636 GENERAL COMPARISON OF THE FILICALES 



continued. The Mixtae have commonly (though not always) a flat re- 

 ceptacle. It is not a constant morphological feature, as shown by the 

 fact that a line of transition from an elongated receptacle to a flat one 

 has been demonstrated in the genera Dennstaedtia and Microkpia, and it 

 is possible that other transitions have also taken place elsewhere. 



The term indusium has been applied to bodies of the most various 

 form, position, and structure borne in relation to the Fern-sorus, and 

 serving for the protection of the sporangia. It is hardly necessary to 

 point out that these, however similar in their function, cannot be regarded 

 as homogenetic throughout : they are often only examples of homoplasy. 

 We call the basal indusium of Cyathea by the same name as the umbrella- 

 like indusium of Matonia or Polystichum, or the marginal flap of an 

 Adiantum. It may be possible, by regarding the indusium as having had 

 a high degree of plasticity, to accept some of its different forms as being 

 modifications of one another, and a reasonable case can be made out 

 sometimes, such as that in the transition from the Dennstaedtiineae to the 

 Pterideae. But such cases as Cyathea and Matonia seem incompatible 

 with any opinion of homogeny of the two structures, especially when it is 

 remembered that in Alsophila and Gleichenia, genera which are respectively 

 allied to the above, an indusium may be entirely absent : and this is in 

 fact the usual condition among the Simplices. We shall then hold the 

 indusium to be an inconstant body, varying in occurrence and in position, 

 and the term will be used to designate outgrowths protective of the sorus, 

 whatever their position and whatever their evolutionary history may have 

 been. 



It has been demonstrated in many cases that the indusium is formed 

 before the earliest sporangia appear : this is probably a case where 

 physiological opportunism, rather than any recapitulatory habit, determines 

 the order of succession. It is impossible to believe that those constant 

 bodies, the sporangia, are of later evolutionary origin than the less 

 constant body, the indusium. In this respect the indusium and the 

 embryonic haustoria are probably alike. It is important to recognise 

 such cases as these, for they go far to break down the dogma, that the 

 prior existent parts in the development of the individual were necessarily 

 prior existent in the evolution of the race. 



The views of Prantl have already been quoted, which involve a 

 comprehensive shifting of sori from the margin to the surface. This has 

 brought about great modifications of the indusium. Starting with a 

 strictly basipetal sorus, with cup-like indusium, slightly two-lipped it may 

 be, as in Loxsoma (Fig. 320), we have seen that the type in Dennstaedtia 

 apiifolia (Fig. 332) is similar in position and structure. The indusial lips, 

 which are equal in the above plants, become unequal in Microlepia 

 speluncae (Fig. 332), the superior lip (s) being both longer and thicker than 

 the inferior (/'), and taking on itself the structural characters and appearanc 

 of a continuation of the leaf; this is repeated in Davallia Griffithiant 



