' The Genus Riella, with Descripticns of new Species trom North 

 America and the Canary Islands * 



Itv M. A. Howe and L. M. Underwood 

 (With Plates ii and 12) 



The genus Riella occupies a unique position among the Hepa- 

 ticae. The striking pecuHarities of its gametophytic phase have 

 attracted the attention of such niorphologists as Hofmeister, Leit- 

 geb, and Goebel, in addition to the interest excited among those 

 who have devoted themselves more exclusively to a study of the 

 systematic relations of the Hepaticae. Riella helicophylla, an Al- 

 gerian species, is alluded to in some of the standard botanical 

 text-books as being peculiar among the liverworts in having a 

 leaf-like lamina or wing disposed spirally in relation to the axis or 

 stem. Later studies of this species, however.f indicate that the 

 supposed helicoid spiral arrangement was exaggerated in the 

 original figures and description and that the spiral appearance is 

 due to the slight torsion of a stem bearing a strongly undulate 

 lamina. Nevertheless, the species of Riella m general are peculiar 

 enough in that the lamina or wing appears at first sight to be 

 attached to one side of the stem ; but the position of the sexual 

 organs, of the root-hairs and of the scale-like appendages shows 

 that the plant is bilaterally symmetrical in the plane of the wing 

 and the conviction is now general that the wing is dorsal in relation 

 to the stem. Goebel % has expressed the opinion that the chief 

 difference between Riella and the other liverworts is that in Riella 

 the development of the thallus is in the vertical instead of in the hori- 

 zontal plane. The species of Riella are all aquatic, commonly 

 growing entirely submerged, and it is doubtless this condition of 

 growth which makes possible the leading peculiarity in form. 



The growing point of a young plant or of a young branch of 



* Read in abstract before the Botanical Society of America, Ninth Annual Meet- 

 ing, Washington, D. C., December 31, 1902. 



fTrabut, Rev. Gen. Bot. 3 : 451. 1891. Stephani, Bull. Herb. Boiss. 7: 659. 

 1899. • 



+ Flora, 77 : 107. 1893. 



214 



