176 THE MORPHOLOGY OF PTERIDOPHYTES 



to have had spores of two different sizes and hence cannot 

 have borne seeds as well. The realization that trunks with 

 this particular type of wood belonged to pteridophytes has 

 come as a surprise to many morphologists, for it has been 

 customary to think of them as gymnosperms. A new taxo- 

 nomic group has been suggested (Progymnospermopsida) to 

 contain the various genera listed above, that have affinities 

 both with the Pteropsida and with the seed-bearing plants. 



While this group may indicate the direction in which the 

 pteridophytes were evolving towards higher forms, there are 

 unfortunately as yet no fossils linking them, in the reverse 

 direction, with their possible ancestors. Discussions still take 

 place as to whether pteridophytes evolved directly from 

 Algae or from Bryophyta, and as to whether, in either case, 

 they had a monophyletic or a polyphyletic origin. Until 

 more fossils are known from the Ordovician, Cambrian and 

 even the Pre-Cambrian, there would seem to be little hope of 

 agreement on these matters. There are some, indeed, who 

 doubt whether 'missing links' will ever be found. In the 

 meantime, relying on what we know with certainty to have 

 existed, we must guess at what their ancestors might have 

 been like. 



Subjective processes of this kind have led to a number of 

 theories of land-plant evolution, of which theTelome Theory 

 has had the greatest number of adherents since it was first 

 propounded by Zimmermann-^ in 1930. According to this 

 theory, all vascular plants evolved from a very simple leafless 

 ancestral type, Uke Rhynia, made up of sterile and fertile 

 axes ('telomes'). In order to explain the wide diversity of 

 organization found in later forms, a number of trends are 

 supposed to have occurred, in varying degrees in the differ- 

 ent taxonomic groups. These are represented diagrammati- 

 cally in Fig. 28 (1-5) and are called respectively (i) plana- 

 tion, (2) over-topping, (3) syngenesis, (4) reduction, (5) re- 

 curving. 



Starting from a system of equal dichotomies in planes 



