How does the leek jinx in Harry Potter respect Gamp's Law?

Question

In Harry Potter and the Prisoner of Azkaban, there is a passage about students having leeks growing out of their ears:

A number of small scuffles broke out in the corridors, culminating in a nasty incident in which a Gryffindor fourth-year and a Slytherin sixth-year ended up in the hospital wing with leeks sprouting out of their ears.

But is this not a violation of one of the "Principal Exceptions to Gamp's Law", stating that food cannot be conjured out of thin air?

Answer 1

Probably fairly well

First, keep in mind that Gamp's Law is from Harry Potter and the Deathly Hallows, and thus presents a later stage of Rowling's thinking than Prisoner of Azkaban.

That said, I think the leek spell is entirely consistent with Gamp's Law.

“Your mother can’t produce food out of thin air,” said Hermione. “No one can. Food is one of the first of five Principal Exceptions to Gamp’s Law of Elemental Transfigur—"

“Oh, speak English, can’t you?” Ron said, prising a fish bone out from between his teeth.

“It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some—”

—Harry Potter and the Deathly Hallows

Hermione says that it is impossible to produce good food out of nothing. This suggests that is possible to produce something that has the appearance of food, but which provides no nutrition. I suspect the leeks coming out of the students' ears were in that category. Just as, presumably, one cannot eat the birds that Hermione conjured in Half-Blood Prince, regardless of their resemblance to real birds.

Answer 2

I like the answer by @Jonah.

However, There is another possible explanation:

Creating food seems to require a source that is already nourishing. The human body certainly fits that category. So the leek might be fueled by the victim of the spell. That would make it transformed blood and/or flesh.