232 BOTANY 



and any 40 -chromosome progeny from them are usually dwarf and 

 sterile. If such plants occur in cultivated varieties, they must 

 reduce the yield to a certain extent. Their occurrence is directly 

 attributable to polyploidy, as in diploid species chromosome 

 aberrations are not of such frequent occurrence. 



Further support for the view that cultivated wheat and oats 

 are allopolyploids comes from the work of Tschermak and Bleier. 

 They investigated crosses between jEgilojps ovata x T. dicoccoides 

 and ^. ovata X T. durum (each of which has 28 chromosomes). 

 They obtained new fertile forms with 56 chromosomes, which 

 they have named Mgilotricum sp. Another point in favour of the 

 hypothesis that wheat and oats are allopolyploids is the fact that 

 most of their characters are found, in whole or part, in species with 

 lower chromosome numbers. This last can be explained by assum- 

 ing that parallel mutations have occurred in different species. But 

 hybridisation, followed by chromosome duplication as the evolu- 

 tionary mode of these genera, affords a simpler and more plausible 

 explanation. 



Gregor and Sansome have described, within the species Phleum 

 pratense L., two intersterile groups of ecological significance. 

 Within each a relationship was found to exist between the growth- 

 forms of a particular habitat and the environmental conditions 

 of that habitat. The authors are of the opinion that the survival 

 of these growth forms is dependent on their genotypic response 

 to the environmental conditions of the habitats which they 

 occupy. The cytological examination shows that Group I is 

 hexaploid (2n = 42), while Group II is diploid (2n = 14). Two 

 chromosome groups (2n = 14 and 28 respectively) have also been 

 found in P. alpinum L. The cross P. pratense 2n x P. alpinum 

 4n gave plants with a 3n number of chromosomes. These hybrids 

 were almost completely sterile, but gave rise to four hexaploid 

 plants. Group I x P. alpinum 4n gave one sterile 5n plant. 

 Fertile hybrids have now been obtained from the artificial hexa- 

 ploid X the natural (Group I) hexaploid. It is tentatively sug- 

 gested that Grou]3 I (6n) may be the result of natural hybridisa- 

 tion of P. pratense Group II (2n) with some other plant, in a manner 

 analagous to that for the artificially produced hexaploid, which 



