I did have to read it a couple times to make sure I was answering ya right mate, but it's all good!
It's actually a good question. If you can get stuff small enough to fit through a channel, be it at the roots or through the leaves, it will be "in the plant". What you can get through there and the effect it then has is way beyond the scope of my knowledge
I remember reading a while back people giving aspirin to their plants for some nutty virus I thought I had. Interesting the amount of people saying it helped the recovery (maybe just a placebo effect on the owners haha). Weed killer is another good example. The plant unknowingly sucks that in and is dead in no time from whatever the stuff does to destroy it. That's the down side. It's basically like flood gates and anything that can fit will fit and get through. There are gazillions of membranes in the human body, no idea how many plants have but there'd be a lot. Once they're through though (molecules, chemicals, parasites, viruses, etc...), it's game on.