610 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
The spores (PI. Ill, fig. 26) were about 65 y in diameter. The difference in size 
from the spores of R. Givynne-Vaughani was constant in all the sporangia of the 
two species so far studied. 
The remarkable and very persistent layer of cells which lines the cavity of the 
sporangium in both species of Rhynia has been referred to throughout as the 
“ tapetum.” Its position corresponds to that of a persistent tapetal layer, but it 
shows peculiarities which call for special remark. When best preserved it is seen 
to be a layer of somewhat flattened, elongated cells, the walls of which remain 
bounding the empty-looking cell cavities. It suggests to our minds a relatively 
rigid tissue, and is perhaps best compared with the sheath of tracheidal cells which 
has been described in some other fossil sporangia.* The appearance of the walls 
of these “tapetal” cells in Rhynia is quite consistent with their being of tracheidal 
nature, though the preservation does not allow of any special thickening being 
recognised. 
Diagnosis, 
The recognition of two species of Rhynia necessitates some modification of the 
generic characters and a differential description of the species. 
Rhynia. 
Plant gregarious, rootless and leafless, consisting of subterranean rhizomes 
attached by unicellular rhizoids, and erect, dichotomously branched, cylindrical aerial 
stems. Stomata present. Stele consisting of a zone of phloem surrounding a strand 
of tracheides. Sporangia cylindrical, without columella, terminal on aerial stems. 
Homosporous. Spores with cuticularised wall, developed in tetrads. 
Rhynia Gwynne- Vaughani. 
Aerial stems tapering upwards, probably about 20 cm. high, and ranging in 
thickness from 3 mm. to under 1 mm. Small hemispherical protuberances of super- 
ficial tissues of the stem occur, and sometimes, in place of them, adventitious branches, 
the stele of the branch not being continuous with that of the main stem. Xylem 
strand of stele slender, only sometimes showing a distinction of smaller central and 
larger peripheral tracheides. Tracheides with broad annular thickening. Sporangia 
about 3 mm. long and 1'5 mm. in diameter. Sporangial wall about '2 mm. thick. 
Spores about 40 y in diameter. 
Locality. — Muir of Rhynie, Aberdeenshire. 
Horizon. — Old Red Sandstone (not younger than the Middle Division of the Old 
Red Sandstone of Scotland). 
F. W. Olivee, “ On a Vascular Sporangium from the Stephanian of Grand Croix,” New Phytologist, vol. i, p. 60. 
