Why is it that the phloem gets black when exposed to radioactivity?

Answer 1

I'm not sure that it does do so normally. Though I suppose if you subjected it to enough radioactivity it would die, unlike the xylem which is already dead. So perhaps that's your answer there.

A lot of tests are done with radioactive materials to demonstrate flow in this or that direction in phloem. Pictures usually show black spots, but those are spots on the radio-sensitive FILM, not the phloem itself. So perhaps that is what you are thinking of.

If phloem did turn black on radiological exposure, I'd expect to find a lot of black trees over by Chernobyl. But instead I seem to recall that most of the plant life seems (quixotically) completely normal. So it goes.