548 Bancroft.—A Contribution to our 
strand, and is similar to that of ft stems. The outer layers are indefinite in 
structure. 
iv. Histology .—In both a and ft petioles the details of cell structure 
follow very closely those described for the corresponding stems. No 
authentic longitudinal sections have been observed, but oblique sections 
show the scalariform and reticulate pitting of the tracheidal walls (see also 
a transverse section, PL XXVII, Fig. 1 , pt). The protoxylem elements, as 
in the stems, are apparently scalariform. In the case of ft petioles the meta- 
xylem elements are frequently imperfectly lignified (PI. XXVII, Fig. 2). 
In some specimens the xylem strand does not occupy the centre of the 
stele, but is situated abaxially; it is probably this fact which caused Hick 1 
to doubt whether the phloem completely surrounds the xylem. In such 
cases it is usually impossible to determine the structure of the external 
stelar tissues, on account of the crushing which they have undergone. In 
favourably preserved examples of a petioles, however, the phloem appears 
to be continuous round the xylem, and to consist partly of narrow cells 
(PI. XXVII, Fig. 1) and partly of larger cells suggestive of sieve-tubes. In 
ft petioles the xylem almost completely fills the area enclosed by the rather 
ill-defined endodermis (PI. XXVI, Fig. 7), and the nature of the small cells 
which immediately surround it cannot be determined. A pericycle may 
sometimes be recognized in a petioles (PI. XXVII, Fig. 1); and the endo¬ 
dermis (PI. XXVI, Fig. 7 ; PI. XXVII, Fig. 1) of both types possesses the 
same characters as that of the stems, although to a less marked degree. The 
contents of the petiolar cortical cells also are similar to those of the stems. 
v. Branching .—In several cases petioles of R. cylindrica type, asso¬ 
ciated with stems of R. cylindrical have been observed to contain small 
lateral traces ; these are apparently produced by the division of a protoxylem 
group of the original petiolar bundle. 
3. Roots. 
i. Development .—Associated with the stems of R. cylindrica are 
numerous small roots, diarch and typically fern-like ; similar structures 
may also be observed in various stages, arising endogenously from both a 
and ft stems 2 (PI. XXVII, Fig. 4 ; PI. XXVI, Fig. 8). They are scattered 
at fairly infrequent intervals, typically occurring singly in association with 
a petiole or branch 3 (PI. XXVI, Figs. 3 and 8; Text-fig. 9); at times, 
however, two roots, at different stages, may be seen in the same transverse 
section of a stem (PI. XXVII, Fig. 4). 
1 1. C., p. II. 
2 Cf. Williamson (’ 78 ), PI. 24, Fig. 87; Hick (’ 96 ), p. 11, and PI. I, Fig. 2 (from the same 
specimen in Q 105, Cash collect., as PI. XXVI, Fig. 8, of the present account). 
3 Cf. Lachmann, J. P. : Contributions a l’histoire naturelle de la racine des Fougeres. Ann. 
Soc. Bot. Lyon, Ser. A, No. 116, 1889. See p. 169. The distribution of the roots in R. cylindrica, 
is not, according to Lachmann’s conclusions, of very primitive type, 
