I’m getting tired of watering all of my housplants. Would it be possible to set up an IV drip system that would eliminate wasteful watering? I mean, putting the needle directly into a root. Would that work?
No. But there are systems known as “drip-feed” that slowly trickle water into the plant’s soil. They are kind of tricky to set up as the flow is very slow and you have to adjust it perfectly, in accordance with temperature and humidity. It really isn’t practical for home use.
Sure, you could hook up a plant with some ivy. Just plant some around the base. 
But otherwise no, except in terms of the “slow drip” mentioned above.
A few weeks ago, I remember reading something in the paper, I think it was in the Hints from Heloise column, about slow-watering plants. The method was to take a thick cotton rope and place one end in a container of water and the other end buried in the soil by the roots of the plant. Apparently the plant can draw the water from the container through the cotton rope (I’m pretty sure it has to be cotton and not a synthetic material). I’ve not yet tried this but the source seemed reliable.
I dont see why you couldnt hook up a plant to an IV – all ya have to do is find a way to place a small cath directly into the plants vascular system…go for it! 
I’ve definately heard of the “rope trick” that d98 describes – more than once in fact – so I suspect it works.
As for the literal IV concept that some of you are scoffing at – don’t be so hasty. I’ve seen trees that are suffering from Dutch Elm disease (or something of that sort) that have tubes full of nutrients (or maybe even antibiotics???) drilled into their trunks. Just like an IV.
I will back sty on this one. Plants have vascular systems. Of course, for the purpose of watering, the wet rope or a water retaining additive to the root area seems less risky.
And of course, some species are forced to give blood every spring. Then, we boil it down, and DRINK it! Sugar maples have every right to find us a disgusting species.
To treat Dutch Elm Disease, it’s fungicide. Other stuff may be used for other purposes. An article on injecting stuff into trees:
http://www.ext.vt.edu/departments/envirohort/factsheets2/tree/aug93pr3.html
I should have said to PREVENT Dutch Elm Disease - the fungicides are mostly preventitive measures, according to that article.
I will reiterate: you can NOT stick an IV into a plant. There is nothing to stick it into. Plants stems and roots don’t have veins and arteries like animals, they have thin layers of cells that move water by osmosis. And you wouldn’t get very far with sticking an IV into a leaf vein, because water only goes UP the plant and is aspirated into the air through the leaves, you’d just end up providing water for the one leaf.
You can inject chemicals into a plant. But that is more like putting a pill into muscle tissue (to continue the human analogy) or perhaps, like an implant like Norplant. Plant injections like the fungicide cited, operates by diffusion, not by injection into “veins.” Usually they just drill, place the fungicides in the hole, and then cork it back up.
As far as maple syrup tapping, it isn’t tapping into any vein. The wound in the tree disrupts the osmotic system and the sap “leaks” from the surrounding tissues. It might seem like you’ve tapped into a vein, but you really aren’t, it’s just normal osmotic flow. Osmosis can move HUGE amounts of water, slowly, but with huge volume.