Knowledge of Rctchiopteris cylindrica , Will. 
i. The Primitiveness of the Stelar Condition. 
R. cylindrica has been described by Tansley 1 as possessing an endarch 
protostele, and as being therefore primitive in this respect. But it has been 
shown that the a forms, at least, are not in the majority of cases typically 
endarch ; they rather tend towards mesarchy, 2 and even, in some specimens, 
towards a differentiation of the xylem into inner and outer zones. 
On the assumption of Tansley’s theory—and there is much in favour of 
the primitiveness of endarchy amongst the ancestors of vascular plants 
as a whole 3 -—the * mesarch ’ a stems are to be considered as having made 
a slight advance upon the primitive condition, particularly in those cases 
where there is differentiation of the internal wood. 4 True endarchy is met 
with in some branches of a stems and in the /3 stems ; its occurrence in 
R. cylindrica^ however, cannot be considered as having any bearing upon 
the primitiveness of the vascular structure ; for in a stem branches it repre¬ 
sents a derived and reduced condition, due probably to decrease of vigour, 
and in j 3 types, if the argument of concentration under the influence of 
a water habitat be tenable, the same condition is again due to reduction. 
From a study of the Fossil Osmundaceae, Kidston and Gwynne- 
Vaughan 5 conclude that the ancestral form of this group must have 
possessed an exarch protostele; and Lady Isabel Browne 6 is of the opinion 
that exarchy represents the primitive condition of the Pteridophyta as 
a whole. Bertrand, 7 however, considers that if the protostele is to be 
regarded as the original stelar type, its protoxylem groups were most likely 
slightly immersed. 
According to any of the above views, it is evident that typical specimens 
of Rachiopteris cylindrica —apart from the cases where endarchy occurs as 
a state of reduction—show some divergence from the ancestral structure. 
A consideration of Bertrand’s theory of the ‘ etoile libero-ligneuse \ 8 or 
£ asterostele’, suggests that such forms as the mesarch a stems of R. cylin¬ 
drica may be more primitive than truly endarch forms, since they may 
represent a stage in the condensation of an ancestral rayed structure like 
that of Cladoxylon ; the endarch condition itself, according to Bertrand, 
denotes a very advanced state of condensation. It has certainly been 
shown that, in a stem branches, endarchy results from the condensation of 
a dispersed condition of the protoxylem groups; as mentioned above, this 
may be due to a decrease of vigour. In (3 stems, endarchy is considered to 
be due to the action of environment, causing concentration of the vascular 
tissues. Since, in these cases, the occurrence of endarchy may be explained 
on physiological grounds, it can hardly be accorded any phylogenetic 
1 1 . c., pp. 14 and 15. 2 Browne (’ 08 ), p. 57. 3 Tansley (’ 08 ), p. 15. 
4 Gordon (TO), p. 400. 5 1 . c., p. 777. 6 1 . c., p. 58. 
7 1 . c., (T2), p. 263. 8 Bertrand (T2), pp. 249-61. 
