65 



A significant conclusion arrived at by Nilsson-Ehle as a result of the 

 behaviour of certain crossings in which Compactum was concerned, is that 

 this sort possesses a special so-called ' inhibiting' factor which is absent in 

 the other sorts with which it was crossed. This factor, he believes, is largely 

 responsible for the extreme shortness of the internodes of this type. Thus 

 the Compactum form used in the above crossings is regarded as being in 

 reality a "land" wheat (common open-headed wheat of the country) 

 its compactness being due to the presence of a certain definite factor. 

 When a Compactum of this constitution is crossed with a Squarehead 

 sort, the crossing behaves quite in the same manner as when a visibly open- 

 headed Land wheat is crossed with Squarehead, or some other sort of 

 average head-density. 



In crossings between Swedish "Land" wheat and Squarehead, the long, 

 open type of the former is found to be dominant. In the F 2 generation this 

 type is therefore in the majority. By crossing Compactum and Squarehead, 

 the long open type again prevails instead of Squarehead. When the Com- 

 pactum factor is designated at C and the " lengthening " factor possessed by 

 the open headed Land wheat as L the crossing may be represented by the 

 following scheme: 



Compactum Squarehead 



C L c 1 



F 9 : Ratio 9CL: 3 C 1 : 3 c L: 1 c 1 



12 Compactum 4 "not-compactum" 



3 "land" wheat: 1 Squarehead 



That the character of the head of " land " wheat is actually distinguished 

 from Squarehead by two or more factors seems clear. 



Referring again to the crossing Compactum X Pudel, line B, of the 28 

 F 3 plants belonging to the " not-Compactum " group, only 2 approached 

 the average head density of Pudel while the remaining heads were longer. 

 This approximates closely the proportion 15: 1 which would be expected if 

 Compactum possessed two 'lengthening' factors which were absent in Pudel, 

 a situation which is regarded by Nilsson-Ehle, after a very careful analysis, 

 to be the case. These two " lengthening " factors he designates as L L 2 each 

 of which may act independently of the other and which in combination may 

 produce longer internodes than those possessed even by Pudel. 



This crossing is expressed in the following manner: 

 Compactum Pudel 



\j Ajj J_j2 ^ A i -1 2 



