VII PTERIDOPHYTA—FILICINE^—OPHIOGLOSSACEJE 255 



arate an inner archesporium from the outer cells, destined to 

 form the wall of the sporangium. Between the young spo- 

 rangia the cells form sterile septa. The cell-groups which form 

 archesporia, and those which develop into sterile septa, are 

 sister-cell groups. 



All of the sporogenous tissue cannot be traced back to the 

 primary archesporial cell, as later secondary sporogenous tissue 

 may be formed by further periclinal divisions in the outer cells 

 of the sporangium. 



A transverse section of the very young sporangiophore is 



A. 



B. 



Fig. 139. — A, Very young; B, older sporangia of O. pendulum, 



X^6o. 



transverse sections, 



somewhat triangular, the broader side corresponding to the 

 outer surface of the sporangiophore. The cells are very irreg- 

 ular in form, and no differentiation of the tissues is to be 

 observed. Sections of somewhat older stages show in some 

 cases, at least, a large epidermal cell occupying nearly the 

 centre of the shorter sides of the triangular section. This cell 

 has a larger nucleus than its neighbours, and is decidedly 

 broader. The next stage was not observed, but a somewhat 

 more advanced one shows a small group of inner cells (shaded 

 in the figure), which appear to have arisen from the primary 



