APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



Since a mechanism for obtaining tetraploids from diploids is known to exist in the four- 

 celled sporangia, there is no reason to regard the Uganda plant as anything other than 

 a derivative by simple chromosome doubhng from the simpler state still retained in 



Fig. 177. Details of chromosome pairing in diploid Ptem cre^zVa L. from sections, x 2000. a. Diakinesis 

 in an eight-celled sporangium with very regular pairing, b. Diakinesis in a sixteen-celled spor- 

 angium showing univalents and multivalents, c. The same at first metaphase showing a univalent 

 a trivalent and other groups. 



Fig. 178. Pairing at diakinesis in a sixteen-celled sporangium of tetraploid Pteris cretica L. 



from Uganda showing complex multivalents. 



the European form of the species. With regard to the triploid we are in the same 

 dilemma as in triploid Dryopteris Borreri, already to some extent discussed in a previous 

 chapter. A hybrid between apogamous Pteris cretica and some related sexual species 

 seems the most probable explanation. 



With regard to diploid P. cretica there are perhaps two alternatives. The first and 

 most probable is that it is a hybrid between two related species with much though 

 not complete homology between the chromosomes of the two gametic sets. Another 



176 



