164 



MORPHOLOGY 



rogeiious cell (fig. 388). The three u|)]icrmost segments and the cap 

 develop the wall of the capsule, which is only one layer of cells thick. 

 At the junction of the cap cell with the last segment is developed the 

 siomium, which is a group of cells so arranged as to permit a cleavage in 

 the wall of the capsule when it begins to dehisce. The annidus is a 



Fig. 386. — Leaflet of Marattia, showing dichotomous venation and synangia 

 (instead of sori). 



band of thick-walled cells which extends from the stomium over the 

 top of the capsule and down on the other side to the stalk (figs. 385, 



39°)- 

 The primary sporogenous cell cuts off a sterile cell from each one of 



the four faces, these four cells developing the tapetum (figs. 388, 389). 



The centrally 

 placed sporoge- 

 nous cell then be- 

 gins a series of 

 divisions until 16 

 mother cells are 

 formed, which 

 means a maximum 

 output of 64 

 spores. Sometimes 



Figs. 387-389. — Sporangium of one of the Polypodiaceae : 

 387, early stage, in which the outer papillate cell has begun to 

 form the apical cell; 388 later stage, in which the cap cell and 

 upper segments invest a central cell (primary spermatogenous), 

 which has cut off one tapetal cell; 389, later stage, in which all there are only 8, 

 the tapetal cells have been cut oft' and have begun to divide; the pvpn 



central cell, by a series of divisions, forms 16 mother cells. ' 



mother cells. The 

 tapetal cells break down, leaving the mother cells free in the enlarged 

 cavity (fig. 390). 



These two kinds of sporangia, one from each extremity of the fern 

 series, indicate not only a passage from the eusporangiate to the lepto- 

 sporangiate habit, but also a striking reduction in the output of spores 

 per sporangium. In this latter feature there is complete intergrading 

 through the intermediate families. 



