67 



The combinations which are theoretically possible when sorts of this 

 constitution are crossed, include all the different types which have actually 

 been found by the above author to arise in practice. Thus, four of the above 

 combinations (L t 1 2 X lj 1 2 ; l t 1 2 X L t 1 2 ; l t L 2 X l t 1 2 ; and l t 1 3 X l t L 2 ) 

 should give a simple 3: 1 proportion i.e., three long-headed types to 1 Pudel 

 type. Four combinations should give 15 long-headed forms to 1 form ap- 

 proximating the type of Pudel: seven combinations should give only long- 

 headed forms (longer and more open-headed than Pudel) and these should 

 represent the longest heads of all produced in this crossing, which also was 

 found to be the case. 



The assumption that two ' lengthening ' factors are possessed 

 by Compactum seems, from the above and other crossings which might be 

 cited, to be quite justified. It is also believed probable that in addition to 

 these two distinct factors which account for the sharp segregation of char- 

 acters, others may exist which produce a secondary or bi-segregation 

 " Nebenspaltung " That such segregation may obtain in connection with 

 other characters involved in a given crossing is believed both by Tedin and 

 Nilsson-Ehle as well as by other investigators to be the case. 



In crossing Compactum (C L t L 2 ) with a Land wheat of the constitu- 

 tion c L x L 2 the segregation must naturally be the simple 3 Compactum: 1 

 Land when Compactum is dominant. When, on the other hand, a Com- 

 pactum of the Constitution C l t 1 2 is crossed with a Land wheat c L t L 2 

 then must arise forms of the Constitution c l t 1 2 (Squarehead type) as " new " 

 entities. This would seem to explain the frequency with which Squarehead 

 types arise out of crossings between certain Compactum and " Land " types, 

 the appearance of Squarehead being due simply to a new combination of units 

 already in existence. 



From the crossing Com.pactum (C L t L 2 ) X Squarehead (c lj 1 2 ) there 

 must arise the 'new' combination C l t 1 2 which lacks both 'lengthening' 

 factors and which must therefore possess shorter internodes than the parent 

 C Lj L 2 (Compactum). 



From the above investigation there emerges the one outstanding obser- 

 vation that the Compactum factor C in the Compactum sort used 

 in the crossings in question, restrains or "inhibits" the L factors so that 

 C L! L 2 and C l x 1 2 may in outward appearance resemble the genuine Com- 

 pactum. While it is not believed that the effect of the L factor is completely 

 suppressed by the presence of C, it is thought to be very greatly reduced. 



Of all the different sorts used as parents in these investigations only 

 T. Compactum (Type I), proved sufficiently well defined and constant in 

 character to enable it to be fairly recognized as a distinct type. The grada- 

 tions between the types which came next in denseness Types II and III, 

 (under which the Squareheads are classed) and the extremely open 

 headed " Land " wheat (Type VII) was found to be so gradual and the lines 

 of demarkation between them so confused by fluctuating modifications, that 

 it was quite impossible to group them into sharply defined types as was 

 attempted at Svalof at an earlier date. Any grouping of types which may 

 be attempted must therefore be a purely artificial one. 



