THE SPORANGIUM 



637 



(Fig. 339), and is still more pronounced in Cystopteris and Lindsaya. 

 Thus the equal lips may be differentiated, the one taking on the structure 

 of the leaf-margin, the other becoming a mere appendage of the surface. 



There is reason to believe that a reduction of the indusium has taken 

 place along more than one line of descent ; one such probable series of 

 reduction may be traced from Cyathea to the very interesting conditions 

 seen in Hemitelia with its one-sided indusium, and in Woodsia and 

 Hypoderris, in which there is an exiguous, fimbriated indusium. It is 

 but a slight step from these to some forms of the comprehensive genus 

 Polypodium, in which, with a similarly superficial sorus, the indusium is absent. 

 Another line of possible reduction may be traced from the Dennstaedtiinae, 

 through Hypolepis, to certain types of Polypodium. The probability is 

 that there is here a .progression from a type with basipetal succession of 

 sporangia protected with a basal indusium, to a mixed type in which the 

 indusial protection is less essential, and the indusium is accordingly abortive. 



THE SPORANGIUM. 



The morphological equivalence of the sporangia of Ferns at large will 

 be generally admitted, whatever their modifications of detail may be. It 

 has been customary to distinguish the Leptosporangiate from the Eusporan- 

 giate types, on the basis of the origin respectively from one or from several 



f g 



FIG. 349. 



Diagrams illustrating the segmentation of Ferns. a=Polypodiaceae (compaiv Kny, 



Wandtafeln XCIV.) b=Ceratopteris (compare Kiiy, Par!.. - ia Paf. XXV., I i^. 3). 



c = Also/>hila (compare Fig. 334). et=Sc -hi^ica (compare Prantl, Tat'. V., Fig. 69), or 

 Thyrsopteris (compare Fig. 329), or Trichomanes (compare Prantl, Taf. V., Fig 92). 

 e,f= Todea (compare Fig. 295). g= Angiopteris (compare Fig. 284). 



parent cells. But comparative observation shows that this distinction is 

 based not on any difference of kind, but only of degree. The transition 

 from one to the other is illustrated by the diagrams (Fig. 349 rt-.v), which 

 show the initial segmentations of the sporangia of various types of Ferns, 

 from the Marattiaceae to the Polypodiaceae. Taking the Simplices first, 



