Question

how scientists prove that the food is transported through the phloem?

Answer 1

because the phloem doesn't contain nucleus and is Hollow .

Answer 2

by doing the experiment with aphid.
they had chosen the aphid which was sucking food from phloem through its probosis and the they cut the aphid and left the probosis in the bark of the tree when they took out the probosis from the bark they found that some honey like substance is stuck the probosis of the aphid but when they examined it they found that it is the glucose which was going down through the phloem.