Marchanticicece. 
163 
the genus Plagiochasma corresponds to Clevea, Reboulia to Sauteria , 
while the highest forms show a distinct approach to the Compositae. 
The air-chambers of the thallus are usually divided up by 
secondary partitions and plates, so that the whole upper region 
appears in section as a spongy mass of green tissue (Fig. 20). In 
some cases, however, the chambers are in a single layer and are 
then either empty (Grimaldia pilosa, Neesiella ) or contain assimilating 
filaments (some species of Grimaldia and of Fimbriaria). 
Fig. 20. Reboulia licmisphcrica. A.—Part of a transverse section of the 
thallus, showing the dorsal zone of green air-chamber tissue and the ventral 
zone of compact colourless tissue. On the midrib are shown the ventral 
scales in section, the small tuberculate rhizoids, and the large smooth-walled 
rhizoids. x 35. B.—A ventral scale, showing two long narrow appendages. 
x35. C.—Part of a scale, showing one of the appendages, several marginal 
mucilage-hairs, some of the oil-cells (shaded), and some tuberculate rhizoids 
springing from the base of the scale, x 150. D.—Part of the dorsal epidermis 
in surface view from above, showing a pore surrounded by six concentric rings 
of cells, the walls of which, as in the rest of the epidermis, show angular 
thickenings, x 150. E.—Part of A., showing the structure of the lateral 
region (lamina) of the thallus ; the compact ventral tissue is here reduced to 
two or three layers of cells, x 150. 
The male receptacles are marked off from the surrounding 
tissue by a groove, from which arise scales that cover the receptacle 
