372 



THE WHEAT PLANT 



8-33 mm., representing the extracted polonicums, heterozygote inter- 

 mediates, and extracted durums respectively. 



The frequencies of the three types agreed with those found for the 

 glume-lengths of the F. 2 of the same cross (p. 396), which suggests a i : 2 : i 

 ratio. 



No grains were found as long as those of the grand-parental polonicum 

 and none as short as those of the durum ancestor, an inward " shift " 

 towards the mean of the two grandparents having occurred similar to that 

 described for glume-length (p. 396). 



The length of the grains of the extracted homozygote polonicum 

 of F 2 and F 3 was 13-14 per cent less than that of the original polonicum 

 parent, the F 2 durum forms being about 8 per cent greater than those of 

 the original durum. 



GRAIN COLOUR. In respect of colour, wheat grains are usually divided 

 into the two classes red and white. In the former a reddish or brownish 

 substance is present in the testa of the seed and absent from the seed-coat 

 of white grains. 



One or two Abyssinian varieties (pp. 195, 196) of wheat have purple- 

 coloured grains, but in these the colour is due to an anthocyan pigment 

 in the " chlorophyll " or " cross layer " of the pericarp. 



i. White x White. The usual result of crossing two white-grained 

 wheats is the production of white-grained descendants in Fj and all 

 subsequent generations. 



Vilmorin, however, obtained red-grained plants among the F 2 progeny 

 of a hybrid between two white-grained forms of T. polonicum. and T. 

 turgidum (p. 397). 



Pitsch also records the appearance of a few yellow- or reddish-grained 

 plants in the sixth generation from the hybrid " Squarehead " x" Chal- 

 lenge," which he states were two white-grained wheats. 



ii. Red x Red. The progeny from this cross is generally red in all 

 generations. 



Tschermak, however, states that in some instances white-grained 

 individuals have appeared among the descendants of hybrids between 

 different red-grained varieties. 



Nilsson-Ehle also records the occurrence of white-grained plants in 

 F 2 of the hybrid " Bore " wheat x" Extra Squarehead," both red-grained 

 parents. 



He suggests that two independent and different factors for red (Rj 

 and R 2 ) are concerned, the respective uniting gametes being R x r 2 and r x R 2 . 



From such a cross, white individuals (r x r 2 ) would appear in F 2 , in the 

 proportion of i white to 15 red. 



iii. Red x White. Very numerous investigations have been made 

 regarding the hybridisation of red- and white-grained wheats. 



