22 BRITISH LICHENS. 



Scurfy thallus. — In this form the surface is torn up into minute 

 flat scales, thickly distributed, giving it the scurfy aspect implied by the 

 name as ordinarily understood. 



Areolate* thallus. — In this form the crust of the thallus is divided 

 into minute spaces, which are either depressed, flat, or convex. These spaces 

 are sometimes rendered more conspicuous by having a border round each, 

 formed by a fissm-e in the crust. This occui's abundantly in the genus 

 Lecidea. (Plate V., Fig. 7.) 



PulveruJent\ thallus. — This is the granulose thallus, having the 

 granules more minutely divided, appearing under a pocket lens as if 

 dusted over the surface with a fine dry powder. This is one of the least 

 developed forms of a Lichen thallus. 



Besides these more marked forms of the thallus there are many 

 variations, the terms for describing which will be found in an admirable 

 glossary in " Leighton's Lichen Flora." 



There ai'e two terms of frequent use in describing the thallus which it 

 may be well to explain here. When a thallus presents a definite outline, 

 clearly marking how far it extends on the rock or bark, it is said to be 

 determinate. When there is no distinct or clearly defined outline, but it 

 dies away in patches, it is said to be effused or indeterminate. 



The size of the thallus varies from a minute point only to be seen by 

 the aid of a magnifying glass up to one or even two square feet in super- 

 ficial extent. We refer exclusively to British Lichens. The genera 

 which contain the largest species are Sticta, Stictina, Peltigera, Parmelia, 

 Usnea, and Ramalina. Occasionally species belonging to other genera than 

 these named, wQl, under particularly favourable circumstances, extend 

 their thallus to an unusual size. 



The colour of the thallus includes all shades of green, varying to 

 yeUow in one direction, to brown in another, and to blue in another. It 

 may be black, or gray, or white ; it may be pale yellow, citron yellow, or 

 orange yellow ; never blue or red, except modified very much by other 

 tints. As a rule, the gonidia contribute largely to the colour of the 

 thallus, and become more conspicuous when moistened by water, hence 

 the predominance of green. 



The textui'e of the thallus may be soft and gelatinous, as in 

 Collema ; spongy, as in Peltigera ; fragile, as in Stereocaulon ; tough, as 

 in Cetraria ; firm and hard as in Roccella; tartarious and brittle, as iu 

 many of the Lecidece. In fact, the variations are infinite. 



Having thus briefly glanced at the external characters of the Lichen 

 thallus that are mostly perceptible to the unaided vision, we shall next 

 proceed to point out the main features of the microscopic structure of 

 the thallus. 



* Prom areola, Latin for a little bed, or quarter, in a garden. 

 + From pulvis, Latin for dust. 



