This is a really good question and something a lot of people usually don't think about. The answer is that all plant cells need oxygen to live, because without oxygen they can't perform aerobic respiration (respiration is the process of breaking down food to get energy). Of course you probably know that when plants perform photosynthesis, they combine water, carbon dioxide, and the sun's energy to produce sugar and oxygen. So the cells in the green parts of the plant, where photosynthesis is taking place, get all the oxygen they need from the oxygen produced by photosynthesis. So cells in the leaves and stems are okay. The trick is the cells down in the roots, where there is no photosynthesis. In most plants, these cells get their oxygen from air in the spaces between dirt particles in the soil (you'd be surprised how much empty space there is in the soil -- mostly because earthworms are always moving around, churning up the dirt). But for plants that live in soggy environments, that's not an option, because water holds a lot less oxygen than air does (we're talking about O₂ here, not the oxygen in H₂0).

So some wetlands plants have developed a tolerance for low-oxygen conditions, and a lot of them have really shallow root systems so they're as close as possible to the air. Mangroves are trees that live in saltwater lagoons, and they have evolved special roots, called pneumatophores (Greek for 'air carrier'), that act like snorkels for the roots. But most plants don't have these special tolerances and adaptations, which is why you can 'drown' your houseplants if you water them too much.

We always hear about how plants use carbon dioxide and give off oxygen as waste, and how animals(like us) use oxygen and give off carbon dioxide as waste, and how this is a very important cycle in nature.

This is all true,but we don't hear about as often is that plants need oxygen to survive, too.

Plant cells perform cellular respiration just like animal cells do, and this process requires oxygen. Plants like rice can grow in wetlands because they have air spaces between their cells, and they can move oxygen-rich air into those air spaces. The reason why farmers grow rice in flooded fields is because the rice will survive there, but many weeds will die from lack of oxygen because they don't have air spaces between their cells. If you seal a plant in an airtight terrarium, it will still grow because it can recycle both the oxygen and the carbon dioxide that it uses. If you could magically remove all oxygen from the terrarium, though, the plant would die.

Plants do need oxygen to survive. They respire (take in oxygen, give off carbon dioxide) the same way that animals do. The difference is that during the day, plants also perform photosynthesis, in which they take in carbon dioxide and give off oxygen. In any given 24-hour period, a healthy, growing plant will give off a lot more oxygen than it consumes, and consume a lot more carbon dioxide than it gives off, so people usually don't think about plants needing oxygen.

Wetland plants need oxygen too. There are a lot of *bacteria* in wetlands that can live without oxygen, but all of the plants (vascular plants, mosses, even algae) definitely require oxygen.