536 Bancroft—A Contribution to our 
Stems of a type are further characterized by the structure of the cortex, 
which presents three areas, though these are not always distinctly recogniz¬ 
able, owing to the tendency of the outer cortex to become crushed and 
eroded (PI. XXVI, Figs. 1 and 2). 
The cells composing from three to six of the innermost cortical layers 
are somewhat rectangular and tangentially elongated as seen in transverse 
section. They have thick walls, and the layers are often arranged more or 
less concentrically 1 in a manner more characteristic of roots than of stems 
(PL XXVI, Figs. 1 and 2). 
The inner cortex passes gradually into the middle cortex, in which the 
Text-fig. 2. A large a stem, in which the xylem is shown in detail. Note the distinct 
differentiation of the xylem into large outer (xo.), and small inner (pci.) tracheides. There are five 
groups of tracheides ( px .) suggesting protoxylem, situated at the junction of the outer and inner 
wood. The cortical layers (V.) of this stem are much crushed, x 40. (From slide 1552, Williamson 
collection.) 
cells are rather loosely arranged and are more rounded in transverse section 
than those of the preceding area (PI. XXVI, Figs. 1 and 2). They are 
irregular in size, and the thickness of their walls is slightly variable in 
different specimens. 
The outer cortex is composed of from four to twelve layers of fairly 
large thin-walled cells (Text-fig. 3), and is thus markedly distinct from the 
middle cortex, forming a delicate tissue which is usually more or less 
crushed, so that the form of the component cells is unrecognizable (PI. XXVI, 
Figs. 1 and 2). The appearance of this zone suggests that it may have 
formed an assimilatory tissue comparable with that of the stems of 
Psilo turn, or the rachis of Stauropteris . 2 Occasionally the whole of the 
1 Williamson (’ 78 ), p. 350; Hick (’ 96 ), p. 3. 
2 Scott, D. H.: The Sporangia of Stauropteris oldhamia. New Phyt., vol. 4, 1905, p. 114. 
See p. 115. 
Bertrand, P. : Etudes sur la Fronde des Zygopteridees. Lille, 1909. See PI. VII, Fig. 48. 
