CUTICULAR STRUCTURE OF PSILOPHYTON. 383 
mechanical nature.” This is in agreement with the conclusions arrived at 
in the present paper. 
Comparisons with Rhynia and Asteroxylon. 
| The exact relationship of Psilophyton to the primitive Pteridophytes of the 
Rhynie Chert has been much discussed since Kidston and Lang instituted the 
class Psilophytales for their reception, taking the name from Dawson > 
genus. Kidston and Lang (1917) at first regarded the affinity of Rhynia 
with Psilophyton as fairly close, and Arber (1921) went so far as to state 
that the two genera were identical. In their later work Kidston and Lang 
(1920, p. 669) consider that Psilophyton is more closely comparable with 
Asteroxylon, and they include it in the family Asteroxylaceæ. They do not 
suggest generic identity, and indeed among known impressions there is one 
which vegetatively resembles Asterowylon much more closely, namely 
Thursophyton. The other Rhynie genus, Hornea, need not detain us, since 
its epidermis is not very well known and it had no emergences of any kind 
and apparently no stomata. 
The epidermal cells of Rhynia and Asteroaylon agree on the whole in sh: ape 
and dimensions, and those of Psilophyton are very similar, but often more 
elongated than in these two genera. The dark median ridge of the cells of 
Rhynia Gwynne- Vaughani (Kidston and Lang 1917, pl. 6. fig. 31) which is 
sometimes also seen in Asterovylon is usually represented in Psilophyton by 
around or only slightly elongated papilla, but there is occasionally a line 
running nearly the whole length of the cell. In the case of Rhynia major 
there are no ridges on the epidermal cells. 
The stomata of Rhynia are similar to those of Asteroaylon and are of 
ordinary form consisting of two crescentic guard-cells apparently without 
any special thickening, whereas in Psilophyton the cuticular thickenings 
suggest a more specialised type of stoma. In the latter genus the stomatal 
apparatus seems to be narrower and often longer than in the Rhynie plants, 
but this might be due to differences in the state of the stoma at the time of 
fossilisation. On the whole the stomata are more numerous than in Æhynia, 
and in this respect Psilophyton agrees better with Asterowylon, of which a 
piece of cuticle with about a dozen stomata is shown in Pl. 37. fig. 5 for 
comparison. A few stomata are rather more highly magnified in fig. 4, but 
though they do not closely resemble those of /’silophyton, it must be remem- 
bered that the method of preservation is very different. 
The arrangements of cells described above as hair-bases have not been 
seen in either Æhynia or Asteroxylon. There remain the emergences to be 
considered. The hemispherical protuberances of /hynia Gwynne- Vaughan: 
have been shown by Kidston and Lang to be usually if not always formed 
underneath a stoma, and it is highly probable that they were a traumatic 
response to the peculiar conditions under which the plant lived, They may 
