342 SYSTEMATIC 



C. *Hymenolichens 



Fungus a Basidiomycete, akin to Thelephora. Algal cells Scytonema or 

 Chroococcus. Thallus crustaceous, squamulose or foliose. Spores colourless, 

 produced on basidia, on the under surface of the free thallus. 



The Hymenolichens* are few in number and are endemic in tropical 

 or warm countries. They inhabit soil or trees. 



Thallus of extended lobes. 



Gonidia near the upper surface i . *Dictyonema Zahlbr. 



Gonidia in centre of tissue 2. *Cora Fr. 



Thallus squamulose, irregular 3. *Corella Wain. 



II. NUMBER AND DISTRIBUTION OF LICHENS 

 I. ESTIMATES OF NUMBER 



Calculations have been made and published, once and again, as to the 

 number of lichen species occurring over the globe or in definite areas. In 

 1 898 Fiinfstiick stated that about 20,000 different species had been described, 

 but as many of them had been proved to be synonyms, and since many 

 must rank as forms or varieties, the number of well-authenticated species 

 did not then, according to his estimate, exceed 4000. Many additional 

 genera and species have, however, been discovered since then. In Engler 

 and Prantl's Pflanzenfamilien, over 50 families and nearly 300 genera fipd 

 a place, but even in these larger groupings opinions differ as to the limits 

 both of genera and families, and lichenologists would not all accept the 

 arrangement given in that volume. 



Fiinfstiick has reckoned that of his estimated 4000, about 1500 are 

 European and of these at least 1 200 occur in Germany. Probably this is 

 too low an estimate for that large country. Leighton in 1879 listed, in his 

 British Lichen Flora, 1710 in all, and, as the compilation includes varieties, 

 it cannot be considered as very far astray. On comparing it with Olivier's^ 

 recent statistics of lichens, we find that of the larger fruticose and foliose 

 species, 310 are recognized by him for the whole of Europe, 206 of these 

 occurring in the British Isles. Leighton's estimate of similar species is 

 about 145, without including varieties now reckoned as good species. In 

 a more circumscribed area, Th. Fries'* described for Spitzbergen about 210 

 different lichens, a number that closely approximates to the 206 recent re- 

 cords by Darbishire* for the same area. 



A general idea of the comparative numbers of the different types of 

 lichens may be gathered from Hue's compilation of exotic lichens^ examined 



1 See p. 152. 2 Olivier 1907. 3 Th. Fries 1867. 



^ Darbishire 1909. ^ Hue iSgi. 



