244 Campbell. — Contributions to the 
number of satisfactory preparations to determine positively, or 
for how long this regularity of growth persists. 
Up to this time the embryo has increased but little in size, 
and has the form of a globular or oval cellular mass in which 
the organs are not differentiated. In proportion as the basal- 
wall diverges more or less from, the horizontal, so does the 
axis of the first leaf which is parallel with it. Occasionally 
the basal-wall is so nearly vertical that the young leaf grows 
upright, penetrating the neck of the archegonium at right- 
angles to its usual course. The embryo now rapidly elongates 
in the direction of the basal-wall, and very soon there may be 
distinguished in the upper portion a division into leaf and root 
(Figs. 27, 28). The original quadrant-wall remains distinct 
for some time, and is especially evident in transverse sections 
(Fig. 29, II II). At the base of the leaf, at this stage, a single 
cell (Fig. 27, x), larger than its neighbours, may sometimes be 
seen. This is the mother-cell of the ligule, which is so con- 
spicuous a feature of all the leaves. 
If we compare the young embryo at this stage with that of 
other pteridophytes, we find that it does not agree exactly 
with any, but on the whole resembles that of the various 
Filicineae that have been examined. The greatest difference, 
apart from the absence of a single apical-cell in the young 
members, is the different position of the stem. This in all the 
true ferns examined arises from one of the lower quadrants, 
which here takes part in the formation of the foot, while the 
stem-apex in Isoetes , as already stated, arises between the leaf 
and root that is from one of the upper quadrants. 
Longitudinal sections of the embryo at a time when root 
and leaf become first clearly recognisable, show that the foot 
is not clearly limited (Fig. 28,/), as the basal-wall of the 
embryo early becomes indistinguishable on account of the 
displacement due to the rapid cell-multiplication in the axis 
of the embryo ; nor is it as conspicuous as in the later stages 
(Figs. 39-41), projecting but little, and not having its cells 
noticeably larger than those of the root and leaf. These now 
(Figs. 28, 30) have the form of a blunt cone, the latter some- 
