POLYPLOroV 



of the morphology of tetrahit with that of the 8-chromosome species, he 

 selected G. piibescens and G. speciosa as the most probable parent species. 

 The hybrid between pubescens and speciosa was highly sterile, but some 

 good gametes were formed. A single Fo plant was obtained, and this 

 proved to be a triploid, apparently the product of an unreduced gamete 

 and a gamete with the speciosa genome. This triploid was backcrossed to 

 a pure pubescens plant, and again a single viable plant was obtained. As 

 this plant was tetraploid, it must have been formed by union of an unre- 

 duced gamete from the triploid parent with a normal gamete from the 

 pubescens parent. This "artificial tetrahit" was fully fertile, both to itself 

 and to natural tetrahit, which it resembles. It breeds true, and is sterile 

 when crosses are attempted with either of the parent species. Thus there 

 can be no doubt that Miintzing has duplicated the natural "synthesis" of 

 Galeopsis tetrahit. 



The Wheats. A polyploid series of great economic importance is that 

 of the genera Triticum, the wheats, and Aegilops, a related grass of no 

 commercial value. Wheats with three different chromosome numbers are 

 known. The Einkorn group comprises three species, of which T. monococ- 

 cum is the most important, and has 7 pairs of chromosomes. These species 

 are not commercially valuable. The Emmer group includes eight species, 

 two of which, T. durum and T. turgidum, are important crop plants. These 

 eight all have 14 pairs of chromosomes. Finally, the Vulgare group com- 

 prises four species, two of which, T. vulgare and T. compactum, are of 

 great economic value. The wheats of the Vulgare group all have 21 pairs 

 of chromosomes. This seriation— 7— 14— 21— strongly suggests a polyploid 

 series with diploid, tetraploid, and hexaploid members. Further, however, 

 it is possible to produce haploid plants of the Emmer and Vulgare groups. 

 In these haploids, there is no pairing of the chromosomes at meiosis, hence 

 it is believed that the homologies between the dijBFerent genomes of the 

 polyploid species must be rather remote. And so these are allopolyploids. 

 If r. nwnococcum is crossed to an Emmer wheat, seven pairs of chromo- 

 somes and seven singletons appear at meiosis, hence the seven chromo- 

 somes of monococcum do have homologues in the Emmer group. If T. 

 nionococcum is crossed to T. vulgare, seven pairs and fourteen singletons 

 appear. Thus it appears that the Einkorn genome. A, is found throughout 

 the genus. Similarly, an Emmer wheat crossed to a Vulgare wheat gives 

 14 pairs and 7 singletons, indicating that all of the Emmer chromosomes 

 are also present in the Vulgare genome. The closely related genus Aegilops 

 has species characterized by 7 and by 14 pairs of chromosomes. When 

 Aegilops is crossed to Einkorn or Emmer wheats, no pairing results, indi- 

 cating that no demonstrable homology exists. But when Aegilops cylin- 

 drica, with 14 pairs of chromosomes, is crossed to a Vulgare wheat, 7 pairs 

 and 21 singletons ( 7 from Aegilops and 14 from the wheat ) appear. This 

 indicates that the third genome of the Vulgare wheat is homologous with 

 one genome of Aegilops, and has evidently been obtained by an inter- 

 generic cross, followed by doubling of the chromosomes to form the fertile 

 allopolyploid species. 



These data may be interpreted in the following way. The Einkorn 



SOS 



