Corn Investigations 
13 
20. A histological study of ten pure line hybrids and their 
pure line parents indicates that the increased growth of the 
hybrid has its basis in both increased size and increased numbers 
of cells. Thru hybridization (1) the stalk diameter was in- 
creased 46 per cent, (2) the number of vascular bundles 45 per 
cent. (3) the bundle diameter 15 per cent, (4) the number of 
pith cells along one stalk diameter 38 per cent, (5) the diameter 
of one pith cell 8 per cent, (6) the length of one pith cell 10 per 
cent. (7) the leaf thickness 14 per cent, (8) the leaf epidermal 
thickness 4 per cent, (9) the number of vascular bundles in one 
cm. leaf width was reduced 8 per cent, which suggests larger 
cellular development within the leaf, and (10) the average 
width of the leaf epidermal cell was increased 4 per cent, which 
suggests greater number as well as greater size of the cells, since 
the total leaf area per plant increased 45 per cent. As an aver- 
age for seven of these hybrids, the stalk volume was increased 
235 per cent, whereas the stalk pith cell was increased only 26 
per cent in volume. Approximately 90 per cent of the increase 
in plant .size due to crossing results from an increase in cell 
numbers and 10 per cent from an increase in cell size. 
21. During seven years, comparative yields are available 
for several different degrees of inbreeding as applied to ear-to- 
row strains. These degrees differ in the likelihood of related 
gametes being involved in the fertilization process. Four suc- 
cessive degrees of complexity in the gametic relationship re- 
sulted in yields of 16.8, 42.2, 49.2, and 54 bushels per acre as 
comjDared with 53.1 bushels for the original corn. 
In another five-year comparison with a different variety, 
seed of ear-to-row strains continuously subjected to different de- 
grees of gametic relationship yielded successively 22.3, 42.7, 
46.8, 51.3, and 64.8 bushels per acre as compared with 63.7 
bushels for the original corn. It becomes apparent from these 
tests that any selection or breeding practice which so restricts 
the breadth of the parental relationship as to increase the likeli- 
hood of identical Mendelian factors being paired upon fertiliza- 
tion is likely to give reduced yields. 
22. In previous statements concerning first generation corn 
hybrids, it was pointed out that in case of pure line hybrids 
great increase in production over the parents resulted, whereas 
this did not hold in case of hybrids between fully heterozygous 
commercial varieties. A similar situation was found in the im- 
mediate effect of crossing upon kernel weight. Both the embryo 
and endosperm of the corn kernel being subject to crossing, we 
