M archantiales. 95 
wall as a cup. Sometimes each epidermal cell grows out into a long 
colourless hair. 
In several species the air-canals become drawn out into wide 
chambers, bounded by cell-plates ; these species have by different 
writers been separated under the name Ricciella, regarded by some 
as a sub-genus of Riccia and by others as an independent genus. 
In R. crystallina, which grows in very moist and shaded places, the 
wide chambers remain open above and form shallow cavities 
separated by a network of thin vertical partitions. In R. fluitans, 
the chambers are roofed over by a layer of cells ; in sterile aquatic 
plants, which float just under the surface-film of the water, the 
chambers are closed, but in the land form a pore is left in the roof 
of each chamber. In some allied species, the chambers are parti¬ 
tioned up, so as to form two or more superposed tiers as seen in 
cross-section of the thallus. The figure given in various text-hooks 
as the land-form of R. fluitans, e.g., in Fig. 379 of the English 
edition of Strasburger’s “Text-book of Botany,” is in reality a 
representation of R. natans ; the land form of R. fluitans differs 
from the water-form only in having rhizoids, sexual organs and 
fruits, but not in the general habit of the thallus. 
Riccia natans is also an amphibious species, and it too has been 
separated as a distinct genus, Ricciocarpus, but the recent investi¬ 
gation of this plant by Lewis (14) and by Garber (8) has shown 
Fig. 5. Riccia natans. Transverse section of thallus showing air-chambers 
and ventral scales. 
conclusively that the older systematists were correct in including it 
in the genus Riccia, The sterile free-floating aquatic form has 
a broad thallus, and the air-chambers are divided up by plates 
