I read it in an old book. But it doesnt say why. Could someone please explain? Thanks.

Floating plants should be fine.

I tie stem plants (e.g. camboba, anacharis, hornwort) to small pieces of slate. and sink that in the gravel at the edge of the UGF. That way they stay in place and I can move them to vacuum the gravel. In one of my tanks, I have a bunch of java fern tied to a large piece of slate. The large piece of slate is perched up on two smaller pieces of slate so most of it's surface area isn't directly on the gravel (isn't impeding water flow).

I don't think it's a good idea to have rooted plants in the UGF gravel, as you need to vacuum the gravel weekly to keep gunk from building up in it. If it gets areas where there water isn't flowing thru the gravel, those areas can get anaerobic and release hydrogen sulfide gas (which is toxic). The rooted plants wouldn't be happy being moved weekly. You could pot them. Some people use plain gravel for the substrate in pots, others use flourite. I use topsoil with a gravel overlayer. You can move them to vacuum underneath. There's some pics on my website below.

"The method of under gravel filteration with the discharge suck up from below the substrate has some detrimental effect. Firstly, substrate fertilizer cannot be used. Secondly, once the bottom sand start to age, it will no longer be efficient as a filter media, and lastly, as the plants grow, their roots will interfere with the flow of water through the substrate. However, this method is actually effective only in the early stages of the planted aquarium. It brings oxygenated water directly to the plants' roots and evens out the temperature difference between substrate and water. Though this improves growth in the early stages, in fact it only brings the aquarium closer to the problems that will develop later. Of course, this principle does not apply to the aquarist who changes the layout frequently."

quoted from:

Aquajournal