meris terns of Ferns as a Phylogenetic Study . 347 
Salvinia , are the only examples among the leptosporangiate 
Ferns other than the Hymenophyllaceae, which constantly 
show this mode of segmentation, and it is to be noted that they 
are all aquatic forms. The prevalent type is that in which seg- 
ments are cut off by walls obliquely inclined alternately to the 
upper and lower surfaces of the leaf : this has been described 
by various writers \ but it will not, I think, be superfluous to 
add drawings illustrating the segmentation of the marginal 
cells, since the drawings hitherto published do not adequately 
represent the extreme regularity and beauty of segmentation 
which may be found in certain Ferns. If transverse sections 
be cut through the young pinna of P ter is cretica , and the 
structure of the young wing be examined, the whole of it will 
be seen to be referable in its origin to the marginal cell (Fig. 
55) : it is composed of segments cut off alternately by walls 
inclined to the lower and upper surfaces, the whole presenting 
an appearance such as that of a section through an apex with 
a conical apical cell 1 2 . In leaves which are unbranched the 
regularity of segmentation is less liable to be disturbed than 
in branched leaves : thus, if transverse sections be cut through 
the young leaf of Scolopendrium vidgare , the tissues may not 
only be seen in the young state to be referable in origin to the 
marginal cells (Fig. 56), but even when the wing has attained 
a very considerable size, the derivation of its tissues by seg- 
mentation from the marginal cells is still clear enough, and 
the segments, even to the number of sixteen, may be recog- 
nised in sections suitably cleared (Fig. 57). Referring to the 
segmentation of cells at the margins of flattened members, 
Prof. Sachs says 3 :■ — r A certain similarity of the cell network 
with that of a slender growing point with an apical cell 
impelled the earlier observers to assume in such cases a 
special type. They regarded cells lying beside one another 
at the margin as a series of neighbouring apical cells. This, 
1 Sadebeck, Entw. and Wachsth. des Farnblattes, Berlin, 1874 ; also Schenk’s 
Handb. I. p. 270. Sachs’ Lectures, Engl. ed. p. 459. 
2 Compare Sachs’ Lectures, Fig. 295 B. 
8 Lectures, Engl. ed. p. 459. 
