ANIMAL AND PLANT BREEDING 323 



dunim-vulgare cross is highly infertile, as a result of irregular segre- 

 gation at meiosis, but the viable gametes tend to have chromosome 

 numbers near to the haploid numbers of either their durum parent (14) 

 or their /ulgare parent (21). By inbreeding or back-crossing, some 28 

 and 42 chromosome plants can be raised, and among the latter are 

 some (e.g. Hope)^ which combine general rust resistance with the high 

 yield and good quality of the vulgare varieties used. 



Some of the new wheat varieties discovered by the Russian collecting 

 expeditions may simplify this task considerably.^ For instance, they 

 describe a species Triticum macha which is like the usual 28 chromo- 

 some wheats but actually has 42 chromosomes, while there is a 28 

 chromosome form, T, persicum, resembUng the usual 42 chromosome 

 group. This T. persicum is claimed to be a tetraploid hybrid between 

 T. dicoccoides n = 14 and the grass Aegilops triuncialis n = 14. 



The breeding of disease-resistant varieties is now one of the most 

 important phases of plant breeding. In the past, entire crops have been 

 wiped out by epidemics. Coffee growing in Ceylon, for instance, had 

 to be given up after the attack by Hemileia on the Coffea arahica then 

 under cultivation; attempts are being made to introduce into the 

 valuable arabica types the resistance of C. canephora, which itself gives 

 a low-quality produa. The destruction of French vines by Phylloxera 

 attacking their roots is another well-known example of the devastation 

 wrought by disease, and in this instance it has hitherto been impossible 

 to combine resistance with quaHty, so that growers have had to graft 

 fine varieties on to the roots of the low grade but resistant American 

 species. 



The existence of different physiologic forms gives trouble in coping 

 with diseases in other plants as well as in wheat. Thus Salaman^ attempted 

 to make Phytophthera-resistant varieties of potato by bringing in fac- 

 tors from the species S. demissum, which is highly resistant (although 

 Phytophthera does not occur in its native regions; an example of 

 adaptation to non-existent conditions !). This attempt appeared success- 

 ful imtil a new strain of Phytophthera arose, against which the demis- 

 sum resistance was inoperative, when the work had to be begun again; 

 it is now once more beUeved that a resistant variety has been produced . 



In some parts of the world, as in Russia, the production of cold- 

 resistant forms is one of the most important problems facing breeders . 

 In this connection a theory of breeding has been developed by Lyssenko* 



1 McFadden 1930. ^ cf. Hudson 1937. 



^ Salaman 1934. * Cf. Anon. I935- 



