CHAPTER XXXIX. 
MIXTAE. 
THERE still remain to be considered the great majority of genera and species 
of living Ferns. It is not proposed here to enter fully into the characteristics 
or the classification of them: it must suffice to indicate certain features 
only which they show, and to place them in general relation to those of 
the other Filicales which the Palaeontological evidence indicates as prior 
to them in time. 
It has been found, as the result of examination of representatives of all 
the remaining genera of living Ferns, that the sorus is of the type which 
is designated ‘“‘mixed”: that is, that sporangia of different ages are aggre- 
gated together without any definite sequence: in fact, that promiscuous 
interpolation of younger sporangia between those already present is the 
tule! This is accompanied by an absence of any definite orientation of 
the sporangia, such as has been seen especially in the Gradatae: also there 
is commonly an elongation of the sporangial stalk, which is often reduced 
to a single row of cells at its base. With this there is a vertical position 
of the annulus, which is interrupted at the point of insertion of the stalk. 
The numerical output of spores per sporangium has never been seen in 
these Ferns to exceed 64, while lower numbers are frequent. These 
characters are general for the remaining Ferns exclusive of those already 
described, and they are accordingly designated collectively the ‘“ Mixtae.” 
There may, however, be very great differences in the number, position, 
and extent of the sori, and in the presence or absence of an indusium; 
and it is upon these characters that their classification has principally been 
founded. But before such classification can be held as more than pro- 
visional the criteria will have to be extended to include the results of wider 
anatomical study, and of comparison of the gametophyte. 
It is improbable that the Mixtae constitute one single phyletic line: 
evidence will be adduced that in more than one distinct line of descent the 
mixed type of sorus was arrived at, and that it was probably derived in most 
1See Studies, iv., pp. 78-87. 
