4 BULLETIN 869, U. S. DEPARTMENT OF AGRICULTURE. 



was not used in any of the crosses, but is included because of an 

 inherited variation found in it. The form used is lax and differs 

 from nutans in having only rudiments of lateral florets. 



RELIABILITY OF EXPERIMENTAL METHODS. 



In this investigation the feasibility and accuracy of density deter- 

 minations were tested in many ways. The length of internode was 

 computed from the measurement of 10 internodes in the middle of 

 the spike. All measurements were taken in millimeters. 



To test the observational accuracy, the populations from wnich the 

 density of three parents was determined were remeasured after a lapse 

 of three weeks. The difference in the measurements of Manchuria 

 was 0.02 ±0.01 mm.; of Zeocriton, 0.04 ±0.01 mm.; and of Hanna, 

 0.12 ±0.02 mm. Differences as small as 0.2 mm. in means of varie- 

 ties, therefore, can not be demonstrated by the method used. As 

 seasonal fluctuations in the means often are as great as this, the 

 method of taking the data is sufficiently accurate. 



The internode measurement was taken in the middle of the spike, 

 not only because of the greater convenience, but because experiments 

 indicated that the internodes in this zone are less variable than in 

 other parts of the spike. Measurements were taken in different parts 

 of the spike on approximately 100 plants of each of the Zeocriton, 

 Pyramidatum, Manchuria, and Hanna parents. Where the spikes 

 were long enough, six different sections were measured, i. e., nodes 

 1-11, 3-13, 5-15, 7-17, 11-21, and the last 10 internodes toward the 

 tip. In Pyramidatum the measurements for nodes 7-18 and 11-22 

 could not be made. The means for these measurements, hi milli- 

 meters, were as follows: Zeocriton, 1.37, 1.47, 1.66, 1.81, 1.95, and 

 2.15; Pyramidatum, 1.98, 2.12, 2.17, and 2.15; Manchuria, 2.88, 

 3.13, 3.35, 3.42, 3.36, and 3.38; Hanna, 3.90, 4.17, 4.40, 4.47, 4.35, 

 and 3.90. 



The Zeocriton is the only variety in which there is a progressive 

 increase in internode length from the base to the tip. If the factor 

 or factors determining this progressive increase segregate in a normal 

 way, the progeny of a cross between this type and one in which this 

 peculiarity is absent or less pronounced, as in Pyramidatum, might 

 contain types easily mismterpreted. The mean of a pure recessive 

 for a main density factor might easily differ by 0.2 to 0.4 mm. from 

 the parent, due to the gain or loss of this marked progressive increase 

 of internode length found in Zeocriton. 



Contrary to results previously reported by Harlan (9), no change 

 in internode length due to the presence of sterile nodes was observed. 





