46 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
The data, therefore, have been examined from the standpoint of 
upper and lower ears. 
Stadler (16) measured the rate of crossing over between and 
Wx on the upper and lower ears of 33 plants. Of these pairs, 20 
gave a higher rate in the upper ear, but the mean difference was less 
than 1 per cent and was not statistically significant. 
From the white- waxy seeds of Dh 416L3L1C5L3L1R23 there were 
eight plants each of which had two ears pollinated simultaneously 
with pollen from the same double-heterozygous plant. The data are 
presented in Table 27. 
The results show a significantly higher crossing over in the second 
or lower ear. The mean difference is 3.23 ±0.55 per cent, a difference 
of 5.8 times the probable error. The individual differences range 
from —0.3 to 6.7 per cent, but none depart significantly from the 
mean. 
The most obvious difference between upper and lower ears, when 
both are pollinated on the same day, is the difference in the length 
of silk at the time of pollination. That this is an important factor 
is indicated by the ears from two other plants in the same progeny. 
These plants also produced two ears, each pollinated by the same 
double-heterozygous plant, but in these cases the lower ear was 
pollinated two days later than the upper ear. No record was made 
of the length of silk, but in normal plants of this type the difference 
in length of silk between upper and lower ears would be more than 
compensated by the growth in two days. (Table 28.) 
Table 28. — Crossing over in upper and lower ears where the lower ears ivere 
pollinated two days later than the upper ears 
Upper ear 
Lower ear 
Difference 
(upper- 
lower) 
Parent plant designation 
Number 
of seeds 
Percentage 
of cross- 
overs 
Number 
of seeds 
Percentage 
of cross- 
overs 
5787X5999 
537 
256 
14. 1±1. 
19. 9±1. 7 
212 
811 
13. 2±1. 6 
16. 9i . 9 
0. 9il. 9 
5789X5941. 
3. Oil. 9 
Total 
793 
1,023 
2. Oi .50 
In both of these plants the usual order was reversed and there 
was a higher crossing over in the upper ear. The mean weighted 
difference of 2 per cent is four times the probable error, and the 
mean of these two ears departs from the mean of the eight ears pol- 
linated simultaneously by more than seven times the probable error 
of the difference. 
If delayed pollination of silks is reducing the crossover class 
in the male gametes, a corresponding difference might be expected 
between the tip and the base of the ear. No such difference was 
found in back-crossed ears with the male heterozygous. A mean 
difference as large as 2 per cent should have been detected in the 
material studied, but the ears are short and perhaps the silks from 
the tip and base of the ear do not differ in age sufficiently to effect 
a differential of this magnitude. Although progeny Dh 416L3L1C- 
5L3L1R23 would seem to furnish excellent evidence that lower ears 
