290 PHYLOGENY 



tree or rock, the presence of the vegetative body being often signalled only 

 by a deeper colouration of the substratum. The researches of Almquist, 

 and more recently of Reinke and Darbishire, have enlarged our conception 

 of the series, and the families Dirinaceae and Roccellacea,e are now classified 

 in Graphidineae. 



Arthoniaceae, Graphidaceae and Chiodectonaceae are all wholly crus- 

 taceous. The first thalline advance takes place in Dirinaceae with two allied 

 genera, Dirina and Dirinastrum. Though the thallus is still crustaceous, it 

 is of considerable thickness, with differentiation of tissues: on the lower 

 side there is a loosely filamentous medulla from which hyphae pierce the 

 substratum and secure attachment. Trentepohlia gonidia lie in a zone above 

 the medulla, and the upper cortex is formed of regular palisade hyphae 

 forming a " fastigiate cortex." It is the constant presence of Trentepohlia 

 algae as well as the tendency to ellipsoid or lirellate fruits that have in- 

 fluenced the inclusion of Dirinaceae and Roccellaceae in the series. 



The thallus of Dirinaceae is crustaceous, while the genera of Roccellaceae 

 are mostly of an advanced fruticose type, though in one, Roccellina, there is 

 a crustaceous thallus with an upright portion consisting of short swollen 

 podetia-like structures with apothecia at the tips ; and in another, Roccello- 

 grapha, the fronds broaden to leafy expansions. They are nearly all rock- 

 dwellers, often inhabiting wind-swept maritime coasts, and a strong basal 

 sheath has been evolved to strengthen their foothold. In some genera the 

 sheath contains gonidia; in others the tissue is wholly of hyphae in nearly 

 every case it is protected by a cortex. 



In the upright fronds the structure is radiate: generally a rather loose 

 strand of hyphae more or less parallel with the long axis of the plant forms 

 a central medulla. The gonidia lie outside the medulla and just within the 

 outer cortex. The latter, in a few genera, is fibrous, the parallel hyphae 

 being very closely compacted ; but in most members of the family the 

 fastigiate type prevails, as in the allied family Dirinaceae. 



d. THALLUS OF CYCLOCARPINEAE. This is by far the largest and most 

 varied series of Archilichens. It is derived, as regards the fungal constituent, 

 from the Discomycetes, but in these fungi, the vegetative or mycelial body 

 gjves no aid to the classification which depends wholly on apothecial 

 characters. In the symbiotic condition, on the contrary, the thallus becomes 

 of extreme importance in the determination of families, genera and species. 

 There has been within the series a great development both of apothecial 

 and of thalline characters in parallel lines or phyla. 



A A. LECIDEALES. The type of fruit nearest to fungi in form and origin 

 occurs in the Lecideales. It is an open disc developed from the fungal sym- 

 biont alone, the alga taking no part. There are several phyla to be considered. 



