54 BULLETIN" 1489, IT. S. DEPARTMENT OF AGRICULTURE 
characters may be isolated for use as selection stocks. It has been 
abundantly proved that strains can be obtained in this way which 
breed true for differences in physiologic functioning in relation to 
temperature (17), moisture (W), and soil nutrient requirements (31), 
as well as for those conditions or processes which may determine 
the difference between resistance and susceptibility to various disease 
organisms (33). Finally, the yields of crosses between selfed lines 
so far reported show conclusively that increased acre yields of corn 
ranging from 20 to 30 per cent or more can be obtained in many 
cases. The comparisons have not been as extensive as they might be. 
Nevertheless, taken in connection with the soundness of the under- 
lying theory, they have been striking enough and consistent enough 
to establish selection within selfed lines as the basis for present-day 
methods of corn improvement. 
Breeding corn by selection within selfed lines always will be ex- 
perimental. Only a few of the lines or combinations resulting from 
the breeding program will represent improvement, and these will 
have to be determined by experiment. Consequently, breeding by 
selection within selfed lines is primarily for the trained plant 
breeder working at the State agricultural experiment stations or with 
associations of farmers or with seedsmen. Few farmers have the 
time or facilities to devote to the intensive and extensive labor of 
hand pollinating and testing over a period of years that is required 
for a comprehensive corn-improvement program of this kind. Occa- 
sional individuals having the means and inclination may find corn 
breeding by these methods an interesting and possibly a profitable 
occupation. They will be the exception. The real burden of finding 
the best combinations will and should fall on the agricultural experi- 
ment stations. These agencies have the necessary facilities for select- 
ing the lines and for comparing the crosses carefully so as to insure 
that only those which really are superior are distributed for com- 
mercial use. 
The function of the farmer will be in the annual production of 
crossed seed of combinations that have been shown to be superior. 
A number of essentials are involved in successfully maintaining the 
individual lines, crossing them in one way or another, and insuring 
that the seed produced is of good quality and germination. Reserve 
stocks of the selfed lines must be maintained to guard against crop 
failure. There must be no question as to which lines or crosses are 
which, either in storage or in the field. Tassels must be removed 
from all pistillate parent plants before they have shed any pollen. 
Finally, the commercial seed must be harvested and handled at a 
time and in a way to insure that it will retain its full vigor of 
germination. The successful accomplishment of these essentials re- 
quires an uncommon willingness and ability to do the right thing 
at the right time. To those who have this willingness and ability the 
commercial production of seed of crosses between selfed lines should 
offer an attractive and remunerative field just as soon as combinations 
that are outstanding for their localities are available. 
The methods of corn improvement based on selection within selfed 
lines should be recognized as still in an experimental stage. Little 
is known as to what makes some lines yield more, in crosses than 
others. Consequently the better lines must be determined largely by 
