THE DETEUMTN\TrO\ OF LICHENS IX THE FIELD 11 



-0. Oil trees. Cortical cells present LeptOgidium. 



On rocks or other lichens on rocks. No cor- 

 tical cells 27. 



27. Filaments sparingly branched. Alga 'SVy- 



tonema Thermutis. 



Filaments with frequent branches. Alga 



Stigonema Spilonema. 



2>S. Very rare plants having perithecia (opening 



by a pore ) Pyrenidium. 



Commoner plants with apothecia (open discs). Leptogium. 



{a) Fruticose. Attached by a basal point to the substratum, tlie 

 divisions of the thallus having a radiate (similar on all sides j 

 structure. 



[h) Algal cells are green except in Roccella with reddish Trente- 

 pohlia. Fanuella pubescens and P. corniculata are included here, 

 because their thalli are not gelatinous and their algal cells are o-veen, 

 though they hive small or short cylinders or lobes. Narrow-lobed 

 Parmelias (e. g. F. encausta, P. aJpicoJa, P. stijgia), with their 

 thalli appressed to the substratum, are included in the Sectional 

 Key A. 



{c) Algal cells blue-green. Soloriua spongiosa (which mav 

 possibly be placed here) has bright-green algal cells in the general 

 thallus, but also contains groups of blue-green algse. 



{d) The thalline divisions are so minute (less than 100 /i diameter) 

 that they can only be seen under a strong lens. The algal cells are 

 blue-green except in Bacodmm (with green Gladophora) and Coeno- 

 goniuni (with reddish Trentepoldia^. 



(e) A parmelioid apothecium is ± shield-shaped, large, and has a 

 thalline margin. 



(/) Some Cladonias, which would be placed here when they are 

 without squamules at the base, are much branched. 



(y) Including A. thrausta. The rare plant, Pligscia infricdta, 

 may be found here. 



{h) When specimens of this genus are without squamules at the 

 base they may be found here. 



(/) Some forms of Flat y sum may possibly come here. 



{j) In Polj/chidium mMscicolum the cylindrical th. is often 

 almost even, but is much-branched. The Scgfonema algal cells in 

 this and other plants can easily be taken as Nostoc, the sheath bein^- 

 often wanting or indistinct. They are more irregular in outline than 

 N'ostoc cells, and seldom form long chains. 



{k) Leptogium microscopicum, which is little branched but is usu. 

 ±nodulose, will generally be found under the group with "minute 

 cylinders " (23) since the thalline lobes are usu. under 100^ thick. 



(/) Scarcely likely to be found here. 



{m) In the rare plant Sgnalissa uitricata the algal cells form 

 chains. 



(«) Some forms of Placgnthium nigrum may possibly be found 

 here. The thallus is scarcely gelatinous though it contains blue-greeu 

 alg;e (Scgtonemu). 



