154 EIGSTI 



species very closely and are not easily separated into the respective popula- 

 tions. 



As we learn more about the role of polyploidy in origin of species we can 

 expect to see an improvement in our approach to the improvement of crops 

 that are in part or exclusively from species that are polyploid (Levan, 1945). 



Agriculture in its historical perspective shows a parallelism between the 

 introduction of new crops and the greater prevalence of species with higher 

 and higher numbers of chromosomes. Thus, within a given group, the newest 

 species introduced to agriculture are usually those with the highest number 

 of chromosomes; or those species with a long record in domestic use gen- 

 erally are diploid. Good cases are found among the species of Triticum, where 

 it is well known that hexaploid wheat, T. aestivum, is the newcomer to agri- 

 culture (Sears, 1948). In fact, there are no species of hexaploids outside culti- 

 vation. At least, up to now there are no wild 42-chromosome types of Triticum 

 on record. The tetraploid, or 28-chromosome Triticum, preceded the hexa- 

 ploid and may have played an important role in the origin of the hexaploid 

 42-chromosomal cultivated forms (Thompson et al., 1943). Finally, the 

 diploids, of which T. monococcum is an example, have been associated with 

 agriculture from its dawning period, in certain centers of civilization. Triticum 

 monococcum is cultivated today in central Europe, but the production is 

 extremely limited in comparison with the acreage now planted with polyploids 

 of Triticum (Sears, 1948). The introduction of more productive varieties 

 often meant the introduction of species that were polyploid. In the future we 

 can look for this trend to continue, and with newer techniques, reviewed by 

 Eigsti and Dustin (1955), for making polyploids, there is great hope for the 

 development of new and more productive crops. 



The tetraploid cotton, Gossypium hirsutum, is another interesting introduc- 

 tion wherein the practice of agriculture played its role in making possible the 

 close contact between the diploid species, one of Asiatic origin and the other 

 American. Both are involved in the present-day tetraploid American Upland 

 Cotton (Beasley, 1940; Stephens, 1950). The diploids had to make contact 

 at some point where the hybridization could take place. Then doubling of the 

 chromosomes within the sterile diploid hybrid resulted in the origin of fertile 

 tetraploids. Man was able to recognize in the tetraploid the increased pro- 

 ductivity. When and exactly where the hybridization took place is unknown. 

 If more facts are established, one can predict that botanists will furnish some 

 of the data to show how the diploids of two continents came together prior 

 to the origin of the American Upland Cotton (Stephens, 1950). 



The superiority of ornamentals and horticulturals (Emsweller, 1949; Der- 

 men, 1952; Mehlquist, 1949; Darrow, 1950) that are known as natural poly- 

 ploids keeps alive keen interest in polyploidy as a practical means for produc- 

 ing new and better flowers and fruits. Kamemoto (1952) reports excep- 

 tionally valuable orchids that are triploid. The outstanding irises judged best 



