Knowledge of Rachiopteris cylindrica , Will. 539 
consisting of several cell-layers in which the elements are small and very 
loosely arranged (PL XXVI, Figs. 3 and 8). In some specimens this band 
of cells has become crushed (PL XXVI, Fig. 9), in others it has given rise to 
radial lacunae (PL XXVI, Fig. 4), the presence of which was noted and 
figured by Hick. 1 Beyond the lacunae, the cells of the middle cortex are 
larger, and tend to become somewhat radially elongated (PL XXVI, 
Figs. 3 and 7); the outermost cells of this area have slightly thickened 
walls (PL XXVI, Figs. 3, 4, and 7). 
The outer cortical cells are apparently of the same type as those of 
a stems (PL XXVI, Fig. 8), although they are usually much more crushed. 
Occasionally the epidermis, with numerous hairs, may be observed, but 
in these cases the thin-walled cortex is not developed. 
In longitudinal section the thicker-walled cortical cells are elongated ; 
the thin-walled elements of the middle cortex are shorter, having their 
end walls more or less horizontal. 
iv. Histology. — The unusually favourable preservation of R. cylindrica 
permits a fuller investigation of its histology than is possible in many 
cases. 
The details of cell structure in a and (3 stems are similar. The long, 
pointed tracheides are pitted on all surfaces; and while the narrower 
walls present a typically scalariform appearance, the broader walls are 
reticulately pitted. The arrangement and extent of the pits may occa¬ 
sionally be observed in transverse section. 2 
The smaller tracheides, considered as representing protoxylem ele¬ 
ments, are usually pitted in a scalariform manner, 3 instead of showing 
the typical annular or spiral thickening ; this fact suggests that the growth 
of the stems was not rapid. 4 
There are a few scattered cells of xylem parenchyma, often with 
their contents preserved, and a continuous zone of phloem surrounds the 
xylem strand (Text-fig. 5). The phloem typically consists of a layer of 
large sieve-tubes accompanied on either side by narrow cells of phloem 
parenchyma. The sieve-tubes usually form a single series, and are as 
distinct in transverse section (Text-fig. 5, st. ; PL XXVII, Fig. 4) as the 
corresponding elements in a living Fern, .such a s Pteridinm or Marsilia? 
It has been impossible to obtain a clear idea of the phloem in longitudinal 
section, and a careful search has failed to reveal any trace of sieve-plates. 
1 l.c., PI. I, Fig. 1 represents the stem in Slide Q 104, Cash collection; PI. XXVI, Fig. 4 of 
the present account is a photograph of the stem in Slide Q 103, cut from the same block as Q 104. 
2 PI. XXVII, Fig. 1 shows the pits in transverse section in the case of a petiole. 
3 Cf. Hick (’ 96 ), p. 8 ; the suggestion of spiral elements is very doubtful. Tansley (’ 08 ), 
pp. 14 and 15. 
4 Tansley (’ 08 ), p. 12. Scott (’ 08 ), p. 329. 
5 Hume, E. M. M. : The Histology of the Sieve-tubes of Pteridium aquilinum , with some notes 
on Marsilia quadrifolia and Lygodium dichotomum. Ann. of Bot., vol. 26, 1912, p. 573. See 
pp. 576 and 577, and PI. LV, Figs. 31 and 32. 
