cannot a bhikkhu receive food offered by bhikkhuni?
In Patidesaniya, one section in the Patimokkha, we find
such a prohibition. Checking in the Vibhanga, where we learn
the historical context of the rule, we found an interesting
story. An elder bhikkhuni of 120 years old went for aims in
the city at the distance of 4 to 5 kms. Upon her return a young
monk was waiting with his empty bowl. Out of respect for monks
as prescribed in the Garudharma, she reverently offered him
her alms received for that day. The young monk got an idea of
not having to go all the way for alms himself and received alms
from the same nun on the following day also. On the third day,
the bhikkhuni went for alms in the city. While roaming in the
city a chariot passed near her path. She took a step aside,
fell down and fainted. The millionaire who was riding that chariot
came out to make inquiry and learned from her that she fainted
out of hunger and tiredness, as she had not eaten for three
days. Upon learning the reason the millionaire criticised the
young monk and later brought this to the attention of the Buddha.
From then on, to protect the nuns from being taken advantage
of, the Buddha laid down the rule for the monks not to receive
alms from bhikkhunis.