278 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



in this group, so that their genetical behavior is still rather incompletely under- 

 stood. Nevertheless, several very important programs of research on the genetics 

 of bryophytes have been carried on through the last half -century. The discovery 

 of sex chromosomes in Sj^h aero car pus by Allen led him to a series of investiga- 

 tions (Allen, 1945) on the genetics of this genus, lasting more than twenty 

 years. The extensive researches of Wettstein on the genetics and experimental 

 morphology of mosses are classics (1942; in Verdoorn, 1932). His data do not 

 indicate clearly that inheritance in mosses follows any simple Mendelian system, 

 and therefore give real incentive for continued work, especially in view of the 

 remarkable techniques that he invented. Burgeff (1943) gives us the results 

 of an extraordinarily detailed monographic investigation of the genetics and 

 genetic mechanisms of Marchantia. Further genetical study of different groups 

 of bryophytes is still very much to be desired (cf. Sinoir, 1952). It would seem 

 that the different behavior of the same chromosomes in haploid and diploid con- 

 ditions, producing respectively a gametophyte and a sporophyte plant of very 

 different appearance, would make an excellent problem for investigation. 



Classification 



At the end of a century during which matters of classification received 

 much thoughtful consideration, we find that the generally accepted subdivisions 

 of the division (or phylum) Bryophyta are three classes, the Hepaticae, the 

 Musci, and the Anthocerotae, although the last-named group is considered by 

 many botanists to be only an order within the class Hepaticae (Fritsch, 1929). 

 Considerable evidence supports the separation of the present order Sphagnales 

 from the Musci, and further study may very well see the acceptance of this 

 group as a separate class (Chalaud, 1945). Detailed studies in any group of 

 plants tends to bring out fundamental differences hitherto unnoticed and to 

 result in an increase in the number of classes or even of divisions, as has oc- 

 curred in Algae (Smith, 1938). The philosophical aspects of systematic bryology 

 have been considered by several authors, and considerable progress made in 

 correlations between the morphology of bryophytes, their classification, and the 

 nature of species and other taxonomic groups. One of the most distressing as- 

 pects of bryophytes to the person who may wish to classify them in an orderly 

 manner is the fact that the gametophyte and sporophyte generations have 

 evolved with no relation to each other, in very different directions, so that in 

 some groups we find a conservative gametophyte and a very variable, rapidly 

 evolving sporophyte, whereas in other groups we find just the opposite situa- 

 tion. Although a relatively small group compared with the Musci, the Hepaticae 

 have received a disproportionate amount of attention, as compared with mosses, 

 from the aspects of phylogeny, evolutionary sequences, and special morphology, 

 perhaps because of the diverse nature of the group and the many families and 

 genera with a single or very few representatives. The results of these studies 

 appear in the standard references on plant morphology, but among the works 

 of especial importance should be cited those of Schiffner (1917) and Fulford 

 (1948). K. Miiller (1948a) offers a thoughtful discussion of tlie species prob- 

 lem and specific criteria in Hepaticae. The numerous problems involved in the 

 classification of Hepaticae have attracted a great deal of attention, and reasonably 

 stable arrangements have been established (Verdoorn, 1932; Evans, 1939). In 



