POLTPODIUM VULGARE 



the morphology but also by the breeding behaviour of all three types when their native 

 localities abut. Thus among the rather limited number of diploids collected at random 

 on journeys as occasion offered, no less than three cases of triploids have been included. 

 A leaf of one of these from the Rhone Valley in Switzerland is shown in Fig. 135^, 

 and the chromosomes of another, from the lower slopes of the Pyrenees near Perpignan 

 in Fig. 139. There is almost complete failure of pairing in the triploids, the shape of 

 the univalents making a very striking contrast with that of the normal appearance of 

 pairs in the putative parent species. These triploids, the third of which came from 

 north Italy, can hardly be other than hybrids between diploid and tetraploid, and they 

 could be the prototypes of the hexaploids before the chromosome number was doubled. 



Fig. 136. Meiosis in tetraploid Polypodium from Norway to show the shapes 

 of the 74 bivalents for comparison with the triploid of Fig. 139. x 1000. 



Where tetraploids and hexaploids grow together, pentaploid hybrids are to be found, 

 and some half-dozen of these have been included in our own collections made in different 

 parts of north and south England and as far afield as Holland. One leaf is shown in 

 Fig. 135^ and the chromosomes in Fig. 138. As may be seen in the diagram (Fig. 137) 

 there are exactly 74 pairs and 37 univalents. 



This pairing is exactly what would be expected if the hexaploid is the allopolyploid 

 between diploid and tetraploid as postulated. The triploid indicates that the diploid 

 and tetraploid are sufficiently different from each other to have practically no chromo- 

 somes in common in spite of their readiness to breed together. On the other hand, the 

 perfect pairing of 74 of the chromosomes of the hexaploid when backcrossed to the 

 tetraploid indicates that the gametic complement of the tetraploid is present intact in 

 the hexaploid, and that the unpaired chromosomes in the pentaploids are therefore 

 almost certainly those of 'var. serratum\ which have already been seen to be non- 

 homologous with these by means of the triploid hybrid. 



136 



