VARIABILITY IN LINKAGE OF CHARACTEKS OF MAIZE 53 
With normal corn plants the silks of the upper ear are the first 
to appear, and if the two ears are pollinated on the same da}^ the 
silks of the upper ears would be the longer. Accordingly, an effort 
was made to test this possibility by recording the length of the 
silks at the time of pollination and cutting back the silks of the 
upper ear until these silks were exceeded by those of the second ear. 
This program was followed on double-heterozygous and double- 
recessive plants and the ears back crossed. Paired pollinations were 
made simultaneously. If the length of the silk was the determining 
factor, it could come into operation only when the male parent was 
heterozygous, whereas if the length merely served as an index of 
age of gamete its influence would be shown when the female parent 
was heterozygous and the pollen all of one kind, namely, double 
recessive. 
The data for both heterozygous male and heterozygous female 
plants are given in Table 32. In examining this table it should be 
borne in mind that the differences shown are between upper and 
lower ears in each case, both ears having been fertilized by pollen 
from the same male parent, thus eliminating differences between 
plants. 
The minus sign is used to indicate those cases where the highest 
crossing over was found on the ears having the longest silks. 
The difference in crossover rates is smaller, though not signifi- 
cantly so, when heterozygous pollen is used than when the female 
parent is heterozygous, but it is apparent that length of silk either 
as such or as an index of age of gamete is not an appreciable 
factor in determining the percentage of crossing over. 
Eight pairs of ears are available on heterozygous female plants 
where there was no difference between the ears in silk length, and 
four such pairs are available where the male parent was hetero- 
zygous. Though the number of pairs in each case is small, they 
afford an opportunity to determine whether the rate of crossing over 
varies more in those pairs where differences in silk length were 
involved than in those where no difference in silk length existed. 
This comparison is made by squaring the differences between 
the pairs and dividing by theV2, since differences between ears 
and not the departure from their mean are involved. 
Where the female gametes are measured and a mean difference 
in silk length of 2.68 inches exists, the standard deviation is 
2.30 ±0.27 per cent, and where there was no difference in silk length 
it is 5.18±0.87. A similar situation is found with respect to the 
male gametes. Thus, where a mean difference in silk length of 2.2 
inches existed the standard deviation is 4.45 ±0.87, and where there 
was no difference in silk length it is 5.22±1.25 per cent. 
From these comparisons it seems clear that silk length is not a 
factor in bringing about differences in crossover rates. 
VARIATIONS IN METEOROLOGICAL CONDITIONS DURING ANTHESIS NOT A FACTOR 
IN RATE OF CROSSING OVER 
Plants of the progeny Dh 416L3L1C5L3L1R23 were flowering 
during a period of 15 days. During this period the mean daily 
temperature showed a fluctuation of 10 degrees F., and on August 17 
there was a rainfall of 0.91 inch, terminating a protracted drought. 
