In both crosses there is a decided deficiency of three- 
and five-ranked ears as opposed to two- and four-ranked 
ears. Also in both crosses there is a tendency for the 
three- and five-ranked ears to be twisted. They are ob- 
viously the result of a structural unbalance or asymmetry 
which induces stresses with visible effects. 
These phenomena, so conspicuous in maize-teosinte 
hybrids, are also readily discernible in maize. There are 
several explanations for them: 
1. Reeves (15) suggests that there is a preponderance 
of ears with rows of spikelets in even numbers because 
the basic unit of the ear is a pair of rows of spikelets 
rather than a single row. Thus an eight-rowed ear is 
much more likely to become a twelve-rowed ear than it 
is to change to a ten-rowed ear. He regards these units 
as hereditary and in a broad sense they undoubtedly are. 
It seems more probable, however, that the unit, a pair 
of rows of spikelets, is a developmental rather than an 
hereditary one for it is not uncommon in maize-teosinte 
hybrids to find two-ranked and four-ranked spikes on the 
same plant. This is not to say that the number of ranks 
is independent of heredity. There is abundant evidence 
that the contrary is true. But apparently the genes do 
little more than determine in a general way whether the 
spike is to be few-ranked or many-ranked, while develop- 
mental factors determine the size of the units by which 
the differences are to be attained. 
2. Any ear strongly influenced by Tripsacum is likely 
to exhibit a spiral phyllotaxy so rigid that only those 
combinations which involve complete spirals will be bal- 
anced and symmetrical and hence are more likely to oc- 
cur than those involving fractional spirals. This is per- 
haps no more than another way of saying that spiral 
phyllotaxy explains and accounts for the fact mentioned 
[ 65 | 
