True. But then, if we want to talk about the banana tree's part individually (though all), we use each and not every.

BBC clarifies it-

We tend to use each if we are thinking about members of a group individually, and every if we are thinking of them in total.

Compare the following:

We gave each child who came to the party a present. We handed them out one by one.
We gave every child who came to the party a present We gave them all a present.

True. But then, if we want to talk about the banana tree's part individually (though all), we use each and not every.

BBC clarifies it-

We tend to use each if we are thinking about members of a group individually, and every if we are thinking of them in total.

Compare the following:

We gave each child who came to the party a present. We handed them out one by one.
We gave every child who came to the party a present We gave them all a present.

Found the exact phrase from the book Hawaiian Heritage Plants where it says each part is usable one or the other way.