This question about ears of corn and ears for listening is
interesting. It turns out that both words have been used in English
for a long time, nearly 1000 years. The OED lists quotes for both
from years around 1000 AD. It also seems that they are independent.
Ear for hearing comes from Latin, auris meaning ear for hearing, and
this ear has given rise to all-sorts of related and modified
meanings, eg "dog-eared page". The ear for grain (can be used for any
cereal) comes from the Latin for acer meaning sharp. This root
apparently also gave rise to "awn", another part of the cereal seed.
The idea that ceral inflorescences are sharp seems also to be
preserved by calling them spikes. The ear as in ear of corn seems to
be used rather narrowly only for that meaning, with few modifications
listed. But anyway ear of corn and ear for hearing seem to have
converged on the same sound, and not by shared meaning.