DAE 
the middle Devonian of northern Scotland (Aberdeenshire). 
Rhynia, of which two species are known, was a small leafless and 
rootiess piant, with a subterranean r 
— 
a" 
uzome. The slender erect 
shoots were sparingly dichotomously branched (7.c., forked), and 
sparingly covered with minute hemispheric projections. There 
was a small, solid, central stele, and the surface was green and 
photosynthetic. The ultimate branches bore large sac-like spore- 
cases (sporangia), with walls several cells thick. They were 
found to be fuil of spores, which were developed in groups of 
four (tefrads), as in all modern spore-producing plants. 
An associated and related genus, [/ornea, is even more simple 
than ARhynia, the particular point of interest being that the spo- 
rangia are obviously nothing but modified endings of the vegetative 
tissues, the spore-forming region corresponding anatomically to 
the inner cortex. Thus the spore-forming parts were but slightly 
differentiated from the vegetative, and were not morphological 
entities. 
The third plant shown, Astero.v\lon, was considerably more 
robust than the preceding, reaching a stem diameter of a centi- 
meter and a height of a foot or more. The single Scotch species 
(muddle Devonian) had a horizontal subterranean hairless rhizome, 
the ultimate branches of which burrowed in the peat like the roots 
of higher plants. The erect shoots had both forked and lateral 
branches, and were thickly covered with small simple leaves. 
These leaves had no vascular tissue, but the central star-shaped 
stele of the stem gave off vascular tissue for leaves (leaftraces), 
which ran through the outer tissue of the stem (cortex) but not 
into the ieaves. The distal branches were smooth and bore large 
pear-shaped spore-cases, whose wa 
— 
Is were thickened toward the 
summit where they split when the spores were ripe. A second 
species of Astcroxylon, from the midd 
e Devonian of Germany, 
was similar to the Scotch species but sturdier. The leaves on the 
distal branches became reduced to spines, and the stele of the main 
stem was not solid but had a central pit 
pea’ 
As 
These plants are grouped by the systematist in a separate order 
—the Psilophytaies, and some students have thought that they 
were transitional between algae and true vascular plants, but this 
view cannot be maintained. Alost botanists consider them to rep- 
