328 LINDSTROM, GENETICAL RESEARCH WITH MAIZE 
A second reason for the high variability in this species arises from the 
fact that maize is naturally cross-pollinated (both by wind and to a less 
extent by insects). This permits any recessive characters or mutants 
(and the latter do occur) to persist for an indefinite period of time des- 
pite their usual, defective nature. Accordingly when inbreeding is com- 
menced they appear in surprising numbers furnishing excellent mate- 
rial for genetic study. 
The monoecious nature of maize renders artificial self- or cross-poll- 
ination relatively simple. A single pollination at the proper time produ- 
_ces from 200 to 900 seeds on the ear. Often the plant bears two ears 
one of which may be selfed and the other crossed. The maize plant 
has practically no serious pests or enemies, and is easily grown to 
maturity. 
Against these advantages there is perhaps one disadvantage from the 
genetical standpoint. The chromosome number is twenty. For linka- 
ge studies ten pairs of chromosomes provide a cumbersome basis for 
such work. However, the chromosome mechanism seems to be remar- 
kably stable, a distinctcontrast to the Oenothera situation. 
There is another possible disadvantage from an evolutionary view- 
point. Maize is really the only true species in its genus, a fact which li- 
mits the field somewhat. However, the successful generic cross of Zea 
mays x Euchlaena mexicana (teosinte) provides an excellent lever for 
determining new principles of inheritance. 
The economic importance of the corn crop in the United States has 
stimulated an enormous amount of plant breeding experimentation. 
This has provided a constant source of genetic problems. A fundamen- 
tal and satisfactory solution of practical corn bréeding has not yet been 
derived however. With a naturally selffertilized crop, pedigree selection 
is satisfactory, but not so with a naturally cross-fertilized crop like 
maize. The ever present problem of inbreeding has prevented real 
SUCCESS. 
A vast amount of genetic research with maize has resulted in the iso- 
lation of nearly one hundred allelomorphic pairs of characters. The mo- 
de of inheritance of most of these is definitely known. Linkage studies 
are gradually placing these characters in groups. Already eight groups 
of linked factors are known. The knowledge that has been gained by 
this genetic analysis in maize is affording some very critical evidence of 
the problem of inbreeding. 
