THE FERNS 139 



last chapter, to the line of descent of the Seed- 

 plants. 



Our modern Ferns, indeed, show little sign 

 of their high connections, and it is only from the 

 study of fossil plants that the evidence has come. 

 The Ferns which have come down to our own day 

 are frankly Cryptogamic, and for the most part 

 make no approach to higher groups. The excep- 

 tion is the family of the Water-ferns, in which, 

 as we have seen, there is a very marked distinction 

 between the two kinds of spores; in fact, the 

 most extreme cases of heterospory among living 

 Cryptogams are in this family. The interesting 

 point is that in the megasporangium only a 

 single spore comes to maturity, filling the whole 

 sporangium, like the embryo-sac of an ovule. 

 Where, as in the little floating water-weed Azolla, 

 an envelope grows up round the sporangium, 

 the resemblance to an actual ovule becomes 

 surprisingly close. There is, however, no forma- 

 tion of a seed, and fertilisation takes place after 

 the spores have been discharged into the water. 

 Probably the Water-ferns have really nothing to 

 do with the evolution of the Seed-plants; they 

 appear to belong to a later branch altogether, 

 which to a certain extent shows an interesting 

 parallelism with the main line of descent. 



Most of the families of our living Ferns can 

 be traced well back into the Mesozoic period; 



