380 MR, W. N. EDWARDS ON THE 
There does not seem to be any definite arrangement of the cells round the 
stomata, which vary in number from 4 to 7, but their walls are sometimes 
rather fainter than those of the ordinary epidermal cells. In some instances 
these cells appear to be very slightly below the general level of the epidermis, 
as if they formed a very shallow pit containing the stoma, but on the whole 
the thickened ridges of the stomatal apparatus seem to be on the same plane 
as the rest of the cuticle. 
The Emergences—The spine-like emergences which are so characteristic of 
the species have usually disappeared from the flattened part of the stem, 
leaving a slit or oval space in the cuticle (indicating that the spines were 
flattened vertically) around which the radiating epidermal cells tend to be 
rather elongated. Part of one of these spaces from which a spine has 
been torn away is shown at the top of fig. 2 (Pl. 37) and another in text- 
figure 3. A few preparations were made of spines attached to the edge 
Fig, 4. 
P. princeps. Spines; left, shows a stoma close to the base, slide V. 678 a; 
right, slide V. 6784, Brit. Mus. Geol. Dept. 
of the flattened stem (text-fig. 4) showing their tapering form with occasion- 
ally a well-marked longitudinal striation, while especially near the tip they 
were of a much darker colour than the epidermal cells, as if they were highly 
eutieularised. No stomata nor veins were seen on the spines examined, and 
rarely any trace of cell-walls, except near the base. The use of the term 
‘emergence " rather than “leaf”? would therefore seem to be justifiable, and 
even if they are to be regarded as rudimentary leaves—the starting point of 
the leaf of the microphyllous Pteridophytes—it seems clear that in Psilophyton 
princeps they were not concerned with assimilation. Since the tip was so fine 
and pointed, it was difficult to obtain exact measurements, but apparently the 
spines did not exceed 2:5 mm. in length in the material at my disposal, and 
were often shorter. Sections of the spines showed only that they were 
vertically flattened and that they were very thin, | 
