Knowledge of Rachiopteris cylindrica , Will. 535 
the groups being arranged round the centre, sometimes at considerable 
distances from it. 1 The stele is surrounded by a cortex, usually showing 
inner, middle, and outer regions (PL XXVI, Figs. 1 and 2) ; the epidermal 
layer, when not crushed, is seen to bear numerous unicellular, or typically 
multicellular hairs, often with somewhat rounded terminal cells (Text-fig. 4). 
This general description applies to all stems referred to R. cylindrica ; 
the stems, however, fall into two distinct groups, representative types of 
which will be described respectively as ‘ a * and c /3 ’. A discussion as 
to the significance of these two types will follow in Section V. 2 
ii. a type . 3 —The stems belonging to this group are characterized by 
the possession of large xylem strands, the average diameter of which is 
0*7 mm. There are typically several (two, or, more commonly, from three to 
five) protoxylem groups arranged round the centre (PI. XXVI, Fig. 1); the 
centrarch condition with one protoxylem group occurs (PI. XXVI, Fig. 2), 
but it is comparatively rare, and results from the fusion of two or more 
groups (Text-fig. 1). The normal condition in stems of a type thus tends 
towards mesarchy, for the occurrence of more than two protoxylem groups 
is quite independent of branching, 4 as an examination of serial sections 
demonstrates (Text-fig. 7). 
The great interest of this type lies in the fact that the mesarch 
condition is accompanied in many cases by a certain amount of differentia¬ 
tion of the xylem 5 into an outer zone in which the lumen of the tracheides 
is of ordinary size (100 /x in diameter), and an inner solid core, in which the 
tracheides are much narrower (Text-fig. 2 ; PL XXVI, Fig. 1). There are no 
parenchymatous cells interspersed among the tracheides. 6 7 The differentia¬ 
tion of the wood is particularly marked in the larger stems (Text-fig. 2), 
transverse sections of which recall the condition seen in Diplolabis RomeriJ 
The protoxylem groups are situated at the junction of the wide and narrow 
elements (Text-fig. 2), as in Diplolabis. The size of the core of small ele¬ 
ments is very variable (cf. PL XXVI, Fig. 1, and Text-fig. 2); it is dependent 
on the size of the xylem strand as a whole, on the number of protoxylem 
groups present, and on their distance from the centre of the strand. 
1 With regard to this point, cf. the statements of Williamson (’ 78 ), p. 350; Hick (’ 96 ), p. 8 ; 
Tansley (’ 08 ), p. 15 ; Scott (’ 08 ), p. 333 ; Brown (’ 08 ), p. 57 ; Seward (’ 10 ), p. 438. The general 
view is that R. cylindrica is typically endarch ; Hick noted the frequent presence of four or five 
protoxylem groups arranged symmetrically round the centre of the stele, but Lady Isabel Browne was 
the first author to comment upon this tendency towards mesarchy. 
2 p. 553 - 
3 Williamson based his original description upon this type : ( 78 ), p. 350, PI. 24, Figs. 80 
and 87. 4 Cf. Browne (’ 08 ), p. 57. 
6 Gordon, W. T. : On the Relation between the Fossil Osmundaceae and the Zygopterideae. 
Proc. Cambridge Phil. Soc., vol. 15, Pt. V, 1910, p. 398. See p. 400. 
3 Hick (’ 96 ), p. 9. 
7 Gordon, W. T. : On the Structure and Affinities of Diplolabis Romeri (Solms). Trans. 
Roy. Soc. Edin., vol. 47, Pt. IV, 1911, p. 711. See p. 719. 
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