370 Sakni.—On the Branching of the Zygopteridean Leaf. 
two strands supplying these axes, the latter cannot, I think, be regarded as 
morphologically equivalent to secondary raches (‘ pinnae ’). There appears 
to be no alternative but to regard them as tertiary raches (‘ pinnules ’), and 
the large strand (‘ piece sortante ’) still enclosed in the cortex of the primary 
axis as the strand of a secondary rachis (‘ pinna ’) which has Hot become free. 
See Fig. i. 
Stress has been laid on this apparently unimportant point, for the reason 
that it has been responsible for a misunderstanding on one point of some 
theoretical importance : the mode of branching of the frond in some Zygo- 
pterideae has been considered 
to be unique among vascular 
plants: fronds have been described 
as bearing four series of pinnae, 
two on each side of the primary 
rachis. In some inexplicable way, 
the morphological significance of 
the embedded pinna-trace has 
hitherto escaped the notice of most 
authors, not only in Stauropteris , 
but also in Dineuron, Etapteris , 
Metaclepsydropsis , and Diplolabis. 
In the two last-named genera 
Dr. Gordon ( 5 ) calls the em¬ 
bedded trace the ‘ pinna-trace-bar’ 
(p. 716, foot-note) and describes 
it as subsequently dividing into 
two { pinna-traces’. In Etapteris 
also, where the homologue of the 
embedded trace is formed by the fusion of two originally separate pieces, 
neither Bertrand nor Kidston and Gwynne-Vaughan ( 9 ) have given it the 
interpretation which appears to be the only admissible one. Dr. Kidston, 
however, in 1908 ( 7 ) correctly regards this strand in Dineuron ellipticum , 
Kidst., and Metaclepsydropsis duplex as a pinna-trace. But the description 
by Kidston and Gwynne-Vaughan ( 1910 ) does not differ in this respect 
from Bertrand’s interpretation. 
The condition in Zygopteris primaria , Cotta, unfortunately remains 
obscure. According to Bertrand ( 1909 , p. 137) it is not possible to say 
whether the pinna-trace is formed as in Etapteris by the fusion of two 
originally distinct pieces, or whether it comes off as a single arc and is thus 
more closely comparable to the condition in Diplolabis , Metaclepsydropsis , 
and Dineuron . l 
The misconception, now for the first time cleared up, apparently dates 
1 The latter would appear to be more probably the case, for Z. primaria and Diplolabis have 
