!93 



i. Collema Wigg. Prod. Fl. Hoist. 89. 1780. 



This genus is most typical of the family, and is readily distin- 

 guished from the following ones by the absence of a cortical tissue. 

 It is a large genus and further careful study may make a subdivision 

 into two or more genera convenient ; the spore-characters seem to 

 make such a division possible. 



The general characters not already mentioned in the description 

 of the family are as follows : The thallus consists of numerous minute 

 lobes in the lower forms, while in the higher the thallus-lobes are 

 large and comparatively few ; its color varies from dark blue to 

 nearly black ; in general the lower surface is somewhat lighter. 

 The resemblance of this thallus to the thalli of other lichens is only 

 apparent ; for instance, the thallus of Pai'melia is also large and lobed, 

 but the histological structure is wholly different, as a comparison will 

 clearly show. 



The thallus of Collema is perhaps more nearly homoimerous than 

 that of any other lichens, yet it is found that the algae (JVostoc) are 

 more numerous near the upper surface. It is evident that the distri- 

 bution of the algae in the thallus can be of little importance in clas- 

 sification. Of still less consequence is the gelatinous nature of li- 

 chens ; the algae which give the gelatinous consistency to the thallus 

 are, however, of prime importance. Older lichenologists who intro- 

 duced the separation into gelatinous and nongelatinous lichens did 

 not recognize the nature of the algae and their relation to the thallus ; 

 for that reason such distinctions are no longer of any value in classifi- 

 cation. 



The apothecia are comparatively small or medium ; in some species 

 they are very numerous, in others very few. A thalloid exciple is 

 present which usually extends somewhat above the margin of the 

 thecial disk. The general contour of the apothecium reminds one 

 strongly of the apothecia of Parmelia and other related genera. The 

 color of the disk is usually brown to very dark ; in form it may be 

 slightly convex, flattened or concave ; the margin of the thalloid ex- 

 ciple is frequently crenate. The colorless hypothecium consists of 

 numerous short-celled hyphae. The cells are somewhat rounded and 

 thin-walled ; the paraphytes are quite slender and unbranched. 



The spore-characters of this group are quite variable ; as a rule 

 the spores are colorless, though they may become somewhat colored 

 in some species ; they vary from simple (comparatively few) to multi- 



