SPORE-PRODUCING MEMBERS 635 
This elongation and structural adaptation of the receptacle becomes much 
more pronounced where there is a continued succession of sporangia, as 
in the Gradatae. The basipetal sequence seen in the Cyatheae, Dicksonieae, 
Dennstaedtiinae, Hymenophyllaceae, would hardly be possible without a recep- 
tacle having intercalary growth: the continual moving upwards of the more 
mature sporangia, so that they can freely shed their spores, leaves space 
for the initiation of younger ones below, while the latter are in their early 
stages close to the source of supply, and protected by the basal indusium. 
This seems to be the raison d’éve of the elongated receptacle in such 
cases: given a basipetal succession, its origin has been a response to the 
need of space, by upgrowth from the base, not the result of “‘ metamorphosis ” 
of any pre-existing vegetative part such as a lamina of a leaf (Prantl). 
An indirect support for this opinion is found in the fact that the 
receptacle disappears in those Ferns where the mixed sorus is acquired ; 
for here the receptacle is commonly flat, though some exceptions do 
occur. A peculiar interest attaches to those closely allied Ferns of the 
Dicksonia-Davallia affinity, in which a transition from the basipetal sequence 
to the mixed sorus is believed to have taken place. In Dennstaedtia and 
Microlepia the receptacle is clearly conical, though it does not show an 
elongation at all comparable to that of the Hymenophyllaceae (Fig. 332). 
Occasionally in Denn. davallioides and in Microlepia hirta, but more com- 
monly in Denn. rubiginosa, the strict basipetal succession which is seen in the 
first stages of the sorus is departed from, though the receptacle still maintains 
in some degree its conical form. But in Ludavadllia, which is without doubt 
closely allied to the above, the sorus is a mixed one, and the receptacle 
is almost flat, though still provided below with a considerable mass of 
tracheides (Fig. 339). Here, since the basipetal succession is not maintained, 
an elongated receptacle is not required, and since this difference occurs 
between Ferns which are certainly of a common stock, it is probable that 
a reduction has taken place. Accordingly, an elongated receptacle is not 
in itself an important morphological feature; it is to be correlated with a 
continued basipetal succession of sporangia, and it is this last which is the 
essential morphological feature in such cases. 
There are, however, instances where there is an enlarged receptacle on 
which the sporangia are borne not in basipetal succession, but in mixed 
order; as an example, the familiar sorus of ephrodium may be quoted, 
with its large receptacle and internal mass of tracheides (Fig. 4). It may 
be urged that these facts are inconsistent with the view expressed in the 
last paragraph, but the large receptacle may here be a survival, which, 
though the sorus has become a mixed one, may still be a convenience ; 
as it certainly is in JVephrodium, where its size allows space for the 
sporangia between the indusium and the leaf-surface. The general view 
may then be formulated thus: the Simplices have as a rule a receptacle 
of small size; the Gradatae have a more or less elongated receptacle, and 
it may attain very considerable dimensions where the succession is long 
