FAMILIES AND GENERA 311 



b. Not gelatinous when moist. 

 XLI. PANNARIACEAE. Algal cells Nostoc, Scytohema or rarely bright-green, 



Protococcaceae. Thallus crustaceous, squamulose or foliose. 

 XLII. STICTACEAE. Algal cells Nostoc or Protococcaceae. Thallus foliose, 



and very highly developed, corticate on both surfaces. 

 XLI 1 1. PELTIGERACEAE. Algal cells Nostoc or Protococcaceae. Thallus 



foliose, corticate above. 



3. Lecanorine group (apothecia with a thalline margin). 



The remaining families have all bright-green gonidia and nearly always apothecia 

 with a thalline margin. The group includes several distinct phyla : 

 XLIV. PERTUSARIACEAE. Thallus crustaceous. Apothecia, one or several 



immersed in thalline tubercles ; spores mostly very large. 

 XLV. LECANORACEAE. Thallus crustaceous or squamulose. Apothecia mostly 



superficial. 

 XLVI. PARMELIACEAE. Thallus foliose, rarely almost fruticose or filamentous. 



Apothecia scattered over the surface or marginal, sessile. 

 XLVII. USNEACEAE. Thallus fruticose or filamentous. Apothecia sessile or 



shortly stalked. 

 XLVIII. CALOPLACACEAE. Thallus crustaceous, squamulose or minutely fruti- 

 cose. Apothecia with polarilocular colourless spores. 

 XLIX. TELOSCHISTACEAE. Thallus foliose or fruticose. Apothecia with 

 polarilocular colourless spores. 

 L. BUELLIACEAE. Thallus crustaceous or squamulose. Apothecia (lecideine 



or lecanorine) with two-celled, thick-walled brown spores (polarilocular in 

 part). 

 LI. PHYSCIACEAE. Thallus foliose, rarely partly fruticose. Apothecia with 



two-celled thick-walled brown spores (polarilocular in part). 



Subclass 2. Hymenolichens. 



There are only three closely related genera of Hymenolichens, Cora, Corella and 

 Dictyonema with Chroococcus or Scytonema algae. 



There is reason to dissent from the arrangement in one or two instances which will 

 be pointed out in the following examination of families and genera. 



B. Families and Genera of Ascolichens 



The necessity for a well-reasoned and well-arranged system of classifica- 

 tion is self-evident: without a working knowledge of the plants that are 

 the subject of study no progress can be made. The recognition of plants 

 as isolated individuals is not sufficient, it must be possible to place them in 

 relation to others ; hence the importance of a natural system. In identifying 

 species artificial aids, such as habitat and substratum, are also often of great 

 value, and a good working system should take account of all characteristics. 



Lichen development is the result of two organisms mutually affecting 

 each other, but as the fungus provides the reproductive system, it is the 

 dominant partner : the main lines of classification are necessarily determined 



