HYBRIDISATION AND WHEAT HYBRIDS 365 



The numbers suggest the 63 : i ratio of a Mendelian three-factor 

 hybrid. 



LAX AND DENSE EARS. Extensive investigations have been made 

 regarding the inheritance of the lax open ears and the denser-eared 

 " Squarehead " forms of T. vulgar e as well as the short dense ears of T. 

 compactum. 



The researches of Nilsson-Ehle have shown that the lax character of 

 T. vulgare is often dependent upon two (or possibly more) independent 

 factors (Lj and L 2 ) over which the single factor (C) for short ear or T. 

 compactum is dominant. 



i. Lax x Lax. Hybrids of two lax-eared forms in which both factors 

 for laxity are present give rise to lax-eared descendants only. Where 

 the lax-eared wheats have the constitution L^ and 1 X L 2 respectively, in 

 the F 2 there arise plants (L x L 2 ) with more open ears than either of the 

 grandparents and Squareheads (l x 1 2 ) in which both factors are absent. 



ii. Lax vulgare x T. compactum. In most cases the compactum char- 

 acter is dominant and the F 2 segregates approximately in the ratio 3 

 dense-eared : i lax-eared. 



In some instances, however, the dominance is incomplete, and group- 

 ing into two classes is not possible owing to the existence of an almost 

 continuous series of intermediates of varying ear-density between the lax 

 and compact ears of the grandparents. 



For further discussion of these characters see pp. 401-405. 



COLOUR OF THE AWNS. The heredity of awn colour has not been 

 closely examined. It probably agrees with that of glume colour, black 

 and red being dominant to white and black dominant to red. 



The development of the dark pigment in the awn like that of the 

 glumes is greatly influenced by climatic conditions, a fact which makes 

 the determination of its inheritance specially complicated. 



PRESENCE AND ABSENCE OF AWNS. The possession of awns or beards 

 is a primitive character and is common to all races of wheats. In addition 

 to fully-bearded plants, so-called " beardless " wheats are abundant in 

 the four races T. vulgare, T. compactum, T. sphaerococcum, and T. Spelta ; 

 in these the awns are absent or reduced to i cm. or less in length. 



The following are the chief results of investigations concerning the 

 inheritance of the fully-bearded and beardless characters : 



i. Bearded x Bearded. The crossing of two bearded forms gives a 

 bearded l generation, and the character is continued in all subsequent 

 generations so long as the wheats belong to the same race. Where the 

 plants belong to different races the F x generation is also usually bearded, 

 but in some cases beardless as well as bearded forms appear in the F 2 

 and later generations : an example of the latter is recorded by Vilmorin, 



