PTERIDOPHYTES 165 



In passing f rom Marattiaceae to Polypodiaceae, it is interesting to note 

 the changes in the character of the sporangium stalk. Beginning with 

 sessile sporangia, there is a transition to very short stalks, and finally 

 the very long slender ones that characterize the Polypodiaceae are 

 reached. This change in the character of the stalk accompanies the 

 reduction in the output of spores, so that perhaps 

 among leptosporangiate ferns the development of 

 an elongated sporangium stalk is of more impor- 

 tance than a large output of spores from a single 

 sporangium. 



Sorus. — The sorus is a feature of most Filicales, 

 being a definitely limited group of sporangia (figs. 

 383, 384). Sori vary in form and in arrangement, 

 and are useful as taxonomic characters. Sporangia 

 do not always form sori; in some cases they are Fig. 390. — Mature 



, . r , , , r . . sporangium of one 



scattered over the surface of the leaf, in other cases of the Polypoiiaceae% 



they form in a continuous band along the margin, showing the incom- 



Indusium.— The indusium is a flaplike outgrowth p' ete vertical annulus 



. , - riifi-i ,ij and the transverse 



from the surface of the leaf which protects the de- deniscence . 

 veloping sorus (figs. 369, 373, 383). It is exceed- 

 ingly variable in form; in some cases forming a pouch, in others an 

 overarching shield (fig. 383), in others a cup, etc. It is sometimes 

 lacking entirely, and in certain cases it is replaced by the inrolled 

 leaf margin, which is then spoken of as a " false indusium." These 

 variations in the indusium are so constant for different groups as to be 

 very useful as taxonomic characters. 



Gametophyte. — The gametophyte of the Filicineae, better known 

 as the prolhallium, is a small green thallus, with rhizoids, and resembles 

 the gametophyte of very simple liverworts (fig. 391). The most primi- 

 tive ferns (Marattiaceae) have the largest and most massive gameto- 

 phytes. In general, the development of the gametophyte passes through 

 the stages indicated for liverworts (p. 98). In germination there pro- 

 trudes from the spore a papillate outgrowth which is cut off by a wall 

 as a cell and develops into the filamentous stage, the length of the fila- 

 ment being related to available light. Following this stage is that of the 

 apical cell with two cutting faces, by means of which the gametophyte 

 is broadened. Then follows the group of initials, by means of which the 

 gametophyte is matured, in the course of which the usual apical notch 

 is developed. The axial region of the gametophyte is much thicker than 



