l68 Rhodora ' [August 



times completely dried up in the summer ; from this he would sepa- 

 rate as a distinct species the submerged M. aquatica. The typical 

 M. sphacdata, on the other hand, is a submerged aquatic, and from 

 this he would separate M. erythrorhiza^ which grows in drier locali- 

 ties. M. aquatica and M. crythrorhiza were both clearly described 

 many years ago by Limpricht, who treated them as varieties only, 

 but implied at the same time that their characters were fully as 

 marked as those separating the typical forms of M. emarginaia and 

 M. sphacclata. The validity of M. aquatica as a species was recog- 

 nized by Lindberg, Massalongo, Stephani and others, and there is 

 now a tendency among European writers to recognize M. crythro- 

 rhiza also. The choice of a name for this second species is not an 

 easy matter. The oldest synonym is the one first quoted, but Schifif- 

 ner discards the name media, provisionally at least, because No. ijy 

 of the Hep. Europ. is a mixture of two species. If we follow him in 

 this course we should choose the name Sullivanti of DeNotaris 

 rather than the later name crythrorhiza of Limpricht. The synonymy 

 of the species is already very complicated and in order to avoid 

 increasing it still further the name M. media is here retained. 



The characters separating M. aquatica and M. tnedia from the two 

 species to which they are respectively allied are drawn almost entirely 

 from the vegetative organs ; they concern the structure of the stem, 

 the shape of the leaves and the peculiarities of the leaf-cells. Most 

 of these characters are relative, and although extreme forms of both 

 species are easily distinguished, other forms are sometimes met with 

 which are difficult to refer definitely. As, however, other species, 

 such as Lophozia lycopodioidcs and Z. Floerkii, are recognized, 

 between which connecting links actually occur, there seems to be no 

 great objection to following this policy in the case of M. aquatica 

 and M. media. Even if they are not yet wholly permanent, they are 

 assuredly species in the process of evolution. 



M. aquatica is a very robust plant and sometimes reaches a length 

 of a decimeter or more. The stems are sparingly branched and 

 very firm in consistency. In a cross-section the cells all appear of 

 about the same size and are polygonal in outline. Those in the 

 middle are thin-walled but pass gradually into the thick-walled cells 

 of the outer portion. The outermost layer is composed of shorter 

 cells than those in the interior but they are usually thick-walled also ; 

 sometimes, however, close to the base of a leaf, the walls of this 



