SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 665 
small scale-leaves ; stele, with the xylem within the zone of phloem becoming stellate 
and giving off leaf-traces ; cortex as in the rhizomes. 
Shoots from over 1 cm. to under 1 mm. in diameter, with simple leaves about 
5 mm. long, without any vascular bundle in the free portion — the leaf-trace 
stopping at the leaf-base. Epidermis with thick outer walls and with stomata ; 
narrow outer cortex ; wide inner cortex usually differentiated into three zones, the 
middle zone having the vertical plates of tissue separated by wide intercellular 
spaces, and extending between the outer and middle zones as radial trabeculae. 
Stele with stellate xylem without any mixture of parenchyma ; protoxylem immersed 
in the ends of the rays ; phloem between the rays .and around the stele. Elements 
of phloem uniform, thin walled and elongated. Thickening of the tracheides, both 
of the protoxylem and metaxylem, spiral. Leaf-traces departing from the arms of 
the xylem in more than one vertical row ; protoxylem surrounded by metaxylem, 
and this by a narrow zone of phloem. Trace ending in the outer cortex in the leaf- 
base. Branching lateral and dichotomous. Small stems of similar structure, but 
with xylem three-rayed, or even a cylindrical strand. 
There is a probability, based on close association of the remains, that the fertile 
region of Asteroxylon consisted of slender, branched, leafless axes with peculiar 
structure and pear-shaped sporangia about 1 mm. long ; sporangia with epidermal 
layer thickened towards the summit, where regular dehiscence took place. Homo- 
sporous. Spores developed in tetrads about 64 m in diameter. 
Locality. — Muir of Bhynie, Aberdeenshire. 
Horizon. — Old Red Sandstone, not younger than the Middle Division of the Old 
Red Sandstone of Scotland. 
The Classification must remain to some extent obscure until the uncertainty 
as to the fertile region is cleared up. We suggest for the present associating Aster- 
oxylon with Psilophyton princeps in a family of the Psilophytales to be called 
the AsteroxylaceEe. 
The Classification of the Vascular Cryptogams or Pteridophyta associated together 
in the Rhynie bed of silicified peat would thus be as follows : — 
PTERIDOPHYTA. 
Class. Psilophytales.* 
Family, rhyniacea;. 
Genus. Rhynia (R. Gwynne-Vaughani, R. major). 
Hornea ( H . Lignieri). 
Family, asteroxylace^e. 
Genus. Asteroxylon ( A . Mackiei). 
* See Part I, Trans. Roy. Soc., Edin., vol. ii, p. 780, 1917. 
