842 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
terminating some of the finer branches. This type of plant-body is simpler than 
anything that was previously known in plants with stomata, tracheidal vascular 
system, and cuticularised spores. Its demonstration is the outstanding contribution 
of fact which the study of the Rhynie deposit has made to the comparative 
morphology of the Vascular Cryptogams, and through them of the land-vegetation 
composed of sporophytes. 
In the comparative study of the Vascular Cryptogams we must now include 
plants with independently living, rootless, and leafless sporophytes ; these may, 
without exaggeration, be described as thalloid branch-systems attached to the soil 
by a basal region bearing rhizoids. As previously mentioned, the existence of a 
gametophyte or prothallus, probably of small size, can be inferred from the pro- 
duction of the spores in tetrads. There is nothing in these very simple sporophytes to 
suggest their origin by reduction from more highly organised types. Their antiquity 
and what we know of the peculiarities of the vegetation of the Early Devonian 
period add to their interest and significance. 
The type of the Rhyniacese is so simple that a gap remains between it and even 
the simplest sporophytes with leafy shoots. The mode of origin of the latter type 
of construction, which has long been a speculative morphological problem, is thus 
not clearly demonstrated by the ancient leafless and leafy types that co-existed at 
Rhynie. This problem will be dealt with further below. 
On the other hand the simplicity of organisation of the sporophyte in the 
Rhyniacese facilitates comparisons with the Bryophyta and the Algse. While 
concentrating attention on the Rhynie plants as most fully known, some other Early 
Devonian plants must be borne in mind in such comparisons. Prominent among 
these plants is the incomplete and less satisfactorily preserved, stalked, columellate 
sporangium or sporogonium which has been named Sporogonites by Halle. Another 
plant that must be mentioned is the peculiar Parka, of uncertain systematic position, 
in which masses of cuticularised spores have recently been shown to fill the areolae. 
As regards Bryophyta, our practical ignorance of undoubted plants of this group 
from any but the more recent geological formations must be remembered. It was 
suggested by Halle that Sporogonites was the sporogonium of a Bryophyte ; in the 
light of our knowledge of Hornea his alternative view that it might be the upper 
portion of a plant on the line of descent of the Pteridophytes appears to be more 
probable. All that can at present be said is that the simplicity of the independent 
sporophytes of the Rhyniaceae makes comparison between the asexual generation in 
Bryophyta and Pteridophyta easier, and to this extent narrows the gap between 
these two great groups. Further than this it is not possible to go, so long as we 
have no knowledge of simple sporophytes permanently borne on the gametophytes ; 
this becomes more than ever the essential distinction between Bryophyta and 
Pteridophyta. 
The organisation of the Rhyniaceae also facilitates comparison with Algae. In 
