Aniphidiploids 457 



In the allotetraploid we have mentioned, the chromosome nmii- 

 ber is twelve. It is four times the haploid number, and because 

 of this 4n condition, the plant is considered to be a tetraploid. 

 However, amphidiploids may be produced from two plants which 

 differ with respect to chromosome number. If a plant with six 

 and a plant with eight chromosomes were crossed and produced 

 an amphidiploid hybrid, the chromosome number would be four- 

 teen, which is not a multiple of the haploid number of either 

 of the diploid parents. We could still regard this plant as an 

 allopolyploid because it has four sets of chromosomes so that 

 in dealing with allopolyploids it is better to consider their poly- 

 ploid nature in the light of the number of genomes rather than 

 of the number of chromosomes. Of course, the problem does not 

 arise when dealing with autopolyploids. 



In our allotetraploid, the genomes of the two parents differed 

 completely from one another. Although it makes the situation 

 clearer to assume two completely different genomes, it might 

 well be questioned whether plants which differed so completely 

 could cross at all. In most allotetraploids the two sets of chro- 

 mosomes are different but not so completely different. Instead 

 of designating the two genomes as A B C and L M P, it would 

 probably be more accurate, in most cases, at least, to designate 

 them Ai Bi Ci and A2 B2 C2, so that the chromosome constitution 

 of the allotetraploid would be AiAi A2A2 BiBi B2B2 CiCi C2C2. 



Amphidiploids 



Some known amphidiploids are the result of hybridization be- 

 tween two species of the same genus; others arose from crosses 

 between species belonging to two different genera. Some am- 

 phidiploids have arisen spontaneously throughout the course 

 of evolution; others have been created by experimentation. Sev- 

 eral amphidiploids that have arisen spontaneously have been 

 duplicated or nearly duplicated by appropriate crosses between 

 species that are believed to have been the original parents of 

 the spontaneous form. 



An interesting amphidiploid is the species Spartina Townsendii, 

 which is believed to have arisen spontaneously as an amphidi- 

 ploid sometime before 1871, which is the first time that it was 

 collected. Morphologically this species resembles both S. alterni- 

 fiora and S. stricta. Huskins's study of the number of chromo- 



