FILICES. 



375 



borne upon slender pedicels. Morphologically they are trichomes, 

 which undergo a special modification. Each sporangium is at first a 

 two-celled trichome ; the lower cell of which develops into the pedicel, 

 while the other becomes divided by partitions parallel to its surface 

 into outer cells, which develop into the sporangia! wall, and an inner 



Fig. Kl.Aspidium Mlix-ma*. A, a section of a leaf through a sorus ; , . the 

 sporangia, borne upon an elevated mass of tissue, the receptacle ; i, I, the indusium, 

 seen in section. B, a section of a young sporangium, showing its central cell divided 

 into four ; r, one cell of the ring, the section being at right angles to its plane. C, a 

 sporangium nearly mature, seen laterally ; r, r, the ring of the sporangium ; d, a 

 glandular hair in the interior of the sporangium are seen the nearly ripe spores. 

 Magnified. After Sachs. 



tetrahedral cell (the so-called central cell), rich in protoplasm ; from the 

 latter a number of spore mother-cells (twelve, according to Reess) are 

 formed, and from each spore mother-cell four spores arise (Figs. 261 

 and 262). In each sporangium some of the cells of the wall are devel- 

 oped into an elastic ring (annnl)is), which extends part way around the 



