608 
MR. F. 0. BOWER ON THE COMPARATIVE MORPHOLOGY OF THE 
each pinna does not correspond to one segment of the two-sided apical cell, still the 
pinnae have a definite relation to the apical cell and its segments, each longitudinal 
row of pinnae originating from one of the two series of segments cut off from the two- 
sided apical cell. In the Osmundacece, since there are but two series of pinnae as 
before, but three series of segments of the apical cell, the relations of the former to 
the latter cannot be the same. In this group the two series of pinnae correspond in 
position to two of the angles of the three-sided cell, and the individual pinnae are 
derived partly from tissues originating from the ventral series of segments, partly from 
the dorsal. 
Again, comparing the members of our series, there is to be traced a progressive 
increase in bulk of the individual pinna. In Ceratopteris the young pinna is a thin 
flattened structure : in many Ferns, as Aspidium Filix-Mas, Polypodium vulgar e , &c., 
the first formed pinnae are more bulky, but still a marginal series of cells may easilv 
be seen upon them : in Osmunda not only are the young pinnae more rounded, but 
also no marginal series of cells are to be found on those first formed, while they remain 
young. In Angiopteris and the Cy cad ace a 1 the pinnae appear as hemispherical papillae 
of tissue, on which no marginal series of cells are to be found. In all these plants the 
pinnae are formed on the more or less developed wings of the pliyllopodium. There 
is thus a progressive increase in bulk of the pinna in its first stages, which may be 
traced on passing upwards through our series. 
Parallel with the increase in bulk of the pinnae there is also an increase in bulk 
of the wings of the pliyllopodium, pinnae, and pinnules, &c., in those cases where a 
winged development takes place. Thus in the Ilymenophyllacece the wings consist 
for the most part of but a single layer of cells, though in some species they consist of 
more than one (Prantl, Hymenophyllaceen, p. 23), still the structure is in all cases 
very simple. In the majority of Ferns a marginal series of cells can be clearly 
recognised on the young pinnae or pinnules, &c., which give rise by their growth and 
divisions to a wing-structure consisting of but few layers of cells : in Angiopteris no 
such marginal series is apparent, and the whole structure of the wing is more bulky 
and complicated than in the Leptosporangiate Ferns : repeated periclinal divisions 
are found in the superficial cells of the wing during development. In Cycas the 
wings arise to external appearance in a manner not unlike those of Angiopteris, but 
there is between the two this important difference : that the periclinal divisions in the 
superficial cells are entirely absent in Cycas. Thus again in the complexity of the 
structure of the wing a progressive advance is seen on passing upwards through our 
series of large-leaved plants, an advance from a simple structure to that more compli¬ 
cated structure which is characteristic of the higher plants.* 
In the above paragraphs a number of characters have been brought forward, showing 
an almost uniform progress of complexity and differentiation of the vegetative organs, 
* The case of tile genus Todea should be mentioned as exceptional, while Todea barbara has wings 
consisting of about nine layers of cells, Todea swperba has wings with only one or two layers, it thus 
