348 Bower . — The comparative examination of the 
from the purely formal point of view, is of course possible, but 
it contributes nothing further to the explanation of the pro- 
cesses of growth.’ With regard to the wings of the leaves of 
Ferns, I cannot agree with this statement. If we accept the 
view that an apical or initial cell represents merely a break in 
the constructive system of the growing point, then the mar- 
ginal cell (m) in Figs. 55j 5^, 57 corresponds in this respect ex- 
actly to an initial cell of stem, leaf, or root, and just as from the 
latter (which maybe one or more in number) by segmentation 
all the fresh tissues of the leaves of the member are derived, 
so also is all the fresh tissue of the wing derived ultimately 
by segmentation from the marginal cells : just as in the apices 
of stems, leaves, or roots, by tracing the origin and subdivisions 
of the segments we gain a knowledge of the manner of 
localisation of growth, so also may this be done in the wings. 
In fact, excepting the difference of form, number, and posi- 
tion, I fail to see that the initial cells of the apex of the stem, 
leaf, or root can with any reason be regarded in a different 
light from those initial or marginal cells which are found on 
the growing margin of the leaves of Ferns. 
It is thus to be seen, both from the observations of others, 
and also from such examples as those above described, that in 
the leptosporangiate Ferns (exclusive of the Hymenophyl- 
laceae) the type of segmentation of the marginal cells is more 
complicated than that of the Hymenophyllaceae ; from each 
of the marginal cells two alternating rows of segments are cut 
off by oblique walls, while in the Hymenophyllaceae only one 
row of segments is derived from each marginal cell. As shown 
by a comparison of Sachs’ diagrams 1 this latter type is asso- 
ciated with a deeper position of the centre of construction, and 
is thus characteristic of a more robust type of structure. 
But among these Ferns the ‘ filmy 5 type of structure is not 
unknown, and I have had the opportunity of investigating the 
development of the leaf of Asplenmm resectum in specimens 
from the Kew collection 2 . The leaf of this Fern is thin and 
1 Sachs’ Lectures, Engl. ed. p. 457, Figs. 295 A and B. 
2 This Fern was brought under my notice by Mr. Staden,of the Royal Gardens, Kew. 
