256 



MOSSES AND FERNS 



CHAP. 



cell by a transverse wall, although this point is exceedingly 

 difficult to determine on account of the great similarity of all 

 the cells (Fig. 139). This group of inner cells (or the single 

 one from which they perhaps come) constitutes the arche- 

 sporium, and by rapid division in all directions forms a large 

 mass of cells whose contents become denser than those of the 



Fig. 140. — Ophioglossum pendulum. A, Section of a young sporangium, the arch- 

 esporial tissue is shaded, the inner cells with dark nuclei being the definitive 

 sporogenous cells, X200; B, transverse section of an older sporangium; sp, 

 sporangeous cells; t, tapetum, X about 35; C, a portion of B more highly magni- 

 fied; D, section of nearly mature sporangial spike, X8. 



surrounding ones, between which and these, however, the limits 

 are not very plain. Later, when the number of cells is com- 

 plete, the difference between them and the sterile tissue of the 

 sporangiophore is much more evident. 



The cells lying outside of the archesporium divide rapidly 

 both by longitudinal and transverse walls, and form the thick 

 outer wall of the sporangium. In longitudinal sections, two 



