BRITISH LICHENS. 21 



The upright thaUua may assume one of two forms known as the 

 fruticulose* mad filamento^ts forms. 



1st. — The fntticiilose form is so named because of its general 

 resemblance to a miniature shrub. It is attached at the base, from 

 which it ascends in one or more stems, with or without branches, which 

 are flat or cylindrical. The most familiar illustration of this form is that 

 of the Iceland moss of commerce (Cetraria Islandica,) (Plate V., Fig. 1,) 

 imported and sold in eveiy chemist's shop. 



2nd. — The filamentous form is that in which the stems arise from a 

 small base of attachment, and consist of thread-like, rigid, or flaccid, 

 upright, prosti-ate, or pendulous filaments, often repeatedly branched. 

 The Beard Moss (Usnea barbata) (Plate V., Fig. 3,) so frequently to be 

 seen on old trees, is an example of this form. 



The prostrate thaUus may be either attached to its support by a small 

 space only, or by the whole of its under surface. As this division includes 

 the major part of Lichens it has been sub-divided into, 1st, the foliaceous, 

 and 2nd, the crustaceous fonns. 



1st. — The foliaceous thallus. — This consists of leaf -like expansions, 

 spreading horizontally, attached by one or more points on the under 

 surface to the substance on which it grows. It is often cut or torn at the 

 margin into more or less irregular lobes, which at times overlap each 

 other. In some species having a foUaceous thallus the attachment 

 beneath is by numerous small filaments, which, though suggesting the 

 idea of rootlets, serve no such purpose ; they are called rhizina.\ 

 Parmelia saxatiUs, one of the commonest British species on rocks and 

 trees, pi-esents these rhizinae in abundance. 



2nd. — The ci-ustaceous thallus. — As the name implies, this forms a 

 more or less thick cx'ust, generally attached by the whole of its under 

 side, but often fi-ee at the margin. Some of the chief forms of this 

 must be particularised. It begins to depart from those already men- 

 tioned by the 



Squamulosel thallus. — This is formed of small scales, either 

 detached or united to each other by the margin, and affixed to the 

 substance on which it gi-ows by its under surface. The outline is very 

 variable, affording important characters in the distinction of species. 

 Sqjiomaria crassa, a common species on the earth in limestone districts, 

 will illustrate this form of thallus. (Plate V., Fig. 6.) 



Grajiulose thallus. — This is formed by a thin layer of iri'egularly 

 sized granules, each granule separated from the other, or united into a 

 continuous surface. It prevails largely in the genus Lecidea. 



*- Fiom frutex, Latin for shrub, /rftJicutose— diminutive. 



t From ptfa a root. 



; Prom squama, Latin for the scale^f a fish. 



