the lamell.e of polttrichtjm 107 



Key to the Gteoups. 



A. Upper margin o£ lamella sti-aight (sometimes finely 



papillate). 

 a. Marginal cells clearly differentiated from the rest ; 



colour yellowish Group II. 



h. Marginal cells not differentiated Group I Y. 



B. Upper margin of lamellae more or less crenate or 



serrate. 

 a. Margin slightly but distincth^ crenate. 



«'. Lamellae with doubled margin, caused by apical 



cell being more or less grooved Group V. 



h' . Lamellae with single margin Group I. 



h. Marginal cells or their thickened walls very 



strongly projecting Group III. 



Notes on the Groups. 



Oroup I. In these plants the apical cells are rounded, giving to 

 the margin an irregular, crenate outline. Both vary very much in 

 the deptli of the lamellae, and also in the thickness of the cell walls. 

 There is practically no difference in the lamellae of value for distin- 

 guishing the two species. 



Group IT. The members of this group are characterised by the 

 yellowish thickened walls of the marginal cells. The species may be 

 distinguished as follows : — 



A. Marginal cells papillate. 



a. Marginal cells longer than broad P. alpinum. 



h. Marginal cells subquadrate P. urnigerum. 



B. Marginal cells not papillate P. sexangulare. 



Oroup III. The cells of the lamelhe of the plants in this group 

 are much alike ; but the species may be separated by the characters 

 of the marginal cells : — 



A. Lumen of the marginal cells not included 



in the projection. Outer thickened wall 



crenif orm P. juniperinum. 



B. Lumen of the marginal cell more or less 



included in the projection. 



a. Lamellae 9-11 cells deep P. piliferum. 



I. Lamellae 6 cells deep P. strict urn. 



Group IV. The lamellae in this group are characterised by the mar- 

 ginal cells being undifferentiated ; the cell walls are of equal thickness 

 throughout. The lamellar characters are of no assistance in separating 

 the species. 



Group V. There is onl}^ one species in this group, P. commune in 

 which the mai-ginal groove of the lamellaj is well marked. But in 

 the varieties minus and perigoniale the groove is less evident, and, but 

 for the thickened and slightly crenulated margin and deeper lamella?, 

 they might be confused with Group IV. 



