16 BULLETIN 925, U. S. DEPARTMENT OF AGRICULTURE. 



chytic character with a partial failure of dominance or they may be 

 considered as instances where the segregation has been incomplete. 



In either event the well-defined difference between these brachytic 

 internodes and the adjoining normal nodes emphasizes the indi- 

 viduality of the node and internode as a metamer. If these cases are 

 looked upon as partial failures of dominance, it is significant that 

 instead of a general shortening of internode length throughout the 

 entire plant single internodes only are affected and become abruptly 

 and decidedly shorter than their mates. This fact indicates that no 

 general lengthening of the internodes is to be expected as the result 

 of contamination by repeated " back crosses " on plants of normal 

 stature. 



These cases would seem to offer some support for the hypothesis 

 of somatic segregation or perhaps bud variation. If such an ex- 

 planation is adopted, a question arises whether an ear borne on a 

 node subtended by a brachytic internode would behave in heredity as 

 a true brachytic. Xo such cases have arisen as yet, and there would 

 be some difficulty in distinguishing them unless the brachysm was 

 pronounced, as in the case shown in Plates IX and X. 



The internode borne on the same node with the ear is almost in- 

 variably shorter than the internodes immediately above and below, 

 so that to be detected brachysm of the internode at this point must 

 of necessity be pronounced. 



Although an ear borne on a brachytic internode might be ger- 

 minalry brachytic, it of necessity must be fertilized by male gametes 

 from an inflorescence produced from a normal node. The resulting, 

 seeds, therefore, would give rise either to plants of normal stature 

 only, or, if the parental plant had been heterozygous for brachysm, 

 one-half of the progeny could be brachytic. The point is of interest 

 in connection with the sudden appearance of braclrysm in the two 

 Chinese hybrids. 



The brachytic segregates in the F 2 of Boone-brachytic hybrids 

 had a mean height in decimeters of 10.12±;0.112, with a coefficient 

 of variability of 11.3±0.79 and a range of 7 to 15 decimeters. The 

 coefficient of variability of the brachytic segregates compared with 

 that of the 14 brachytic progenies shows that there was no signifi- 

 cant increase in variation in the segregates, the increase being only 

 1.2±:0.87. A somewhat larger increase is obtained if the coefficient 

 of variability of the brachytic segregates is compared with that of 

 the brachytic progeny grown in 1918; the difference then becomes 

 4.i ; zhl.l3. or 3.7 times the probable error, but since these progenies 

 were not grown the same year the propriety of comparing their 

 variability is questionable. TThile -the average height of the bra- 

 chytic segregates was greater than that of brachytic plants directly 

 descended from the original variety, the difference of 1.61 ±0.87 is 



