30 STRUCTURE AND LIFE-HISTORY OF HAY-SCENTED FERN. 
its exposed face. As segments are cut off from it alternately on the two 
sides, it itself becomes narrower, and finally very slender (figs. 131, 135). 
Each segment divides first on the ventral side by a radial anticline which 
cuts off a narrow cell running from the periphery to the center of the 
leaf-rudiment (figs. 132-134, 140). A similar wall near the dorsal margin 
of the segment meets the first nearly at right angles, close to the center 
of the leaf-rudiment. The two narrow cells may be called sections (Johnson, 
1898) and the walls section-walls. The remaining triangular portion of 
the segment is a primary marginal cell. The segment enlarges and a wall 
parallel to the segment walls (transverse anticline) divides the primary 
marginal cell into two equal secondary marginal cells (figs. 132-134). In 
each of these two new section-walls occur, one ventral, one dorsal. Thus 
there is a regular alternation of section-walls with a halving of the mar- 
ginal cells. At least six or seven section-walls are formed in the region 
which is to become petiole or rachis. The ultimate marginal cell is then 
cut across by a periclinal wall (figs. 142, 143). Its specific activity is ended 
and its outer portion breaks up into epidermal cells. The relation of the 
sections to the inner tissues of the petiole and rachis (vascular tissues, etc.) 
differs in different places, and was not worked out in detail. 
No sooner has the leaf-rudiment become a conical projection on the stem 
than it begins to grow more actively on the dorsal side—to bend over 
toward the stem-initial—to become circinate. The initial cell of the leaf 
lies with one point Cin cross section) towards the stem initial and the ven- 
tral (upper) surface of the leaf, the other point in the dorsal (lower) surface 
of the leaf. 
The circinate vernation comes about through the rapid growth in thick- 
ness of the dorsal sections of the segments. The cells divide about twice 
as rapidly as in the ventral sections, and are larger (figs. 136, 139). A 
little later, divisions occur on the ventral side to about the number of those 
on the dorsal, but the curved position is maintained by greater elongation 
of the dorsal cells. Finally, when the leaf unfolds, the ventral cells elongate 
to equal those of the dorsal surface. 
The activity of the initial cell of the leaf is, as in most ferns, limited. 
After the rudiments of five to eight pairs of pinne are visible and about 
three pairs of segments are cut off in advance of any visible differentiation 
into pinne, the initial ceases to exist as such. Probably it simply begins 
to divide into sections as a segment would do. Fig. 138 shows a leaf-tip 
at this stage, where the initial has divided twice in succession on the same 
side. There is no evidence at all for a “‘transverse’’ division of the 
initial. After the initial is lost the leaf-apex is occupied by a group of 
marginal cells (figs. 137, 144) which grow and section and divide into 
halves for a long time. Since it is probable that each segment of the 
initial (while it lasts) develops a single pinna in the region of the lamina, 
we may say that the lowest eight to eleven pairs spring from as many 
