16 BULLETIN 869, U. S. DEPARTMENT OF AGRICULTUEE. 



appeared to give homozygous dense progeny-. Of the entire 22 

 plants, representing all types of F 2 densities, nine proved about as 

 variable in F 3 as the F 2 generation. 



Seven F 3 selections which appeared to be breeding true, as deter- 

 mined by the frequency distribution and coefficient of variability, 

 were tested in the F 4 generation. This was done by selecting 10 heads 

 of different densities and growing the progeny of each separately. 

 Where all heads gave similar results, they are combined in the table 

 and are given as the result of 10 plants. 



The F 3 line 325-5, of which only 26 plants were available for study, 

 gave a mean of 3.15 ±0.02 mm. in 1917, with a low coefficient of 

 variability. On testing this line in 1918, when data from 213 plants 

 were available, a somewhat higher mean was obtained, or 3.43 ±0.01 

 mm. Its coefficient of variability is also somewhat larger than in 

 the homozygous parental forms. Selection 325-15 proved pure in 

 F 4 with the exception of the progeny of one plant which gave as 

 great a variability as the F 2 generation. Why one plant should 

 behave so differently from the nine others is difficult to explain. 

 The possibility of a natural cross must not be overlooked, although 

 observations show that these are very infrequent. An occasional 

 error is also a possibility, although precautions were taken to elimin- 

 ate these as far as possible. 



The F 4 means for the seven lines which gave evidence in F 3 and F 4 

 indicating that they were homozygous are as follows: 325-5 (10 

 plants), 3.43 ±0.01 mm.; 325-13 (10 plants), 3.47±0.01 mm.; 

 325-16 (9 plants), 3.74 ±0.01 mm.; 325-18 (10 plants), 2.24 ±0.01 

 mm.; 325-20 (10 plants), 2.47 ±0.02 mm.; 325-21 (10 plants), 

 3.95±0.01 mm.; 325-22 (10 plants), 3.72±0.01 mm. 



Of these, five have mean densities which are not very different 

 from that of the Jet (lax) parent, while the means of the other two 

 are similar to that of the Pyramidatum parent. The most dense 

 and the least dense of the five lax homozygous segregates have mean 

 internode lengths of 3.43 ±0.01 mm. and 3.95 ±0.01 mm., respectively. 

 As great a difference as this in any one season would not be expected 

 in a sort homozygous for similar characters. It is not much greater, 

 however, than seasonal variation in the means of several of the pure 

 2-rowed forms, which seem more susceptible to such variability 

 than the 6-rowed parents. Inheritance of such a reaction difference 

 might possibly explain the results here represented. Whatever expla- 

 nation may be given for these new means, here, as in the Man- 

 churia X Svanhals cross, no homozygous forms were produced 

 which differed materially in density from the density of one or the 

 other parent. 



