200 Increase in Number of Species 



although not in the woody plants. The annual herbs and woody 

 plants, however, do have many polyploid series at an incidence 

 that is extremely high compared with the animals. 



A high proportion of natural polyploids in plants are hybrid in 

 composition. Winge (1917) first theorized that new species of 

 plants could arise as the result of hybridization followed by poly- 

 ploidy. His theory was verified experimentally by Clausen and 

 Goodspeed ( 1925 ) by the artificial synthesis of the polyploid 

 species Nicotiana digJuta from the hybrid between N. tabacum and 

 N. gliitinosa. Unknown to these investigators, a similar case was 

 already on record in the genus Primula. In Kew Gardens a perennial 

 hybrid plant of a cross between Primula verticillata and P. foribumla 

 was viable but sterile. After some time a polyploid shoot arose which 

 produced fertile polyploid flowers (Digby, 1912). Since that time 

 botanists have discovered many more cases of this sort (Stebbins, 

 1950), and it now appears that the great proportion of naturally 

 occurring polyploids arose from hybrids, in other words are allo- 

 polyploids. 



The occurrence of simple polyploids (aufopolijploids), which 

 arise by doubling of the parental chromosomes without hybridiza- 

 tion, is much less frequent. In her fascinating account of the cytology 

 and evolution of ferns, Manton (1950) found several cases of 

 autopolyploidy but many more in which the evidence pointed to 

 the hybrid origin of the polyploid species. Stebbins cautioned in 

 this regard that even species suspected of being autopolyploids 

 because of chromosomal behavior may actually be allopolyploids 

 in which many chromosomes of the parental species of the h)'brids 

 are similar in basic structure. 



All possible polyploids are not successful species. Stebbins (1950) 

 listed many experimentally produced ones which are either sterile 

 or virtually inviable, or both. In the laboratory, Manton (1950) 

 produced the autotetraploid of the fern OsmumJa rcgaJis (a form 

 which has not been collected in nature ) and found that it was much 

 less vigorous than the naturally occurring diploid form. She con- 

 cluded that autopolyploidy in the genus Osmuuda would give rise 

 to no persistent types and therefore would be of no exolutionary 

 significance. 



Allopolyploidy in plants has unusual exolutionary significance 

 in two respects. Because of it, whole sets of polyploid species have 

 evolved to form a series of polyploid levels. A simple example in 

 which the species differ only sliglith' is the European triad of fern 

 species in the genus Pohjpodium (Manton, 1950). In these forms 



