l8o THE MORPHOLOGY OF PTERIDOPHYTES 



and Horneophyton as examples of this type of plant. How- 

 ever, they were certainly not the ancestors of pteridophytes. 

 As Leclercq^^ emphasizes, they occurred much too late in 

 the fossil record for this to be possible, and represent the 

 last surviving examples of that particular growth form. As 

 Andrews^* suggests, the emphasis that has been placed on 

 Rhynia has drawn attention away from the great diversity 

 of form that is now known in Silurian and Middle Devonian 

 plants, and has led to an uncritical acceptance of the thesis 

 that vascular plants are a monophyletic group. 



So far, these speculations as to the course of pteridophy te 

 evolution have centred around the sporophyte, since it is 

 this phase of the hfe cycle that is represented in the fossil 

 record. Even more speculative is the evolution of gameto- 

 phytes, concerning which there are the two diametrically 

 opposed schools of thought referred to, near the end of 

 Chapter 3, as 'Antithetic' and 'Homologous'. Mention was 

 there made of abnormal gametophytes of Psilotum, contain- 

 ing vascular tissues. The significance of this interesting dis- 

 covery was somewhat diminished for a time, however, when 

 it was shown that they were diploid ; but in relation to dis- 

 cussions of antithesis and homology chromosome counts 

 are, in a sense, *red-herrings'. This is made apparent by the 

 phenomena of apogamy and apospory, cases of which have 

 been recorded many times in pteridophytes since they were 

 first observed in 1874. 



Apogamy is the development of a sporophyte directly 

 from the gametophyte without the intermediate formation 

 and fertihzation of gametes. The resulting sporophyte, 

 therefore, has the same haploid chromosome count as the 

 gametophyte. By 1939'^^ apogamy had been recorded among 

 ferns, in Pteris, Dryopteris, Pellaea, and Trichomanes, where 

 it is frequently preceded by the appearance of tracheids in 

 the gametophyte. More recently ^^ it has been recorded in 

 Thelypteris, Pteridium, Phyllitis and several species of Lyco- 

 podium. In the case of Phyllitis, the haploid apogamous 



