APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



sporangia are more abundant than in Cyrtomium falcatum, and details of chromosome 

 pairing in such sporangia are therefore available for all the polyploid types. The 

 greatest interest naturally attaches to such pairing in the diploid which in this case is 

 hkely to be the oldest type, a view with which its apparently rehct and disjunct 

 occurrence in Europe is in full accord. 



The first impression given by chromosome pairing in the sixteen-celled sporangia of 

 Pteris cretica is its irregularity. Pairs are numerous and absorb the majority of the 





Fig. 176. Meiosis in the polyploid series of Pteris cretica L. from sections, x 1000. a. First meiotic 

 metaphase in an eight-celled sporangium of the diploid {2n - 58). b. The same in a sixteen-celled 

 sporangium, c. Triploid var. albolineata {^n = c. 90) an eight-celled sporangium, d. The same in 

 a sixteen-celled sporangium, e. The wild tetraploid from Uganda (4« = c. 120) an eight-celled 

 sporangium. /. The same in a sixteen-celled sporangium. 



chromosomes, but there are also some trivalents together with a few probable quadri- 

 valents and a residue of unpaired chromosomes remains. Various views are shown in 

 Fig. 177^ and c. 



In contrast to this, the sixteen-celled sporangia of triploid and tetraploid show fewer 

 unpaired chromosomes and more multivalents involving larger numbers of chromo- 

 somes. Some of these, as found in the Uganda tetraploid, are shown in Fig. 178, in 

 which a very large multivalent group is seen at diakinesis. Another expression of the 

 same thing is the reduced number of lagging univalents visible in side views of meta- 

 phase; this is perhaps detectable by comparing the bottom row of Fig. 176 in which 

 this stage in diploid, triploid and tetraploid are placed side by side. 



With regard to the interpretation of these observations, the presence of multivalents 

 in tetraploid P. cretica need not surprise us, since some are also to be found in the diploid. 



175 



