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 th< 

 rule. 1 This is accompanied by an absence of any definite orientation o 

 the sporangia', such as has been seen especially in the Gradatae : also there 

 is commonly an elongation of the sporangial stalk, which is often reducec 

 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 alread) 

 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 beer 

 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 wide 

 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 th 

 mixed type of sorus was arrived at, and that it was probably derived in mos 



1 See Stttdies, iv. , pp. 78-87. 



