XLI] ABORTIOx\ OF THE INDUSIUM 129 



possess it to that seen in Dryopteris is amply shown by the richness of the 

 synonymy of these Ferns. This view has been generally adopted as phy- 

 letically probable (Von Goebel, OrganograpJiie, II, Aufl. 2 Teil, p. 1150). 



Fig. 660. Aspidium [Polysticlnint) lobatitiii Sw. A 

 secondary segment, showing the prltate indusium. 

 (After Luerssen. X5.) 



Abortion of the Indusium 

 Within the genus Dryopteris the indusium is apt to be reduced in size, or 

 to be wholly absent in forms corresponding closely in other details to those 

 which have it normally developed. The result is that the sporangia form a 

 naked hemispherical or slightly elongated mass, attached to the leaf-surface, 

 such as is held to be typical of the old genus Polypodium. Here the question 

 will arise whether the indusium has really been aborted, or whether the state 

 actually seen is primitive, there never having been any indusium present in 

 the ancestry. The Oak Fern {Dryopteris Linnaeand) and the Beech Fern 

 {^Dryopteris phcgopteris) are examples, and the natural interpretation of them 

 seems to be that they are Dryopteroids that have lost their indusium by 

 abortion. In the genus Dryopteris at large, as it is represented in the Mono- 

 graph of C. Christensea (Copenhagen, 191 3, onwards), a very considerable 

 number of species are found to be ex-indusiate, while in others the indusium 

 is noted as "minute" or "fugacious." The effect of this is to suggest that the 

 indusium is an inherited feature no longer of high biological importance, 

 and that it is here seen in course of elimination. Our own native species 

 represent extreme types, on the one hand retaining the indusium in its full 

 development, as in Dryopteris filix-mas or Polystichum aculeatnin; on the 

 other illustrating its complete abortion, as in Dryopteris Liniiaeana or 



BHI 9 



