PTERIDOPHYTA 



315 



their development they agree closely with Lycopodium, and, as in 

 that genus, the tapetum is the innermost of the three layers of cells 

 forming the wall of the sporangium. The tapetal cells remain 

 intact, and form an epithelial layer in contact with the developing 

 spores, to which they doubtless furnish food in a manner analogous 

 to that found in the corresponding cells of the ovule in the Seed- 

 plants. 



Up to the separation of the individual sporogenous cells, micro- 

 sporangium and macrosporangium develop alike ; but while all the 

 sporogenous cells in the microsporangium produce tetrads of spores, 

 in the macrosporangium 



this is true only of one of A ^ B 



them, the others remaining 

 undivided, and finally be- 

 ing destroyed by the devel- 

 oping macrosporic tetrad, 

 whose spores reach a very 

 large size. 



THE ISOETINE^E 



The Isoetineae are so 

 different from the other 

 Pteridophytes that there is 

 much difference of opinion 

 as to where they should 

 be placed. While they are 

 most commonly associated 

 with Selaginella, and un- 

 doubtedly show certain 

 structural resemblances, 

 they also have some points 

 in which they seem to 

 approach more nearly the 

 eusporangiate Ferns, with 

 which they are sometimes 

 associated. Whether they 

 are assigned to the Fili- 

 cales or Lycopodiales, they 

 must be placed in a dis- 

 tinct order. There is a single genus, Isoetes, with perhaps fifty 

 species, of which sixteen occur within the United States. 



The sporophyte (Fig. 282) is very similar in all of them, and is 

 usually aquatic, although there are a number of terrestrial and am- 

 phibious species. The stem is very short, and completely concealed 



FIG. 282. A, Isoetes Bolanderi. Sporophyte, 

 slightly reduced. B, base of leaf with macro- 

 sporangium, ma (X 3) ; I, ligule. C, I. echino- 

 spora, male gametophyte, reduced to a single 

 antheridium and a vegetative cell, v (X 700). 



