Pteropsida 



Sporophyte with roots, stems and spirally arranged 

 leaves (megaphylls) often markedly compound and 

 described as 'fronds' (although some early mem- 

 bers showed Uttle distinction between stem and 

 frond). Protostehc, solenostehc or dictyostehc, 

 sometimes polycyclic (rarely polysteUc). Some 

 with limited secondary thickening. Sporangia 

 thick- or thin-walled, homosporous or hetero- 

 sporous, borne terminally on an axis or on the 

 frond, where they may be marginal or superficial on 

 the abaxial surface. Antherozoids multiflagellate. 



Some botanists widen the definition of the Pteropsida to 

 include, not only the megaphyllous pteridophytes, but also 

 the gymnosperms and angiosperms, on the supposition that 

 all three groups are related. While this may well be so, it 

 seems preferable to retain the distinction between pterido- 

 phytes and seed-plants and to restrict the definition of the 

 Pteropsida so as to exclude all but the ferns. Even so, the 

 group is an enormous one, with over 9,000 species, and 

 shows such a wide range of form and structure that it is 

 almost impossible to name one character which is diagnostic 

 of the group. The reader will have noticed that almost all of 

 the characters Usted at the head of this chapter are qualified 

 in some way. 



It will readily be appreciated that, in such a large group, 

 the correct status of the various subdivisions is very largely 



114 



