105. Leptogium] 28. COLLEMACEAE 161 



105. Leptogium Ach.; S. F. Gray, Nat. Arr. Brit. PI. 1:400. 1821. 



Transforming the algal host colonies into small to large, more or less irregular, 

 foliose bodies, thallus showing an upper and lower cortex, each commonly com- 

 posed of a single layer of cells, rarely with poorly developed algal and medullary 

 layers, adhering to the substratum or attached by short rhizoids; apothecia some- 

 times wanting, immersed to adnate or sessile, the disk flat to convex, brownish to 

 brown, the algoid exciple sometimes disappearing; hypothecium hyaline to brown- 

 ish; hymenium hyaline or brownish above; paraphyses rarely branched; asci 

 clavate to broadly clavate; spores 8, hyaline, transversely and longitudinally sep- 

 tate. 



The algal host is Nostoc. 



A. Thalloid body very small to small, minutely lobed or more or 

 less crustlike 



B. Thalloid body granulose or crustlike 

 C. Spores 7-10 fi wide 



D. Thalloid body forming a broken areolate crust . . . 4. L. dactylinum 

 D. Thalloid body crustlike, more or less covered 



with coralloid branchlets 9. L. perminutum 



C. Spores 8— IS fi wide 



D. Apothecia more than 0.4 mm. across, disk reddish brown 



E. Plectenchymatous throughout 1. L. byssinum 



E. Plectenchyma limited to the cortical regions . . . 8. L. caesiellum 

 D. Apothecia 0.2-0.4 mm. across, disk dull brown 



or black 7. L. rhyparodes 



B. Thalloid body minutely squamulose, lobed, or 

 coralloid, rarely becoming crustlike 

 C. On rocks 



D. Thalloid body bearing coralloid branchlets along 



the margins 4. L. dactylinum 



D. Thalloid body with ascending margins . . . . . .5. L. Schraderi 



C. On soil and old wood among moss 



D. Spores 24-36 X 9—15 n 3. L. tenuissimum 



D. Spores 18-29 X 8-12 /x 6. L. minutissimum 



A. Thalloid body small to middle-sized or large, more or less 

 distinctly lobed 

 B. Rhizoids conspicuous on the under surface 



C. Plectenchyma limited to the cortical regions 

 D. Rhizoids scattered or clustered 



E. Bearing clustered rhizoids 12. L. Hildenbrandii 



E. More or less covered with delicate 



rhizoids below 11. L. Burgessii 



D. Rhizoids forming a densely tomentose nap below . . 10. L. saturninum 



C. Plectenchymatous throughout 2. L. plectenchymum 



B. Rhizoids inconspicuous or absent 



C. Spores usually less than 30 n in length 



D. Thalloid body more or less granulose or coralloid above 

 E. Thalloid body thicker, dark green to lead-colored 

 F. Thalloid body much lobed 



G. Margins crenate 13. L. chloromelum 



G. Margins erect and curled or 



wrinkled 13. L. chloromelum stellans 



F. Thalloid body less lobed, lobes gyrosely folded 



and margins slashed 13. L. chloromelum conchatum 



E. Thalloid body thinner, lead-colored or sky-blue 



F. Thalloid body lead-colored 14. L. tremelloides 



F. Thalloid body sky-blue 14. L. tremelloides azureum 



D. Thalloid body smooth to more or less wrinkled above 

 E. Thalloid body small, lobes more or less imbricated 

 F. Lobes broad, usually much imbricated 

 G. Spores 3-septate transversely, 



18-27X7-12/* 15. L.arizonicum 



