THE BRYOLOGIST 
Vol. XXV September, 1922 No. 5 
REGENT STUDIES ON CERTAIN SPECIES OF RICCIA* 
Alexander W. Evans 
The genus Rtccia, as ordinarily understood, includes the two subgenera 
Euriccia and Ricciella. In typical Eiiriccia the air chambers are very narrow 
and clearly extend in a sa'ngle layer from the compact ventral tissue of the 
thallus to the upper surface; in typical Ricciella the chambers are much brcader 
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 beirg the case the distinction 
in the chambers become reduced to a difTerence in width, and^eventhisdistinction 
has been proved inconstant as a differential character. Juel (5) has shown, for 
example, that R. Btschoffit Hiiben., 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. 
The July number of The Bryologist was published July 31, 1922. 
