The Experimental Polyploids 279 



tetraploid, 4;? = 28; upon doubling of the 21 -chromosome triploid, a 

 hexa})loid (42-chromosome) species originates.^'' In this way species 

 h\bridization, followed by doubling of the chromosomes, fulfils the 

 principle of the Winge hypothesis. Among the wheats (Triticinae) 

 there is an excellent chance to show how this mode of evolution 

 accounts for speciation as well as the production of mankind's most 

 \aluable economic crop species, hexaploid wheat, (42-chromosome 

 Triticum aesthnim L.) .•*^ However, on a purely numerical basis and 

 without a knowledge of the only known case to support his assumption, 

 the A X B hypothesis was outlined to explain the origin of species with 

 high chromosomal numbers. The data which Winge needed were 

 published by Digby for Primula keioensis.^^ 



The facts of cataclysmic evolution became clearer, for new tetra- 

 ploids Avere discovered"'^ or synthesized continuously from 1926. 

 These include Miint/ing's synthetic Galeopsis tetraliil;''^ Primula 

 kewensis, arising under culture at Kew Gardens;*"^*^ Karpechenko's 

 Raphanobrassica,-^ a doubled intergeneric hybrid between radish 

 and cabbage. Finally Spartina fownsendii}^ a new polyploid of recent 

 historic times, is a new species which invaded a habitat not previously 

 occupied. The mud flats along the channel coastline of England 

 abound with this new species, but records show that prior to 1870 no 

 plants were present in this area.^^ 



Two important conclusions emerge from the numerous studies 

 dealing with polyploidy and evolution. (1) Polyploid species are 

 abundant in nature: by one estimate as many as 50 per cent of the 

 flo\\'ering plants are in some dui^licated form. (2) Valuable economic 

 crop species (food, fiber, and others) are polyploid, e.g., bread wheat, 

 cotton, oats, sugar cane, tobacco, grapes, berries, nuts, and many other 

 horticultural and floricultural species. In the first instance our 

 problem may be called cataclysmic evolution in nature; in the second, 

 evolution under domestication.^"^ 



Polyploid agricultural species originated through the years in 

 nature without man's guidance, but under his hand and through his 

 selection they may have become quite different species than if left 

 to natural processes of selection. When man eliminates certain types 

 and nurtures the environment for his choice plants, the situation is 

 not com))arable to nature's elimination process and selection that goes 

 on conijjetitively without cultivation. Nevertheless, the problems of 

 evolution in nature and imder domestication'*'* are very closely inter- 

 related. That is why closer integration of theoretical and j^ractical 

 work seems advisable in j^olyploidy research. Increasing the in- 

 formation about the origin of jjolyploids in nature improves our posi- 

 tion in the planning (jf a ne^v hybridization program.'-'' Furthermore, 

 the data from countless selections by the practical breeder could be 

 valuable for analysis with purely theoretical objectives in mind.^^ 



