602 
MR. F. 0. BOWER ON THE COMPARATIVE MORPHOLOGY OF THE 
those plants which are universally accepted to be higher in the scale. And first it 
will be well to consider those of the vascular plants which, in the characters of the 
vegetative organs, as also in other respects, appear to constitute a natural series, viz. : 
(1) the Leptosporangiate Ferns, exclusive of the Osmundacece ; (2) the Osmundacece; 
(3) the Marattiacece; (4) the Cyccidacece. These may, for convenience, be called the 
largq-leaved vascular Cryptogams and Gymnosperms. 
In the Hymenophyllacece, in which group of Ferns the conformation of the leaf is 
simplest,* the phyllopodium is not clearly differentiated from the appendicular mem¬ 
bers of higher order. It arises at first as a flattened organ, referable in its external 
conformation (according to Prantl’s figure), as well as in the arrangement of the cells 
at its apex, to a single plane; by increase in bulk of its central part, below the 
extreme apex, and by continued growth at the margins, it becomes a typical winged 
structure. Prantl (l.c., p. 59) regards it as probable that in the simplest forms 
there was “an entirely unbranched leaf, traversed only by a midrib, a form which 
probably really exists in the simplest species of Hymenophyllum .” I think, however, 
judging from the rather incomplete data given by Prantl, that it is more probable 
that the leaf was originally a flattened expansion without a midrib, and that the 
appearance of the median thickening was of subsequent occurrence. This is, however, 
pure theory, and, in the absence of intermediate forms between the Ferns and the 
Muscinese, it is incapable of decision. 
The branching of the phyllopodium in the Hymenophyllacece is chiefly, if not 
exclusively, dichotomous. On advancing from the simpler to the more complex forms, a 
transition may be traced from the typically dichotomous to the sympodial develop¬ 
ment (Prantl, he., p. 58), and this is accompanied by an increasing prominence of the 
phyllopodium, which is thus a pseudo-axis. Though prominent to the eye, the 
phyllopodium is not in these cases clearly differentiated in the first instance from the 
less strongly developed branches of the dichotomy : it is often winged like them to 
its extreme base, while it shows no sheathing development, nor other peculiarity of 
conformation at its base. 
Passing on to the other Leptosporangiate Ferns (exclusive of the Osmundacece ), 
though the apex of the phyllopodium is often more bulky than in the Hymeno- 
phyllacece, it still retains the two-sided apical cell, so characteristic of flattened organs, 
and thus in the arrangement of its meristem is referable to one plane. When mature, 
it is typically a winged structure throughout its length, and though in Ferns with 
much branched leaves the wings are often but slightly developed in the lower parts of 
the phyllopodium, in Ferns which have more simple leaves the winged development 
may be readily followed to the point of insertion on the axis. As regards the 
* The question may for tlie present he left open as to the real relation of the Hymenophyllaceffi to 
other Leptosporangiate Ferns, i.e., whether they are rudimentary forms or reduced representatives of a 
higher development. It is sufficient for the present that the leaf as there represented is structurally the 
simplest among the large-leaved forms. 
