GENERAL CHARACTERS OF BRITISH LICHENS. 45 



filaments is the hypothallus ; its filaments, in progress of de- 

 velopment, become continuous with those of the medullary 

 layer of the Lichen-thallus, and possess similar characters. 

 Like them they may consist of a series of elongated cylin- 

 drical cells placed in apposition, and are then marked by 

 septa at irregular intervals and usually possess thickened 

 walls; or, the septa of the constituent cellules having dis- 

 appeared, the filaments are hollow, branching, thin-walled, 

 and simple. In the latter case, however complex the net- 

 work may appear, it can only be morphologically regarded 

 as the extension and subsequent ramification of the mem- 

 brane or wall of a simple cell. The hypothallus is in most 

 Lichens evanescent ; after it has served as the basis of the 

 thallus, it disappears. In some species it is persistent, and 

 may be recognized as a delicate filamentous network, — 

 frequently having a dendritic and crystalline appearance, 

 sometimes pale-coloured, more usually black, — below or sur- 

 rounding the thallus. It is characteristic of the Rhizocar- 

 pous section of the Lecidea, and may be easily seen in Le- 

 cidea geograpkica in the form of a black dendritic radiating 

 fringe, surrounding the yellowish-green thallus. When 

 this species grows on pure milk-white quartz, as it frequently 

 does on our Scotch mountains, this thalline fringe is very 

 conspicuous. 



