CORN" BREEDING 
45 
times. An extension of this idea to single and double crosses has 
suggested another possible advantage for the double cross. The 
plants of a single cross between long-time selfed lines are exceedingly 
uniform in time of blossoming and earing as well as in other ways. 
The plants of a double cross are less uniform. Any advantage that 
such variation might possess, therefore, would be favorable to the 
double crosses. 
So far there have been no critical comparisons between single and 
double crosses the parent lines of which have all been reasonably 
productive. The question as to which kind of crosses will yield more, 
therefore, is unsettled. Furthermore, the question probably will 
COMMERCIAL F/ELOS* ^lCXD)X(AXB^DXC)X(BXAl\ 
Y£ARS 
Fig. 8. — Diagram showing the method of producing single and double crossed seed corn. 
Seed of the single crosses A X B and C X D is obtained the first year from the two 
isolated single-crossing plats in which the A and the C plants are detasseled. Pure 
seed of B and of D also is obtained from these plats for use in the second and third 
years' single-crossing plats. The single crosses obtained the first year are combined 
into the double cross (C X V D) X (A X B) during the second year, the double-crossed 
seed being used for commercial planting the third year. No seed is saved from the com- 
mercial fields. By alternating the direction in which the single crosses are made each 
year, supplies of pure seed of all four lines may be maintained. The production of 
single-crossed seed requires the maintenance of only one single-crossing plat each year 
remain an open one for some time. The method to be used will be 
based on questions of economy, practicability, and the like. Both 
methods have the disadvantage that crossed seed must be produced 
anew for each season's planting by methods similar to those described 
under the discussion of varietal hybrids. Double crossing requires 
that three separate crosses rather than one must be made each year. 
Unless the double cross has outstanding advantages, therefore, it 
would seem that the single cross is more likely to find favor. 
The use of what have been called synthetic varieties (27) was 
suggested to obviate the necessity of using Fi crosses each year. A 
synthetic variety is one created by intercrossing enough selfed lines 
