298 PHYLOGENY 



The marginate apothecium has appeared once and again as we have 

 seen. It is probable however that its first development was in this group of 

 lichens, and even here there may have been more than one origin as there 

 is certainly more than one phylum. 



aa. COURSE OF DEVELOPMENT. At the base of the series, the thallus is 

 of the crustaceous type somewhat similar to that of Lecidea, but there are 

 none of the very simple primitive forms. Lecanora must have originated 

 when the crustaceous lecideine thallus was already well established. Its 

 affinity is with Lecidea and not with any fungus: where the thallus is 

 evanescent or scanty, its lack is due to retrogressive rather than to primitive 

 characters. 



bb. LECANORACEAE. A number of genera have arisen in this large family, 

 but they are distinguished mainly if not entirely by spore characters, and 

 by some systematists have all been included in the one genus Lecanora, 

 since the changes have taken place within the developing apothecium. 



There is one genus, Harpidium, which is based on thalline characters, 

 represented by one species, H. rutilans, common enough on the Continent, 

 but not yet found in our country. It has a thin crustaceous homoiomerous 

 thallus, the component hyphae of which are divided into short cells closely 

 packed together and forming a kind of cellular tissue in which the algae are 

 interspersed. The dorsiventral stratose arrangement prevails however in the 

 other genera and a more or less amorphous " decomposed " cortex is fre- 

 quently present. The medulla rests on the substratum. 



With the stouter thallus, there is slightly more variety of crustaceous 

 form than in Lecideaceae: there occurs occasionally an outgrowth of the 

 thalline granules as in Haematomma ventosum which marks the beginning 

 of fruticulose structure. Of a more advanced structure is the thallus of 

 Lecanora esculenta, a desert lichen which becomes detached and erratic, and 

 which in some of its forms is almost coralline, owing to the apical growth of 

 the original granules or branches: a more or less radiate arrangement of 

 the tissues is thus acquired. 



The squamulose type is well represented in Lecanora, and the species 

 with that form of thallus have frequently been placed in a separate genus, 

 Squamaria. These squamules are never very large; they possess an upper, 

 somewhat amorphous, cortex; the medulla rests on the substratum, except 

 in such a species as Lecanora lentigera, where they are free, a sort of fibrous 

 cortex being formed of hyphae which grow in a direction parallel with the 

 surface. In none of them are rhizinae developed. 



cc. PARMELIACEAE. The chief advance, apart from size, of the squamulose 

 to the foliose type is the acquirement of a lower cortex along with definite 

 organs of attachment which in Parmeliaceae are invariably rhizoidal and 



