THE PTERIDOPHYTA : FILICALES, THE FERNS 



537 



Development of the Sporangium 



Characteristic of the family as a whole are the sporangia, which have walls 

 several cells thick, and are in Marattia united into solid, compound structures 

 called synangia (Fig. 535). Each synangium is in two halves, like a small 



Fig. 535. — Morattia fraxinea. Portion of a fertile pinna 

 showing marginal rows of synangia, each seated on 

 a vein. 



Open pod, about 2 mm. long, with ten to twelve sporangia fused side by side 

 in each half. The halves are at first closely pressed together, but when 

 ripe they separate like an opening book (Fig. 536). The tvvo halves are united 



Fig. 536. — Marattia fraxinea. Section of pinna 

 showing two mature dehiscent synangia. 



beneath to a placenta, and below them arises a small membraneous indusium 

 which never covers the synangium. Since the sporangia are completely 

 united each sporangium appears only as a loculus in the compound structure, 

 but each is individually much larger than the sporangium of a Dryopteris 

 and produces about 2,500 spores as against forty-eight in the latter type. 

 This large spore output is held by Bower to be a primitive character. 



There is no annulus and the sporangia dehisce along a ventral line of 

 thin-walled cells, that is to say, inwards in the synangium. 

 i8a 



