M archantiacece. 
171 
the remaining genera there are two furrows. In Wiesnerella, with 
six to eight involucres, and in Dumortiera, with as many as sixteen, 
Fig. 27. Preissia commutata. A.—Transverse section of thallus. The two 
areas enclosed by dotted lines indicate the position of the mycorhiza, and the 
depth of the air-chamber zone is shown by the upper dotted line extending 
across the thallus. At the margins there is a membrane one cell thick and 
four or five cells broad, x 20. B.—A ventral scale, showing its pointed 
appendage, the club-shaped mucilage hairs on the free margin, and some of 
the tuberculate rhizoids springing from the base of the scale. x35. C.—Part 
of A., x 60, showing the upper zone of air chambers and the lower zone of 
compact tissue. In the latter, part of the mycorhiza is shown with fungal 
hyphae, and one of the latter is seen entering by a smooth-walled rhizoid on 
the side of the midrib. On the low'er side are shown the overlapping scales 
and the bundles of tuberculate rhizoids. D.—Part of C., x 120, to show the 
structure of an air-chamber. The floor of the chamber consists of green cells, 
and from it arise branched green filaments. Chlorophyll is also present in the 
epidermal cells. E.—Part of the epidermis in surface view, from above, 
showing tw r o rings of cells around the pore, with the thin membrane covered 
with resinous grains. The dotted lines indicate the position of the low'est ring 
of cells, x 200. E and F.—The lowest ring of cells in surface view', from 
below. In E the pore is open, in F it is almost completely closed, x 200. 
the receptacle is like that of Fegntella in external appearance; but 
in Preissia and Bucegia, which have four involucres, the rhizoid- 
