meris terns of Ferns as a Phylogenetic Study . 329 
which segmentation immediately proceeds so as to define a 
two-sided conical apical cell (Fig. 30) ; the position of this is 
such that one of its edges is directed towards the apical cell 
of the axis, the other away from it, and accordingly a 
longitudinal tangential section through the axis would, if it 
traversed a young leaf, show the conical apical cell of the 
young leaf, and the succession of its segments. I succeeded 
in obtaining such a preparation of a leaf in a very early stage 
(Fig. 31), and this shows that the apical cell is defined by the 
first cell-divisions after the convex projection of the young 
leaf beyond the surface of the axis ; Jf a median section of the 
creeping rhizome be cut in a horizontal plane it would traverse 
the apical cell from edge to edge (Figs. 18, 32). When 
viewed from above, the segments are seen to be cut off from 
the apical cell in regular alternate succession (Fig. 30), and 
they are subsequently sub-divided by cutting off their adaxial 
and abaxial edges, so that the central part of each segment 
remains ; this divides chiefly by walls parallel to the segmental 
walls, and the resulting cells are readily recognisable as the 
marginal cells (m, Fig. 30) : it is important to note that this 
marginal series of cells, which plays so prominent a part in the 
ultimate conformation of the leaf in this as in other Ferns, is thus 
to be recognised as being initiated in the very first segments 
cut off from the apical cell ; in other words, the frond is here a 
typically winged structure to its extreme base . It is also 
worthy of note that here, as in most leptosporangiate Ferns, 
the external area of the segments is relatively small as 
compared with the apical cell, while in the higher members 
of the series, if an apical cell be present, the segments appear 
relatively large (see page 335). 
The young frond which thus originates, soon assumes the 
circinate curvature ; it may even be seen in Fig. 30 that the 
growth is less active on the adaxial than on the abaxial side ; 
but the curvature is very clearly seen in longitudinal sections 
(Fig. 32), which also demonstrate that cell-division is more 
active on the convex side, accompanied by occasional forma- 
