452 



MOSSES AND FERNS 



CHAP. 



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 all directions. Bower -^ 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 arche- 

 sporium are difficult to follow, as the contents of the sporo- 

 genous cells are not strikingly different from the inner tapetal 

 ones. These are derived from the cells adjacent to the axial 



Fig. 238. — A, Longitudinal section of young sporangiophore, showing the primarjr sporangial cell {sp), 

 X 260 ; B, C, longitudinal sections of young sporangia, x 260. The archesporial cells are shaded. 



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 complete 



1 Bower (15), p. 497. 



