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.t 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 Ferns 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 formed. 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 vecepzacle first, it is required as a means of transfer of nourish- 
ment to the sporangia arfd 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 Aaw/fussia and 
Ptychocarpus unitus, there may be a massive receptacle with a vascular 
extension into it, while in Afarattia Kaulfussit 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. 
Lu¢., p. 12. 
