340 Bower . — The comparative examination of the 
from a comparison of the apices of the root and the stem in 
the same series of plants. 
The Development of the Wings of the Leaf. 
The leaves of the large majority of Ferns are typically 
winged structures throughout their length : the wings are, 
however, variable in their devolopment ; in the upper part of 
the leaf they are usually large and expanded, whereas in 
the lower part, they may be reduced in size, and only be 
recognised in the mature state as giving a slightly angular 
form to the transverse section, or merely as light-coloured 
and very slightly projecting longitudinal ridges : or they may 
widen out at the extreme base forming an almost sheath-like, 
or even stipular development. Though the mode of origin of 
the wings is known in some few forms, as far as I am aware 
their morphology and development have never been the 
subject of comparative study in the whole series of the 
Ferns, notwithstanding that they are so constant and marked 
a feature. 
As with the stems and leaves of other vascular plants, so 
the leaves of the Ferns may develop either (i) in a simple 
cylindrical manner: this is very rare, but is seen in Pilularia\ 
(2) as flattened, spathulate structures, without prominent 
wings or midrib, as in Drymoglossum ; or (3) with an enlarged 
midrib, and thin, longitudinally-running wings on either side : 
this is the type for the very large majority of the Ferns. 
These wings may not only be traced down the phyllopodium, 
but may sometimes even be seen to be continuous to the 
stem itself: this is, however, the case only in Ferns with an 
elongated, creeping stem, as in Pteris aquilina , where they 
may be recognised as continuous light-brown lines, on either 
side of the rhizome, or in species of Acrostichum (A. scandens , 
Smith, and A. tenuifolium , Baker), in which case however they 
appear as discontinuous white streaks : with this may be 
compared such examples of winged stems among the Angio- 
sperms as Symphytum , or Vitis pterophora . 
