SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 621 
to recall the rhizoids, the simple vascular strand, the intercellular spaces and stomata, 
and the thick cuticle to indicate how these needs are met. 
The simplicity of the stelar anatomy limits comparison with other vascular plants, 
but may be compared with the rhizome of the Psilotacese. In the xylem of the 
central cylinder of the Rhyniacese we find two grades of construction illustrated : 
(a) the strand of equivalent and similar tracheides in many stems of Rhynia Gwynne- 
Vaughani ; ( b ) a differentiation of the strand of xylem into smaller inner tracheides 
and larger peripheral tracheides, as in some stems of Rliynia Gwynne-Vaughani 
and regularly in R. major and in Hornea. This differentiation suggests comparison 
with the xylem of the stem-stele and leaf-traces of a number of Pteridophyta in 
which the protoxylem is surrounded by metaxylem (centrarch). 
The sporangia of Rhynia and Hornea will be compared below with the sporogonia 
of Bryophyta and the stichidia of the Red Sea-weeds. What in other Pteridophyta 
may correspond to their simple terminal position will be better discussed in a later 
paper when the more complicated types of Psilophytales have been considered. 
A peculiarity of the sporangia of Rhynia and Hornea is that they are relatively 
large and indehiscent. The thickened epidermis seems an advance in specialisation, 
though apparently useless for dehiscence. It looks as if the thickened epidermis, 
serving for the protection of the spores, had preceded any arrangement for the 
opening of the sporangium in these plants. The sterile columella of the sporangium 
of Hornea may correspond to the sterile projection from the base of the sporangium 
in some species of Lycopodium. 
Comparisons with Bryophyta. 
The difficulty in comparing Rhyniacese with Bryophyta lies in our ignorance of 
the sexual generation and the relations of the young sporophyte to it in the ancient 
Pteridophyta. The simple morphology of the vegetative region of the plants of 
Rhynia and Hornea as well as their sporangial structure appear, however, to bring 
us a step nearer to a comparative morphology of the sporophyte of Vascular Crypto- 
gams and the bryophytic sporogonium. It is sufficient at present to indicate the 
general resemblances that can be traced between the fertile regions of these Old Red 
Sandstone plants and the sporogonia of Hepaticse, of Anthoceros, and of certain 
Mosses. Whether the sporogonium is a simpler parallel development to the 
sporophyte or is a reduced equivalent of such a plant as that of the Rhyniacese must 
remain an open question. The possibility that these questions, which have hitherto 
had to be treated in a purely speculative fashion, may be at least partially answered 
by the discovery of further plants of early Devonian age,~is strengthened by the 
experience of the last few years. 
