Reprinted from The Bryologist, Vol. XXV, September, 1921. 



MBRAKY 



NEW YOtK 



BOTANICAL 



UAKDEN 



RECENT STUDIES ON CERTAIN SPECIES OF RICCIA* 



Alexander W. Evans 



The genus Riccia, as ordinarily understood, includes the two subgenera 

 Euriccia and Ricciella. In typical Euriccia the air chambers are very narrow 

 and clearly extend in a single layer from the compact ventral tissue of the 

 thallus to the upper surface; in typical Ricciella the chambers are much broader 

 and seem to be arranged in two or more layers when cross sections of the thallus 

 are examined. Largely on the basis of these differences Ricciella has sometimes 

 been considered a distinct genus. In all probability, however, its broad chambers 

 are really in a single layer, and the superimposed arrangement which they seem 

 to show is due to the obliquity of their course. This beir.g the case the distinction 

 in the chambers become reduced to a difference in width, and eventhisdistinction 

 has been proved inconstant as a differential character. Juel (5) has shown, for 

 example, that R. Bischoffii Huben., a widely distributed species of the Old World 

 has narrow chambers in the median portion of the thallus and broad chambers 

 toward the margin; and Von Gaisberg (4, p. 264) has made similar observations 

 in the case of R. Gougetiana var. armatissima Lev., a Mediterranean form. The 

 latter author cites in addition certain species of Ricciella in which some of the 

 air spaces are almost as narrow as those of typical Euriccia. In view of these 

 transitional forms connecting Euriccia and Ricciella, it seems wisest to follow 

 the usual custom and consider the groups subdivisions of a single genus. 



Until a few years ago Riccia fluitans L. was universally regarded as a clearly 

 defined species of Ricciella with a wide geographical distribution. It was said 

 to occur in two distinct states or forms: an aquatic form without rhizoids, which 

 never produced sexual organs and sporophytes; and a terrestrial form with 

 rhizoids, which produced sporophytes with some degree of frequency. The 

 fertile terrestrial form was said to develop when the aquatic form was deposited 

 on the mud, upon the evaporation of the surrounding water, while the aquatic 

 form was said to develop when the terrestrial form was flooded. Most writers 

 considered that the aquatic form, in spite of its sterility, represented the typical 

 condition of the species, and the terrestrial form was often separated as the 

 variety canaliculata (Hoffm.) Lindenb., in spite of the apparent ease with which 

 one form could be converted into the other. R. fluitans, moreover, was regarded 

 as the only member of the subgenus Ricciella that was able to live indefinitely 



♦Contribution from the Osborn Botanical Laboratory. 



