90 
F. Cavers. 
been again examined by Porsild (26) and by Muller (24), and it would 
appear that Montague’s account of this species was quite correct. 
The nature of the wing itself has formed the subject of 
conflicting statements. Hofmeister (17) compared the Riella plant 
to a Marchantia or a Metzgeria in which the thin lateral wing had 
developed only on one side of the midrib, and Goebel has recently 
adopted this explanation in a somewhat modified form, basing his 
interpretation chiefly on observations made with young plants. 
Goebel’s “ Profilstellung ” theory of the organisation of Riella 
assumes that this genus differs from all other Liverworts in that 
the development of the thallus is in the vertical instead of in the 
horizontal plane ; that the whole tissue of the young plant arising 
from the spore is at first meristematic and consists of a single layer 
of cells; that later an intercalary growing-point arises either at one 
place or at two opposite places on the margin of the plate; that the 
new cells formed on the upper or distal side of the growing-point 
then contribute to the growth of the one-layered wing, while those 
formed on the lower or proximal side go to form the cylindrical 
stem. Solms (30), however, upholds Leitgeb’s view that the plant 
is dorsiventral and that the wing is simply a dorsal outgrowth ; and 
it is difficult to see how Goebel’s explanation could be applied to 
the case of R. bialata. The extreme plasticity of the Hepatic 
plant-body renders elaborate interpretations of the “Profilstellung” 
kind quite unnecessary, just as this plasticity completely baffles the 
attempt sometimes made to distinguish between “ thallus-lobes ” 
and “ leaves ” in this group. Schiffn^r (29) has described and 
figured the occurrence of leaf-like organs on the marginal portion 
of the wing in R. Battandieri. 
A comparison of the descriptions and figures in the literature 
of the genus, and the examination of specimens of most of the 
species hitherto discovered, shows that there is on the whole a 
definite correlation between the breadth of the wing and the size 
of the leaves; the broader the wing, the smaller are the leaves, and 
vice versa. The form of the plant depends very largely upon the 
external conditions of growth, especially the depth at which it grows 
in the water and the light which falls upon it. Most of the species 
grow erect in fairly deep water, and have a broad wing, usually 
thrown into folds, and relatively small leaves ; these erect species 
are attached to the mud by rhizoids at the base of the stem. Some 
species, growing in shallow water, have a creeping stem, attached 
along its whole length by rhizoids; here the leaves are relatively 
