V. AFFINITIES AND PHYLOGENY OF RICCARDIA 



The classification of the anacrogynous Jungermanniales is by 

 no means firmly established. A number of recent writers asso- 

 ciate Riccardia with Metsgcria and Hynicnophytitm in a special 

 tribe or family, separated from the other thalloid genera by their 

 short and specialized sexual branches and by the possession of an 

 elaterophore extending downward from the apex of the capsule. 

 To this group Cavers has given the family name Aneuraceae 



(5, P- 195)- 



In the large genus Mctzgeria the thallus is characterized by broad 

 unistratose wings and a thickened median portion containing a 

 strand of elongated cells. It thus resembles the ultimate branches 

 of such species of Riccardia as R. crispa and R. hymenophyl- 

 loidcs, and it was natural for the older writers to refer species of 

 this character to Met:^gcria. In Riccardia, however, branches 

 with broad wings are found only in the more highly differentiated 

 forms and are usually associated with subterete and wingless axes 

 of a higher order. In Mctzgcria, on the other hand, the entire 

 thallus is winged, a branch is essentially like the axis from which 

 it arises, spreading from it dichotomously, and it is usually imprac- 

 ticable to distinguish branches of different ranks. The genus is 

 further characterized (in most species at least) by distinctive 

 appendicular organs in the form of long unicellular hairs, appar- 

 ently derived from rhizoids. These are sometimes restricted to 

 definite regions, such as the margins of the wings, but are often 

 more generally distributed and may even cover over both surfaces 

 of the thallus. The closest approach to these filamentous struc- 

 tures in Riccardia is found in such species as R. criocaiila. 



Although the vegetative branches of Metsgcria are normally 

 lateral in origin, just as in Riccardia, the sexual branches differ in 

 being invariably ventral, growing out from the thickened median 

 portion of the thallus. The male branches are far more delicate 

 than the vegetative branches but show well-differentiated wings, 

 which are strongly involute and cover over the antheridia. The 

 latter are borne on the dorsal surface of the thickened median 

 portion, as in Riccardia, but are not situated in depressions. The 



