SHOWING STRUCTURE, FROM THE RHYJS’IE CHERT BED, ABERDEENSHIRE. 849 
The structure of even the more simple Rhyniaceae in this respect differs 
markedly from what has been observed in any known Algae, and no pertinent com- 
parisons can be made with these plants. It is also unnecessary to do more than 
mention without entering into detailed comparisons the simple strands of narrow 
lignified cells found in the midrib of the thallus in a few genera of Hepaticae. 
In the Rhyniaceae the structure of the simple vascular strand or stele is similar 
throughout the plant. A feature of interest is the distinction of central and 
peripheral xylem seen in the steles of Rhynia major and Hornea and in the stouter 
strands of xylem of Rhynia Gwynne-Vaughani. Though the central xylem of 
these plants is not clearly protoxylem, it is perhaps justifiable to compare the 
arrangement with the centrarch construction of the leaf-traces of Asteroxylon and 
many species of Lycopodium and the stem-steles of some existing and extinct 
Vascular Cryptogams. 
The complicated vascular system of the stem of Asteroxylon is not easy to 
adequately compare with the simple vascular structure of the Rhyniaceae. This 
difficulty is intimately connected with the gap between the leafless condition 
of the latter and the leafy condition of the stem of the former plant which 
has been emphasised in the preceding section. Another difficulty that may 
be pointed out is that those regions of Asteroxylon in which the vascular 
strand is simplest (rhizome, branch-traces) have a solid strand of xylem 
with no indication of protoxylem. Protoxylem is clearly present in the 
leaf-traces and appears in the arms of the stellate xylem of the stem-stele. The 
attempt to interpret the complex xylem of the leafy stem of Asteroxylon at once 
introduces the problem of the respective parts played in its construction by a cauline 
strand and by decurrent leaf- traces. Plowever this may be, the resulting stele is 
remarkably comparable with the stele of Lycopodium, although the type of thicken- 
ing of its tracheides is quite peculiar to A steroxylon. The resemblance to the stellate 
stem-steles of some Zygopteridese, though not so close, must also be mentioned. It 
may be pointed out that there is also a possible comparison between the leaf-trace 
“ receptors ” and the central mass of metaxylem of the Asteroxylon stele and the 
receptors and the apolar connecting bar in the petiolar bundles of the Zygopteridese. 
These comparisons are the more significant in the light of the various points of 
resemblance between Asteroxylon and both Stauropteris and Lycopodium. 
Such general questions concerning stelar anatomy are too wide to be entered 
upon here, and may be left with these brief comparative remarks. 
(d) Position and Nature of the Sporangia of Pteridophyta. 
The Rhynie plants appear to afford some definite information on the nature 
and original position of the sporangia in the Pteridophyta. In the absence of 
evidence of actual origin in descent of one great group of plants from another, 
we can only recognise the way in which the sporangia of Rhynia and Hornea 
