22 BULLETIN 1489, U. S. DEPARTMENT OF AGRICULTURE 
their inheritance in this plant. There is ample evidence, however, to 
indicate that such characters as resistance to corn smut (35, 73), to 
the root and stalk rots (32, 33), to toxic soil conditions (31), and 
the like are transmitted exactly as are more obvious characters. 
SIZE CHARACTERS 
Striking differences in size such as those between dwarf and normal 
plants have been considered under " Abnormalities." The present 
group includes only such differences in the size of plants and plant 
parts as occur among ordinary plants. These differences, in general, 
are made up of many smaller differences each of which may be in- 
herited independently of the others. For example, one strain of corn 
may be taller than another because its plants have more nodes (joints) , 
because some or all of the internodes are longer than the correspond- 
ing ones in the other strain, or for both of these reasons. The in- 
ternodes may be longer because more cells are formed, because the 
individual cells are larger, or because there are both larger cells and 
more of them. Finally, there may be more cells because cell divi- 
sion is more rapid, because it continues over a longer time, or be- 
cause it both continues longer and is more rapid. All of these con- 
ditions are determined in their inheritance by genetic factors, some 
of which may be entirely independent and some of which may be 
interrelated. This is but a simple statement of the more obvious 
heritable characteristics that may affect the size of the plant. Only 
when it is remembered that the expression of such a complex of 
characteristics is limited finally by an ever-changing environment 
can the difficulties of analyzing size inheritance be appreciated. 
In spite of these complexities, it has been possible to show by 
careful research that a number of size characters are inherited just 
as would be expected if they were determined in a manner entirely 
similar to that of other characters. There is no reason, therefore, 
for considering the inheritance of size characters as distinct from 
that of the other groups except as a matter of convenience (21). 
The important point is that all of the characters of size and func- 
tion that affect yield are controlled in their inheritance in the same 
way as are the simpler, more obvious characters of seed coloration, 
form, and the like. Characters affecting yield are far more complex 
in their interdependency and as a rule can not be dealt with indi- 
vidually ; but fundamentally inheritance is the same : Mendelian. 
HYBRID VIGOR AND PRODUCTIVENESS 
The decrease in vigor and yield that follows inbreeding and the 
increased vigor that so frequently follows crossing have long been 
recognized in corn. It is only recently, however, that a satisfactory 
explanation of these phenomena has been forthcoming. This ex- 
planation rests upon the principles of Medelian inheritance and the 
fact that, in general, dominant genetic factors determine characters 
more favorable for growth than do their respective recessive allelo- 
morphs. 
In the preceding pages, when there was any difference between 
two allelomorphic characters, the character determined by the domi- 
nant allelomorph was more favorable to growth than its contrasted 
