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; cf. also fig. 63o in 

 Sadcbeck, 1898). This papilla is the rudiment of the first and central spo- 

 rangium. Neighboring cells elongate 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 in a 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 scalarifonn tracheids (fig. 155). These last constitute morphologic- 

 ally the end of the neighboring venule, which appears to terminate under 

 the sorus just />nw/</thc placenta. The apparent ending is really a branch 

 in the indusium. From the placenta arise a few paraphyses (figs. 169, 

 173) and a 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 dig. 157). Then a transverse wall cuts across the top, leaving 

 a central tetrahedral cell (the primitive archesporium) surrounded by four 

 wall-cells digs. 154, 156). The three lowest and earliest-formed cells 



