1907] BINFORD—SPORANGIUM OF LYGODIUM 221 



the under side it is only one cell wide. In this species the length of 

 the annulus cells varies somewhat, so that the line which is the border 

 of the ring is rather irregular. 



The number of wall cells included within the annulus at the apical 

 end is twelve to seventeen. 



The sporogenous mass. — The first division wall of the arche- 

 sporium is usually perpendicular to the longest axis of the central 

 cell {-jig, ji), w^hich is at first perpendicular to the leaf surface. Other 

 divisions are quite irregular, but some appear curved and parallel 

 with the first walls of the initial sporangial cell (fig. 28). The out- 

 line of the sporogenous mass becomes very irregular {jigs. 27, 28^ 2g), 

 and the tapetal cells thus dip down into it; this makes possible a 

 better nourishment of the sporogenous mass. 



The sporogenous mass is rather large and made up of large cells; 

 I have counted as many as seventy-two mother cells, but sixty-four 

 seems to be the usual number. The number of spores per sporangium 

 is 242 to 258 by actual count. Some of the mother cells become 

 sterilized and function as nutritive cells {fig. 34). The mother cells 

 and newly formed tetrads occupy but a small portion of the cavity of 

 the sporangium {fig. jo). After the spores are formed there is a great 

 increase in their size, so that they finally fill the large sporangium. 



^ + 



The average diameter of the spore mother cell, newly formed tetrad, 

 and mature spore are respectively 0.025"^^, 0.028"^"^, and 0.075 

 Occasionally a dwarf spore is found. 



Sterile sporangia. — The growth of the spores is apparently a 

 great drain upon the pinnae, so great that the young sporangia at the 

 apex are drawn upon for nutriment, and the sporogenous mass in 

 these sporangia is arrested in its development. They contain no cells 

 which take the stain in the manner characteristic of sporogenous 

 tissue; but the cytoplasm takes on a very loose fibrillar structure and 

 there is little material in the cell that stains at all. The outer layer 

 of the tapetum, which usually remains a thin layer of tabular cells, 

 takes on a very great development, the cells bulging into the cavity of 

 the sporangium like great haustoria; figs. 35, j6, and j/ show various 

 stages in this development. The cells of the wall at this time become 

 filled with food material and stain densely. 



Relationships. — In the evolutionary line from Marattiaceae to 



mm 



