8 52 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
specialised branches' of some Red Sea-weeds, in which the numerous tetrasporangia 
are enclosed by a wall-like outer layer of the branch and may surround a central 
axis that would correspond to the columella. The specialised tetraspore-bearing 
branches are called stichidia. They not only occur as terminal organs of the thallus, 
but may stand in definite relation to laterally placed vegetative branches. 
Advancing knowledge of the spore-producing structures in the Early Devonian 
plants may be expected to throw further light on such comparisons, which it is not 
advisable to press too far at present. The facts appear, however, to fully warrant 
the suggestion that the structure known as the sporangium in the Vascular Crypto- 
gams may be regarded as corresponding to the tip of a branch of a thalloid sporophyte 
enclosing the sunken tetrasporangia represented by the spore-mother-cells. On this 
view the sporangium of the Pteridophyta would not be an organ sui generis, although 
in considering its further modifications in the Pteridophyta it may be treated as if 
it were a primary member of the plant. 
In this section we have placed together our conclusions as to the main bearings 
of the new facts obtained by the study of the Vascular plants of Early Devonian age 
found at Rhynie on the comparative morphology of certain extinct and existing 
plants. While it appears inadvisable to omit such a general discussion, we would 
explicitly point out that we regard all such comparisons as to a certain extent 
tentative. They are thus distinct from the addition to our definite knowledge of 
the plants which it is the primary object of this series of papers to make known. 
In conclusion a word may be said as to the bearing of the Vascular Cryptogams 
in the Rhynie peat on the geological age of the deposit. That some plants of similar 
morphological character exist in both the Lower and Middle Old Red Sandstone of 
Scotland seems to be certain, although no clearly defined species has yet been 
recorded as common to both. In the Middle Old Red Sandstone neither Psilopliyton 
nor Artliro stigma, two characteristic Lower Old Red Sandstone plants, have yet 
been recorded on satisfactory evidence. On the other hand, no such leafy plant as 
Thursophyton Milleri, which is a most characteristic species of the Middle Old Red 
Sandstone, has been discovered in the Lower Old Red. If the leafy shoots of 
Asteroxylon Mackiei and those of Thursophyton Milleri are compared, taking the 
probable sizes of the plants into consideration, the similarity is very striking. The 
similarity is indeed so great as to suggest that they may be only different conditions 
of preservation of one species. It seems at present desirable, however, to treat them 
as distinct in the hope of additional light being thrown on their relation to each 
other. In any case it is clear that Asteroxylon Mackiei in its leafy shoots exhibits 
a close similarity to Thursophyton Milleri, and, even if not specifically identifiable 
with it, represents a characteristic type of plant which so far as at present known is 
absent from the Lower Old Red Sandstone. These facts taken together lead us to 
believe with but little reservation that the Rhynie chert-band is almost certainly of 
Middle Old Red Sandstone age. 
