The Genus Riella, with Descriptions. of new Species from North 
America and the Canary Islands * 
By M. A. Howe anp L. M. UNDERWOOD 
(WITH PLATES II AND 12) 
The genus Ae//a occupies a unique position among the Hepa- 
ticae. The striking peculiarities of its gametophytic phase have 
attracted the attention of such morphologists 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. vedla 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,{ 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 Aze//a in 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. Goebelt has expressed the opinion that the chief 
difference between Aze//a and the other liverworts is that in Azed/a 
the development of the thallus is in the vertical instead of in the hori- 
zontal plane. The species of Rve//a 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. 
t Trabut, Rev. Gén. Bot. 3: 451. 1891. Stephani, Bull. Herb. Boiss. 7: 659- 
1899. 
t Flora, 77: 107. 1893. 
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