THE THALLUS 291 



aa. COENOGONIACEAE. There are two types of gonidial algae in this 

 family, and both are filamentous forms, Trentepohlia in Coenogonium and 

 Cladophora in Racodium. The resulting lichens retain the slender thread-like 

 form of the algae, their cells being thinly invested by the hyphae and both 

 symbionts growing apically. The thalline filaments are generally very 

 sparingly branched and grow radially side by side in a loose flat expansion 

 attached at one side by a sheath, or the strands spread irregularly over the 

 substratum. Plectenchyma appears in the apothecial margin in Coenogonium. 

 Fruiting bodies are unknown in Racodium. 



Coenogoniaceae are a group apart and of slight development, only the 

 one kind of thallus appearing; the form is moulded on that of the gonidium, 

 and is, as Reinke 1 remarks, perfectly adapted to receive the maximum of 

 illumination and aeration. 



bb. LECIDEACEAE AND GYROPHORACEAE. The origin of this thalline phylum 

 is distinct from that of the previous family, being associated with a different 

 type of gonidium, the single-celled alga of the Protococcaceae. 



The more elementary species are of extremely simple structure as 

 exemplified in such species as Lecidea (Biatora) uliginosa or Lecidea granu- 

 losa. These lichens grow on humus-soil and the thallus consists of a spreading 

 mycelium or hypothallus with more or less scattered thalline granules con- 

 taining gonidia, but without any defined structure. The first advance takes 

 place in the aggregation and consolidation of such thalline granules and 

 the massing of the gonidia towards the light, thus substituting the hetero- 

 merous for the homoiomerous arrangement of the tissues. The various 

 characters of thickness, areolation, colour, etc. of the thallus are constant and 

 are expressed in specific diagnoses. Frequently an amorphous cortex of 

 swollen hyphae provides a smooth upper surface and forms a protective 

 covering for such long-lived species as Rhizocarpon geographicum, etc. 



The squamulose thallus is well represented in this phylum. The squa- 

 mules vary in size and texture but are mostly rather thick and stiff. In 

 Lecidea ostreata they rise from the substratum in serried rows forming a 

 dense sward; in L. decipiens, also a British species, the squamules are still 

 larger, and more horizontal in direction; they are thick and firm and the 

 upper cortex is a plectenchyma of cells with swollen walls. Solitary hyphae 

 from the medulla pass downwards into the support. 



Changes in spore characters also arise in these different thalline series, 

 as for instance in genera such as Biatorina and Buellia, the one with colour- 

 less, the other with brown, two-celled spores. These variations, along with 

 changes in the thallus, are of specific or generic importance following the 

 significance accorded to the various characters. 



In one lichen of the series, the monotypic Brazilian genus Sphaerophoropsis 



1 Reinke 1895, p. no. 



19 2 



