THE THALLUS 301 



species and forms a parallel development. Even in this phylum there are two 

 series : one with colourless spores and mostly yellow or reddish either in 

 thallus or apothecium, and the other with brown spores and with cinereous- 

 grey or brown thalli. The dark spores are in many of the species typically 

 polarilocular, though in some the median septum is not very wide and no 

 canal is visible. Practically all of the lighter coloured forms contain parietin 

 either in thallus or apothecia or in both ; it is absent in the dark-spored series. 



Among the lighter coloured forms it is difficult to decide which of these 

 two striking characteristics developed first, the acid or the peculiar spore. 

 Probably the acid has the priority: there is one common rock lichen in this 

 country, Placodium rupestre (Lecanora irrubata), which gives a strong red 

 acid reaction with potash, but in which the spores are still simple, and the 

 fruit structure in the biatorine stage. Another species, PI. luteoalbum, with a 

 purplish reaction in the fruit only, shows septate spores but with only a 

 rather narrow septum. The development continues through biatorine forms 

 to lecanorine with a fully formed thalline margin. Among these latter we 

 encounter PI. nivale which is well provided with acid but in which the spores 

 have become long and fusiform with little trace of the polar cells or central 

 canal. We must allow here also for reversions, and wanderings from the 

 straight road. 



From crustaceous the advance is normal and simple to squamulose forms 

 which in this phylum maintain a stiff regularity of thalline outline termed 

 "effigurate"; the squamules, developing from the centre, extend outwards in a 

 radiate-stellate manner. There are also foliose thalli in the genus Xanthoria 

 and fruticose in Teloschistes. The cortex in the former horizontal genus is of 

 plectenchyma, and no peculiar structures have emerged. In Teloschistes the 

 cortex is of compact parallel hyphae (fibrous) which form the strengthening 

 structure of the narrow compressed fronds ( T. flavicans}. 



In the brown-spored series there is a considerable number of species that 

 are crustaceous united in the genus Rinodina, all of which have marginate 

 apothecia. One of them, Rinodina oreina, approaches in thalline structure the 

 effigurate forms of Placodium ; while in R. isidioides, a rare British species, 

 there is an isidioid squamulose development. 



Among foliose genera, the tropical genus Pyxine is peculiar in its almost 

 lecideine fruit, a few gonidia occurring only in the early stages; its affinity 

 with Physcia holds, however, through the one-septate brown spores with very 

 thick walls and the reduced lumen of the cells. The more simple type of 

 fruit may be merely retrogressive. 



Physcia, the remaining genus, is mainly foliose and with dorsiventral 

 thallus. A few species have straggling semi-upright fronds and these have 

 sometimes been placed in a separate genus Anaptychia. Only one " Anap- 

 tychia" Ph. intricata, has a radiate structure with fibrous cortex all round ; 

 in the others the upper cortex alone is fibrous of long parallel hyphae 



