634 GENERAL COMPARISON OF THE FILICALES 



Prantl had already noted it, and held that the translocation had occurred 

 along several distinct phyletic lines within the Polypodiaceae. 1 There is 

 no evidence at hand of the converse progression from the surface to the 

 margin. But though a probability is thus established of progression of 

 the sorus from the margin to the lower surface, this does not prove that 

 the former position was prior for the Ferns at large. It must be remembered 

 that Marattiaceous types with sori intra-marginal are recorded as far back as 

 the Culm ; and it is quite possible that they may have originated from forms 

 .with sori superficial from the first. The question is accordingly an open one 

 whether all Ferns sprang from types with marginal sori, though it seems 

 certain that in some the superficial position has been secondarily acquired. 



The sorus itself consists of a receptacle upon which the sporangia are 

 inserted, and of the sporangia themselves, while various accessory growths 

 may be present also, and are called by the collective name of indusium. 

 According to the construction of their sori the F"erns have been seen to 

 fall into three main groups : the Simplices, Gradatae, and Mixtae. In the 

 Simplices the sporangia are all simultaneous in origin : in the Gradatae 

 there is a basipetal succession of the sporangia, but there is no intercalation 

 of younger sporangia between those already initiated : in the Mixtae new 

 sporangia are intercalated without order between those first forme.d. There 

 is reason to believe the Simplices to be the most primitive type, the Gradatae 

 to occupy a middle position, and the Mixtae to be the most advanced, and 

 that either of the latter might be derived phyletically from the first. The 

 differences in order of origin of the sporangia in these three types have 

 entailed variety in adjustment of the sorus, especially in regard to the 

 protection and nutrition of the sporangia, and the distribution of the spores 

 when ripe. 



Taking the- receptacle first, it is required as a means of transfer of nourish- 

 ment to the sporangia and as a basis for their support in such a position 

 that the spores can be scattered when mature. These requirements will 

 have to be considered separately in the case of each of the three groups. 

 In the Simplices the sporangia are usually produced in such moderate 

 numbers that there is room for them all in a single radiating series round 

 the centre of attachment : there appears to be little need in them for a 

 raised receptacle. It is true that in some cases, such as Kaulfussia and 

 Ptychocarpus unitus, there may be a massive receptacle with a vascular 

 extension into it, while in Marattia Kaulfussii it may be elongated into a 

 stalk below the sorus; but in many of the Simplices the receptacle is 

 hardly developed as such, the sporangia receiving their supplies directly 

 from the leaf through their own massive stalks. The Gleicheniaceae show 

 certain points of interest for comparison : those species which have few 

 sporangia in the sorus have only a comparatively small receptacle; but in 

 G. dichotoma, where the sporangia are more numerous, the receptacle is 

 slightly elongated, while it is well provided with vascular tissue. 



L.C., p. 12. 



