, 

 by the medullary hyphae. Further growth of the squamule results in (3) 

 the foliose thallus with all the adaptations of structure peculiar to that form. 

 In all of these, the principal area of growth is round the free edges of the 

 thallus. A greater change takes place in the advance to (4) the fruticose 

 type in which the more active growing tissue is restricted to the apex, and 

 in which the frond or filament adheres at one point only to the support, a 

 new series of strengthening and other structures being evolved at the same 

 time. 



The lichen fungi associate, as has been already stated, with two different 

 types of algae: those combined with the Myxophyceae have been designated 

 Phycolichenes, those with Chlorophyceae as Archilichenes. The latter pre- 

 dominate, not only in the number of lichens, but also in the more varied 

 advance of the thallus, although, in many instances, genera and species of 

 both series may be closely related. 



B. Comparative Antiquity of Algal Symbionts 



One of the first questions of inheritance concerns the comparative an- 

 tiquity of the two gonidial series: with which kind of alga did the fungus 

 first form the symbiotic relationship ? No assistance in solving the problem 

 is afforded by the type of fructification. The fungus in Archilichens is 

 frequently one of the more primitive Pyrenomycetes, though more often a 

 Discomycete, while in Phycolichens Pyrenomycetes are very rare. There 

 is, as already stated, no corelation of advance between the fruit and the 

 thallus, as the most highly evolved apothecia with well-formed thalline 

 margins are constantly combined with thalli of low type. 



ForsselP gave considerable attention to the question of antiquity in his 

 study of gelatinous crustaceous lichens in the family Pyrenopsidaceae, termed 

 by him Gloeolichens, and he came to the conclusion that Archilichens 

 represented the older combination, Phycolichens being comparatively young. 



His view is based on a study of the development of certain lichen fungi 

 that seem able to adapt themselves to either kind of algal symbiont. He 

 found' in Euopsis {Pyrenopsis) granatina, one of the Pyrenopsidaceae, that 

 certain portions of the thallus contained blue-green algae, while others con- 

 tained Palmella, and that these latter, though retrograde in development, 



1 Forssell 1885. 



