32 STRUCTURE AND LIFE-HISTORY OF HAY-SCENTED FERN. 
are just developing. The pinna from which this was taken was 1.5 cm. 
long, and unrolling. 
While it is unfolding (May 4, 1906, Baltimore, Maryland), the fertile leaf 
acquires its sori. In origin the sori are strictly marginal. At the point 
where a sorus is to develop, a marginal cell of the lamina, at the tip of a 
rudimentary veinlet, after giving rise to a mass of vein and lamina cells, 
grows out into a short, rounded papilla (figs. 151, 158; of. also fig. 63p in 
Sadebeck, 1898). This papilla is the rudiment of the first and central spo- 
rangium. Neighboring cells clongate to form a mound, the placenta. New 
sporangia at once begin to develop around the first one. Meanwhile, about 
four or five cells removed from the central marginal cell, the leaf-tissues 
begin to rise up ina ring (indusium) around the placenta (fig. 158). The 
ventral (upper) part of this ring soon becomes much thicker than the 
opposite side—as thick, indeed, as the lamina itself. Vascular tissues, also, 
are formed for a short distance into this lobe (fig. 155). 
As growth proceeds, the sorus reaches its ultimate position on the under 
side of the leaf. One is found on the lower outer venule of each lobe of 
each pinnule (fig. 5). At maturity the indusium is circular and cup- 
shaped. One side of it is continuous with the margin of the leaf (fig. 155); 
elsewhere it rises abruptly from the surface. In its lower parts it con- 
sists of inner and outer epidermis, with a few parenchyma cells and air- 
spaces between. Here stomata occur both within (fig. 120) and without 
(fig. 119). The epidermal cells are wavy-margined (fig. 115). The 
indusium tapers above to two cells, then to one cell in thickness. On its 
sides and margin it bears hairs, both glandular and acicular. The margin 
is irregular. In the bottom of the indusium cup and on the side nearest 
to the base of the pinnule is a low, rounded placenta. It is covered with 
epidermis, beneath which is a layer of parenchyma, and then a group of 
short scalariform tracheids (fig. 155). These last constitute morphologic- 
ally the end of the neighboring venule, which afpears to terminate under 
the sorus just devond the placenta. The apparent ending is really a branch 
in the indusium. From the placenta arise a few paraphyses (figs. 169, 
173) anda number of sporangia. The paraphyses are obtuse hairs, com- 
posed of about three cylindrical cells each. 
The sporangia arise in centrifugal succession from superficial cells of 
the placenta. The mother-cell bulges out considerably and is cut by an 
oblique wall from the middle of its base to one side of its summit (fig. 
159). A second oblique wall strikes across from one side of the summit 
to the first wall (fig. 160). A third wall, striking both of the preceding, 
leaves an upper cell with a spherical outer surface and a triangular pyra- 
midal base. Three more divisions follow in the upper cell, parallel to the 
first three (fig. 157). Then a transverse wall cuts across the top, leaving 
a central tetrahedral cell (the primitive archesporium) surrounded by four 
wall-cells (figs. 154, 156). The three lowest and earliest-formed cells 
