384 MR. W. N. EDWARDS ON THE 
possibly have been due to the presence of irritating vapours from fumaroles, 
and in any case did not form part of the original equipment of the plant. 
There is no close resemblance between these intumescences composed of thin- 
walled cells, and the highly cuticularised spines of Psilophyton, which are 
not definitely arranged in relation to stomata. Arber (1921, p. 25, fig. 7) laid 
much emphasis on the supposed resemblance between an axis of Rhynia 
figured by Kidston and Lang showing the external surface with intumescences 
and a fragment of Psilophyton figured by Halle (see above) showing spine- 
bases. It is to he noted that in the latter the actual spines were not present 
and that the radiating arrangement of epidermal cells round the spine-bases 
(shown in Halle’s figure) is not found in the case of the Rhynia outgrowths. 
We have already seen that the general resemblance of the epidermal cells of 
Rhyma and Psilophyton extends also to Asteroxylon. The structure of the 
epidermis and emergences does not, therefore, suggest such a very close 
relationship between Rhynia and Psilophyton as was claimed by Arber. The 
emergences of Psilophyton are more like the simple leaves of Asteroxylon 
in appearance, but there are important differences: the leaves of the 
latter, which are larger and are sometimes nearly 1 em. in length, are 
provided (though sparingly) with stomata, and sections as figured by 
Kidston and Lang (1920, pl. 5. fig. 39) show a papillate epidermis, whereas 
the spines of P. princeps are perfectly smooth. In Asteroxylon there is an 
extension of cortical tissue into the leaf, though the leaf-trace only reaches to 
the base and does not run into the free portion, but the spines of P. princeps 
seem to be so highly cuticularised and so slender that there can have been 
very little cortical tissue in them, if any. 
Though there are differences in the epidermal structure which may be of 
importance, the above comparisons support on the whole the view that 
Psilophyton is more nearly related to Asteroxylon than to Rhynia. This view 
was based by Kidston and Lang on the general organisation of Psilophyton, 
but the internal anatomy of the genus is not yet sufficiently known for a 
decisive answer to be given to the question of its exact systematic position. 
Comparisons with other early Devonian impressions are not at present 
possible, and attempts to macerate specimens of Thursophyton and other 
Scottish Old Red Sandstone plants have not so far met with success. 
Summary. 
An investigation of the cuticle of Psilophyton princeps Dawson from the 
original locality i in Canada has revealed the presence of stomata on the epi- 
dermis of the stem. This epidermis resembles in a general way that of the 
petrified members of the Psilophytales from the Rhynie Chert, but the stomata 
differ in the presence of cuticular thickenings and the epidermal cells are 
usually papillate and not ridged. In the numerical distribution of stomata 
on the stem, Psilophyton agrees with Asteroxylon rather than Rhynia. There 
