204 Alexander IV. Ex'ans, 



Proljably most students of the Hepaticae would agree that such 

 species of Riccardia as R. pingids represent generalized types, 

 from which the more complicated types have been derived by pro- 

 gressive modification and specialization. Taking the genus as a 

 whole, therefore, a flat dorsiventral thallus, with little evidence of 

 histological or other differentiation, might be regarded as a sort 

 of evolutionary center from which various lines have diverged. 



This conception, unfortunately, throws no light on the phyto- 

 geny of the generalized types themselves, and the large amount of 

 morphological work which has been done on the Hepaticae leaves 

 this matter in equal obscurity. It is wholly uncertain, for example, 

 whether an apparently simple thallus of this character is really 

 primitive or whether it has been derived from some more complex 

 type of gametophyte. If the latter is assumed, at least three 

 possibilities present themselves. First, it may have descended 

 from some leafy anacrogynous form. In this case the lateral 

 leaves would have disappeared altogether, while the ventral slime 

 papillae would represent the final vestiges of two rows of under- 

 leaves. In certain species of Cephalozia, such as C. bicuspidata, 

 the underleaves on vegetative axes are represented by similar ves- 

 tigial papillae in a single row, corresponding to the different type 

 of apical cell found in this genus. Second, the simple thallus may 

 have descended from some more complex marchantiaceous thallus. 

 In this case the elaborate system of dorsal air-chambers would 

 have undergone degeneration or simplification, leading to the com- 

 plete obliteration of the chambers, and the two median rows of 

 ventral scales would have failed to develop beyond the earliest 

 papilliform stage. A similar simplification has evidently taken 

 place in the marchantiaceous genera Dmnortiera and Mono- 

 selenium and is usually regarded as an adaptation to a wet environ- 

 ment. Third, the long-lived thallus may represent the elaboration 

 of the thalloid juvenile stage of some leafy form. The genera 

 Pteropsiella and Mctzgerlopsis are apparently examples of such a 

 derivation, the juvenile thalloid condition persisting until the 

 appearance of the sexual branches. It is possible that later inves- 

 tigations, especially upon the early stages of different gameto- 

 phytes, will help clear up some of these questions. 



Another feature of the thallus which increases the difficulty of 

 tracing out its phylogeny is its constant dorsiventrality. Accord- 



