557 
Knowledge of Rachiopteris cylindrica , Will 
R. cylindrical and B. ramosa and hirsuta as forming a series in which the 
foliar traces show progression from a simple structure in the oldest species 
to a more complex development in the later species ; for, as Bertrand 1 now 
admits, the simple trace of B. antiqua may readily be derived from a basal, 
generalized form—a rounded or oval mass of wood having a single central 
pole—by a slight anterior displacement of the protoxylem. According to 
this view, the foliar traces of the Botryopterideae, Osmundaceae, Psaronieae, 
and Zygopterideae may be referred to a common ancestral type. 2 
The branching of the petiolar bundles in B. antiqua , 3 B. ramosa , and 
B. hirsuta is lateral, and there are indications that it is of the same type in 
R. cylindrica . The secondary traces are usually smaller than the parent 
strand, and their protoxylem is apparently provided by the division of one 
of the petiolar groups. In the case of the triarch petioles of B. ramosa and 
B. hirsuta , the central protoxylem group is not concerned in the production 
of branch traces. 
With regard to stem structure, R. cylindrica is essentially very similar 
to the three British species of Botryopteris. In each case the stem is 
protostelic, the differences depending chiefly upon the varying position of 
the protoxylem groups. It is not, however, possible to trace an evolutionary 
series like that exhibited by the foliar bundles. 
In B. antiqua the position of the protoxylem is variable and indefinite 4 ; 
many scattered peripheral elements occur in the root-bearing zone of the 
stem. These are no doubt comparable with the small elements giving rise 
to roots in the other species. In the leaf-bearing zone, there are only one 
or two mesarch groups ; the same number occurs in B. ramosa , where they 
are, however, placed towards the centre of the xylem strand; B. hirsuta 
apparently presents a similar condition. It has been shown that the 
vascular strand of R. cylindrica may possess a single endarch group, or 
from two to five groups in a mesarch position, the chief distinction between 
this and the other species consisting in the slight and varying differentiation 
of the internal xylem in typical specimens. Whether this differentiation is 
indicative of a higher or lower development of the vascular strand is open 
to much discussion ; its theoretical significance will be discussed in the next 
section. 5 
The similarity in general stem structure in these species extends to 
the pitting of the tracheides, which is of scalariform and reticulate type. 
Scalariform pitting seems to be predominant in B. antiqua , and reticulate 
pitting in B. ramosa and B. hirsuta ; whether there is any significance in 
1 1. c. (12), p. 233. 
2 Bertrand ( 12 ), p. 299. 
Kidston, R., and Gwynne-Vaughan, D. T.: On the Fossil Osmundaceae. Pt. I. Trans. Roy. 
Soc. Edin., vol. 45, Pt. Ill, 1907, p. 759. See pp. 777 and 778. 
Gordon ( 11 ), p. 733. 
3 Benson ( 11 ), p. 1048. 
4 ibid., p. 1046. 
6 P* 5 6i « 
