THE TROPICS. 
317 
We can test the proportions of the different forms of 
gametes produced by a heterozygote by crossing it—as 
Mendel did—by a pure recessive. Thus Aa (gametes A and 
a) crossed by a will give Aa+aa, appearing under the 
forms A and a in equal numbers, if the theory is correct. 
Example IV .—A number of white grains of Flint corn (Zea 
Mays)—of doubtful ancestry as regards colour characters— 
were sown near similar yellow and blue grains, and produced 
plants which yielded (F^ some 90 per cent, of white grains 
and 10 per cent, or less of scattered blue and yellow grains. 
The latter are doubtless cases of xenia due to cross polli¬ 
nation from the surrounding plants. 
A number of these yellow grains were sown in a plot in 
which pollination was effected by plants produced from the 
white grains of the same cobs. The latter proved to be true 
extracted recessives, since considerably less than 1 per 1,000 
of the whole number of grains produced by them (F 2 ) were 
yellow or blue, and these are to be accounted for by the 
escape of pollen from the cross-bred plants grown in the 
same plot. The tassels were carefully removed from all the 
plants on this plot (other than the white-grained recessives) 
previous to the opening of the male flowers, but a little 
pollen appears to have escaped from one or two plants ; not 
enough, however, to have had any appreciable effect upon 
the numbers here considered. Nine plants, offspring of 
yellow grains, produced the following (F 8 ) :— 
