Why is it so?

A little while ago I was reading a joke about Martha Stewart vs Maxine (the grumpy old woman) and there's a bit that goes:

Martha: wrap celery in aluminium foil when putting it in the fridge and it will stay crisp for weeks.

Maxine: celery, what's that?

Now I'd never heard of that so I thought I'd give it a go and it actually works but I can't find any info as to why it works. What is it about foil that keeps celery crisp? Does it work for other veggies? Enquiring minds want to know.