8o 



MORPHOLOGY 



species of Dermatocarpon and the primary thallus of the Cladoniae. Most of 

 these squamules are of a firm texture and more or less round in outline; in 

 some species of Cladonia, etc., they are variously crenate, or cut into pinnate- 

 like leaflets. Squamulose lichens grow mostly on rocks or soil, occasionally on 

 dead wood, and are generally attached by single rhizoidal hyphae, either 

 produced at all points of the under surface, or from the base only, growth 

 in the latter case being one-sided. In a few instances, as in Heppia Guepini, 

 there is a central hold-fast. 



A frequent type of squamulose thallus is that termed "placodioid," or 

 " effigurate," in which the squamulose character is chiefly apparent at the 

 circumference. The thallus is more or less orbicular in 

 outline ; the centre may be squamulose or granular and 

 cracked into areolae ; the outer edge is composed of 

 radiating lobules closely appressed to the substratum 

 (Fig. 42). 



All lichens with this type of thallus were at one time 

 included in the genus Placodium, now restricted by some 

 lichenologists to squamulose or crustaceous species with 

 polarilocular spores. Many of them rival Xanthoria parietina in their 

 brilliant yellow colouring. 



" 



Fig. 42. Placodium 

 murorum DC. 

 Part of placodioid 

 thallus with apo- 

 thecia x 2. 



Fig. 43. Lecania candicans A. Zahlbr., with placodioid thallus, 

 reduced (S. H., Photo.'). 



There are also greyish-white effigurate lichens such as Lecanora saxicola, 

 Lecania candicans (Fig. 43) and Buellia canescens, well-known British 

 species. 



