230 BOTANY 



clear that the duphcation or multipHcation of chromosome 

 number has played a large part in the evolution of the species of 

 wheat and oats and is of the greatest use in agriculture. 



The hexaploid species of both oats and wheat behave cytologi- 

 cally and genetically like simple diploids, i.e., when the germ-cells 

 are formed, the 42 chromosomes pair to give 21 bivalents. It has 

 usually been assumed that the hexaploid species have been 

 formed by the reduplication of the chromosome set of one original 

 diploid species, i.e., by auto polyploidy, and that the present wide 

 diversity of hexaploid cereals, as well as the differentiation of their 

 chromosomes which causes them to form 21 pairs instead of seven 

 sets of six, has been brought about by gene mutation subsequent 

 to the doubling. According to Huskins this is an improbable 

 state of affairs, and their diversity and behaviour are just what 

 would be expected if they had arisen by allopolyploidy. The 

 hexaploid species have probably arisen by two steps. In the first 

 stage there was hybridisation of two diploid species followed by 

 chromosome doubling to produce a tetraploid form and then the 

 hybridisation of this or similar tetraploids with diploids. This 

 again, followed by chromosome doubling, led to the hexaploid 

 species. 



Nilsson-Ehle and other workers have discovered that different 

 varieties of wheat may have one, two or three pairs of factors for 

 red colour of grain. It has also been shown by Akerman that 

 three independent pairs of factors affect the development of 

 chlorophyll in oats. In wheat it appears that head-type, dwarfing 

 and chlorophyll development may be determined by one, two or 

 three pairs of factors, and that the production of awns and hairs 

 and the winter or spring habit of growth, by either one or two 

 factors. In oats there is good evidence of as many as three pairs 

 of factors affecting grain colour and ligule development and of two 

 pairs affecting pubescence and side or open type of panicle. This 

 is in marked contrast to the condition exhibited by barley and 

 rye, in which du23licate factors are rare and single factor differences 

 common. 



These duplicate factors may arise in a diploid either through 

 parallel gene nuitation in different chromosomes or through 



