300 PHYLOGENY 



various adaptations of structure in these two types of thallus have already 

 been described^ 



In Parmelia itself there are indications of this line of development in 

 P- stygia, with short stiff upright branching fronds, and in P. pubescens, 

 with its tufts of filaments, but these two species are more or less dorsiventral 

 in structure and do not rise from the substratum. In Cetraria also there 

 is a tendency towards upright growth and in C. aculeata even to radiate 

 structure. But advance in these directions has stopped short, the true line 

 of evolution passing through species like Parmelia physodes with raised, and 

 in some varieties, tubular fronds, and the somewhat similar species P. Kamt- 

 schadalis with straggling strap-like lobes, to Evernia. That genus is a true 

 link between foliose and fruticose forms and has been classified now with 

 one series, now with the other. 



In Evernia furfuracea, the lobes are free from the substratum except 

 when friction causes the development of a hold-fast and the branching out 

 of new lobes from that point. It is however dorsiventral in structure, the 

 under surface is black and the gonidial zone lies under the upper cortex. 

 Evernia prunastri is white below and is more fruticose in habit, the long 

 fronds all rising from one base. They are thin and limp, no strengthening 

 tissue has been evolved, and they tend to lie over on one side; both surfaces 

 are corticate and gonidia sometimes travel round the edge, becoming fre- 

 quently lodged here and there along the under side. 



The extreme of strap-shaped fruticose development is reached in the 

 genus Ramalina. In less advanced species such as R. evernioides there is a 

 thin flat expansion anchored to the substratum at one point and alike on 

 both surfaces. In R. fraxinea the fronds may reach considerable width (van 

 ampliatd), but in that and in most species there is a provision of sclerotic 

 strands to support and strengthen the fronds. One of those best fitted to 

 resist bending strains is R. scopulorum {siliquosa) which grows by preference 

 on sea-cliffs and safely withstands the maximum of exposure to wind or 

 weather. 



The filamentous structure appears abruptly, unless we consider it as 

 foreshadowed by Parmelia pubescens. The base is secured by strong sheaths 

 of enduring character; tensile strains are provided for either by a chondroid 

 axis, as in Usnea, or by cortical development, as in Alectoria; the former 

 method of securing strength seems to be the most advantageous to the plant 

 as a whole, since it leaves the outer structures more free to develop, and there 

 is therefore in Usnea a greater variety of branching and greater growth in 

 length, which are less possible with the thickened cortex of Alectoria. 



ee. Physcjaceae. There remains still an important phylum of Lecano- 

 rales well defined by the polarilocular spores^. It also arises from a Biatora 



1 See p. loi. 2 See p. i88. 



