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 Ffemitelia 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 
Polypodtum, in which, with a similarly superficial sorus, the indusium is absent. 
Another line of possible reduction may be traced from the Dennstaedtiinae, 
through Ayfolepis, to certain types of olypodium. 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 
ad 
Fic. 349. 
Diagrams illustrating the segmentation of Ferns. a= Polypodiaceae (compare ,Kny, 
Wandtafeln XCIV.) 4=Ceratopteris (compare Kny, Parkeriaceen Taf. XXV., Fig. 3). 
c=Alsophila (compare Fig. 334). @=Schizaca (compare Prantl, Taf. V., Fig. 69), or 
Thyrsopteris (compare Fig. 329), or Tvichomanes (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 @-¢), which 
show the initial segmentations of the sporangia of various types of Ferns, 
from the Marattiaceae to the Polypodiaceae. Taking the Simplices first, 
