302 



PHYLOGENY 



but that character appears in nearly every one of the horizontal species as 

 well, sometimes in the upper, sometimes in the lower cortex. 



In Physcia the horizontal thallus is of smaller dimensions than in Par- 

 melia, and never becomes so free from the substratum: it is attached by 

 rhizinae and soredia appear frequently. Very often the circular effigurate 

 type of development prevails. 



It is difficult to trace with any certainty the origin of this series of the 

 phylum. Some workers have associated it with the purely lecideine genus, 

 Buellia, but the brown septate spores of the latter are of simple structure, 

 though occasionally approaching the Rinodina type. There are also 

 differences in the thallus, that of Buellia, especially when it is saxicolous, 

 inclining to Rhizocarpon in form. It is more consistent with the outer and 

 inner structure to derive Rinodina from some crustaceous Placodium form 

 with a marginate apothecium, therefore from a form of fairly advanced 

 development. As the parietin content disappeared — perhaps from the pre- 

 ponderance of other acids — the colouration changed and the spores became 

 dark-coloured. 



Many genera and even families, such as Thelotremaceae, etc., have 

 necessarily been omitted from this survey of phylogeny in lichens, but the 

 tracing of the main lines of development has indicated the course of evolu- 

 tion, and has demonstrated not only the close affinity between the members 

 of this polyphyletic class of plants, as shown in the constantly recurring 

 thalline types, but it has proved the extraordinary vigour gained by both 

 the component organisms through the symbiotic association. 



The principal phyla', developing on somewhat parallel lines, are given in 

 the appended table : 



Archilichens 



' Dr Church (1920) has published a new conception of the origin of lichens.\ See postscript at 

 the end of the volume, p. 421. \ 



