INHERITANCE IN THE BARLEY SPIKE. 3 



with spikes as lax or as dense as those of the parents, with a series 

 lying between these extremes which could not be satisfactorily classi- 

 fied without further test. In some crosses the F 2 generation curves 

 plotted from the measurements showed two peaks and in others three. 

 In a cross of zeocritonx nutans groups of plants were centered about 

 internode lengths of 2.2 and 3 millimeters, respectively. The 65 

 plants constituting the more dense group were tested in the F 3 

 generation by seeding all individuals with internode lengths ranging 

 from 1.8 to 2.6 millimeters. Of these 65 plants, 55 proved homozy- 

 gous and 10 were heterozygous. Thus, 55 out of a total of 209 plants 

 grown in F 2 bred true for densities near that of the dense parent, or 

 a close approximation of a 1 : 3 ratio. No genetic analysis is given of 

 crosses which appear to have three groups in F 2 , or lax, dense, and 

 intermediate forms. 



Study has been made of the inheritance of density in wheat and, 

 although apparently pertinent, it is not comparable to one made in 

 barley, for the reason that the dense wheats are clubbed at the tip 

 and thus introduce a condition which makes comparison difficult. 

 Gradations were found in F 2 between the parents. Nilsson-Ehle (20) 

 explained these on the basis of two kinds of factors, a positive factor 

 for compactness which partially inhibited the action of one or more 

 lengthening factors. Parker (21), in a more extensive study in which 

 the statistical method was used, concludes that numbers such as 

 Nilsson-Ehle used were inadequate to demonstrate his hypothesis. 

 In Parker's studies segregation occurred in F 2 , but it seemed impos- 

 sible to determine the number of factors involved. 



PURE-LINE VARIETIES USED IN THESE STUDIES. 



With the exception of the Jet variety, the pure lines used in crosses 

 in the studies here reported are quite typical representatives of the 

 three degrees of density much used by taxonomists in the 6-rowed 

 barley. Their relationships are most easily made apparent by use of 

 the taxonomic key which follows. The variations in density are well 

 shown in Plate I. 



KEY TO BARLEY VARIETIES USED IN DENSITY STUDIES. 



Hordeum vulgare pallidum (6-rowed, hulled, awned, white). 



Sub variety typica, spike lax, pure-line Manchuria. 



Subvariety parallelum, spike dense, pure-line Reid Triumph. 



Subvariety pyramidatum, spike very dense, pure-line Pyramidatum. 

 Hordeum distichon palmella (2-rowed, hulled, awned). 



Subvariety nutans, spike lax, pure lines Hanna and Steigum. 



Subvariety erectum, spike dense, pure-line Svanhals. 



Subvariety zeocriton, spike very dense, pure-line Zeocriton. 



Jet is a naked, black, 2-rowed barley of about the same spike 

 density as Steigum. Although Hanna and Steigum belong to the 

 same group, Steigum is slightly more dense than Hanna. Dejiciens 



