Corn Investigations 
101 
thickness was 14 per cent greater, (11) the average thickness of 
the upper and lower epidermal cell was 4 per cent greater, (12) 
the number of vascular bundles in one centimeter cross section 
of the leaf was 8 per cent lower, (13) the width of epidermal 
cells on the upper and lower leaf surface averaged 4 per cent 
greater. This, in view of a 46 per cent increase in leaf area, 
suggests a larger increase in cell numbers than in cell size. 
Fig. 23. — Cross section of corn leaf taken midway between midrib and 
margin. Arrangement of cells and vascular bundles is shown. Some 
histological leaf measurements for corn hybrids and their inbred 
parents are given in Tables 38 and 39. 
From the above comparisons it may be concluded that cross- 
ing of homozygous (pure line) plants is accompanied by a 
marked increase in both size and number of histological units. 
The homozygous plants appear to be more sluggish in their 
physiological activities than are heterozygous (hybrid) plants, 
since it takes practically the same length of growing period to 
mature a much smaller number of cells which are also smaller 
in size. 
An interesting point in connection with heterosis, or in- 
crease of growth resulting from crossing pure lines, is, how 
much of this increase is due to increase in cell size and how 
much to increase in cell numbers. 
Most of the cells in a corn plant are so irregular in shape 
and so irregularly arranged that it is very difficult to determine 
their relative sizes. Therefore most of the cell measurements 
were confined to pith cells of the stalk and epidermal cells of 
the leaves. 
