Evolution of Branching in the Filicales. 19 
primary construction of these plants” (loc. cit., 1915, p. 47). With 
this condition as a starting point lie has postulated a gradual 
retreat of the branch from its subtending leaf, as well as a stronger 
development of tbe branch, to arrive at the dichotomous type of 
branching now known to be so common in the Zygopterideae. 
While it is agreed that in the ontogeny the axillary buds in the 
Ophioglossacese (and elsewhere) are laid down at the growing point 
of the axis, and are not adventitious, it seems difficult to admit 
that the production, by the growing point, of a branch rudiment 
regularly in connexion with each leal rudiment, was the primitive 
state of affairs. The latter condition naturally suggests itself 
as being a specialized case of that in which there was no such 
regularity ; and I venture to suggest that the origin of this relation, 
which is admittedly very ancient, may perhaps be traced to some 
possible biological advantage accruing from it. One of these was 
probably tbe protection of the young bud during its period of 
dormancy. However, while the protection afforded to the branch 
initial by its axillary position is obvious in such cases as Zygopteris, 
the Hymenophyllaceae, and the Ophioglossaceae, some other 
explanation must be sought for cases where the bud is abaxial 
to the leaf. 
In his well-known recent paper on Zygopteris Grayi Dr. Scott 
dicsussed the morphology of the organ conveniently termed by him 
the “undivided trace.” 1 Of the two views discussed, the one that 
regards the associated leaf as the first leaf of the branch, may be 
dismissed at otice, not only in view of the variation above mentioned, 
but also because the leaf is always orientated with regard to the 
main axis, whether the branch is adaxial or abaxial to the leaf. 
From the form and structure of the undivided trace in the 
Shore specimen of Zygopteris Grayi Dr. Scott has been led to 
decide in favour of the second view, which regards the strand in 
in question as being from its base upwards of a foliar, and not 
axial, nature. However, as he himself suggests, “ we may .... 
regard the axillary stele as ‘ adherent ’ to the adaxial side of the 
leaf-trace ” (loc. cit., p. 59), and this is the position here adopted. 
While it is true, as Dr. Scott remarks, that this way of looking 
upon the subject does not throw any new light on the facts, 
and the difference is ultimately one of words, from a comparative 
• Scott, Annals of Botany, 1912, Vol. XXVI. (i), pp. 57—60. Since the 
branching of Zygopteris Grayi and the Hymenophyllaceae is analogous to that 
in the other Ferns in which the branch is closely related to the leaf, the term 
“undivided trace” may be extended to the corresponding organ in these 
Cases, whether the branch is adaxial or abaxial to the leaf. 
