537 
Knowledge of Rachiopteris cylindrica , Will. 
outer cortex appears to be obliterated, and in these cases, the cortex 
as a whole presents a fairly uniform appearance, the rounded cells of the 
middle cortex being the most conspicuous feature. 
In longitudinal section the inner and middle cortical cells are 
elongated, their end walls being straight or oblique ; no specimen has 
been observed which gives a clear idea of the outer cortical cells in 
longitudinal section. 
In some specimens large cavities occur here and there in the outer cor¬ 
tex and particularly where this adjoins the middle cortex (PI. XXVI, Figs, i 
and 5 )* Their general appearance suggests that they may have been 
secretory in function, though no trace of contents has been observed. On 
Text-fig. 3. The outer layers of an a stem, just above a branch, showing the thin-walled cells 
of the outer cortex ( oc.) } and the irregular epidermal cells ( ep .) still uncrushed ; me., middle cortical 
cells, x 400. (From slide K 20 m, University College, London.) 
the other hand, they may merely be due to breaking down of the cells 
before or during petrifaction. 
The epidermal cells are rarely recognizable 1 in stems of a type. In 
some cases, however, at a place of branching, the structure of the epi¬ 
dermis is well shown on the inner or ‘ separation ’ surface of both pro¬ 
ducts of stem division (Text-fig. 4, a and b). The epidermal cells are large 
and rather irregular in shape, as seen in transverse section, while in longitu¬ 
dinal section they appear to be slightly elongated ; frequently they produce 
multicellular hairs (Text-fig. 4, b and c). 
A point of interest is the absence of the thin-walled cortical cells under 
the epidermis, in the region just above the separation of a branch (Text- 
fig. 4, a and b). These seem to develop gradually, becoming intercalated 
between the thicker-walled cells and the epidermis. Text-fig. 3 shows 
1 Hick (’ 96 ), p. 2. 
N n % 
