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 (fit^s. 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 secfion-a'a/ls. 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 (in 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 pinnae are visible and about 

 three pairs of segments are cut off in advance of any visible differentiation 

 into pinna\ 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 



