SPOROPHYTE. 31 



segments. The remaining- 22 to 40 pairs of pinnae come out after the 

 single initial is lost. In either case their actual history is the same. 



The apical growth of the leaf, with or without a single initial, gives 

 rise directly to a slightly flattened and circinately curved rod of embryonic 

 cells (figs. 140, 141, 144). Each margin is occupied by a row of marginal 

 cells (fig. 137). Where a pinna is to develop, about six consecutive 

 marginal cells elongate to form a papilla. By sectioning" and halving they 

 rapidly increase in number of cells and mass of tissue. The apex of the 

 papilla and its manner of growth are exactly like those of the tip of the 

 leaf after the loss of the initial cell (figs. 145-147). On the sides of this 

 protuberance similar outgrowths form the pinnules, and on the pinnules, 

 in a similar manner, the lobes of the pinmile arise, and on these again, in 

 like manner, the ultimate crenations of the leaf -margin (figs. 5, 148, 149). 

 From the inner ends of the oldest sections in each part of the leaf the 

 vasctilar tissues are derived (fig. 149). In every case also there are in- 

 active marginal cells between those groups which grow out to form pinnae, 

 pinnules, lobes, alid teeth. These sluggish cells ultimately give rise to 

 the tissues along the rachis between the pinnae, or along the ribs between 

 the pinnules, or in the notches of the pinnules (fig. 148). In the lamina 

 proper, away from any vein, the sections of the marginal cell are broad 

 and shallow, extending only half the thickness of the leaf (fig. 150). Each 

 of the sections is halved parallel to the surface of the leaf. The outer half 

 is an epidermal cell; the inner half remains single or divides again in the 

 same plane and forms parenchyma (fig. 150). The ultimate marginal 

 cells constitute the margin of the mature leaf. Stomata are formed while 

 the epidermal cells are still polygonal in outline, and while the leaf is un- 

 folding. A curved wall cuts out the stoma mother-cell on one side of a 

 young epidermal cell (figs. 152, 153). The mother-cell becomes oval and 

 is divided longitudinally into the two guard-cells. 



With 20 to 25 pairs of rudimentary pinnae, no stomata, no air-spaces in 

 the parenchyma, and no signs of fructification, the leaves emerge from 

 the soil. It would be, therefore, quite a mistake to suppose that in Denn- 

 stccdtia Punctilobula the unfolding of the leaves "consists merely in an ex- 

 pansion of the leaf with comparatively little cell division' ' (Campbell, 1895, 

 p. 325; 1905, p. 333), in spite of the rapidity with which the unfolding 

 takes place. One-third to one-half of the blade of the leaf must be made 

 outright during this time. In eastern Pennsylvania and Maryland the leaves 

 appear above ground in the latter half of April (Cockeysville, Maryland, 

 April 21; Oxford Valley, Pennsylvania., April 29, 1905). By June 4 (Loch 

 Raven, Maryland, 1905) spores are nearly mature. At first the petioles, 

 green in all the aerial part and clothed with white hairs, elongate and unroll. 

 Then the leaf spreads out from below upward . A comparison of figs . 2 and 5 

 of the mature leaf and fig. 152 of a pinnule 3 mm. long from an unrolling 

 leaf will give an idea of the change that goes on. In fig. 152 the stomata 



