474 



MOSSES AND FERNS 



CHAP. 



most cell of the axial row is the archesporium, and gives rise to 

 the sporogenous cells by repeated divisions, at first at right 

 angles to each other, later in aU directions. Bower ((15), p. 

 497) thinks that all the sporogenous cells are not to be traced 

 back to the single archesporial cell, but that the inner of the 

 two cover cells also takes part in spore-formation. The exact 

 limits of the archesporium are difficult to follow, as the contents 

 of the sporogenous cells are not strikingly different from the 



Fig. 277.— a, Longitudinal section of young sporangiophore, showing the primary 

 sporangial cell isp), X260; B, C, longitudinal sections of young sporangia, X260. 

 The archesporial cells are shaded. 



inner tapetal ones. These are derived from the cells adjacent 

 to the axial row, and from the cells of the latter just outside the 

 archesporium. The wall of the sporangium is mainly formed 

 from the cells adjacent to the axial row of cells. All the cells 

 grow and divide rapidly, so that the sporangium soon projects 

 strongly from the margin of the sporophyll, whose upper part 

 becomes broad and flattened, while the stalk increases but little 

 in diameter. The wall of the sporangium at first is three or 

 four cells thick, Finally it is reduced to but a single completer 



