VARIABILITY IN LINKAGE OF CHARACTERS OF MAIZE 63 
The behavior of a number of progenies for several generations 
shows that the rate of crossing over is often inherited, but this is 
not invariably true ; and it seems necessary to conclude that occasion- 
ally environmental factors profoundly modify the rate. 
A separation of the plants into those in which the aleurone factor 
R is homozygous from those in which this factor is heterozygous 
discloses the fact that plants heterozygous for R have a lower rate of 
crossing over than those homozygous for R. 
A comparison of the rate of crossing over in plants heterozygous 
for the sweet endosperm factor Su with the rate of crossing over 
in plants homozygous for Su indicates that this factor may be asso- 
ciated with the crossover rate. 
Plant reciprocals in 13 progenies show that the rate of crossing- 
over is not alike in the two sexes. In some progenies the rate is 
higher in the male and in others in the female. 
Crosses providing four classes of seeds but no linkage indicate that 
the proportion of the several seed classes is not alike in the 
reciprocals. 
Pollen stored for seven hours produces a much higher proportion 
of waxy seeds than the fresh pollen. Although stored pollen pro- 
duces waxy seeds in excess of equality, fresh pollen produces a signifi- 
cant deficiency of waxy seeds. On the other hand, the crossover per- 
centage is not altered as a result of storing pollen, even though the 
viability of the pollen is reduced by about one-third. 
Pollen produced early in the ontogeny of the plant has the same 
gametic proportions as that produced late. 
From a consideration of the rate of crossing over on ears varying 
in length of silk, it seems clear that the length of silk is not an im- 
portant factor in the survival value of the several gametic classes. 
In three progenies the crossover rate was not alike in upper and 
lower ears when the female parent was homozygous recessive. In 
two of these progenies the rate was higher on the lower ears and in 
the other the rate was higher on the upper ears. 
Ears in which the upper and lower halves were classified separately 
show a higher rate of crossing over in the tip than in the base, but 
this difference is found only when the female parent is heterozygous. 
In some progenies crossing over in the male is found to be corre- 
lated with the number of seeds resulting from the use of this hetero- 
zygous pollen on homozygous recessive plants, but this is not true for 
all progenies. Such a correlation indicates a lower survival value 
for the crossover gametes of the male. Measures of vigor other than 
number of seeds were not found to be correlated with the crossover 
rate, indicating that the condition of the plant has no effect on the 
frequency of crossing over. 
The distinction between factors that produce a measurable effect 
and those that do not depends in no small measure on the number 
involved in the experiments. With the factors here considered, the 
sizes of the populations varied widely in the several experiments, and 
there can be little doubt that if more refined tests were made many 
of the factors dismissed as being without effect would be found to 
change the crossover rate a small though significant amount. 
None of the factors considered provide an explanation of the 
occasional wide departure from the mean crossover rate. 
