Which is true, it is essential, just some plants dont use it themselves.
Holoparasitic Plants
Holoparasitic plants are parasites like the semiparasitic plants which means they have to be directly connected to an autotrophic host plant. But these plants have no green parts and can't do any photosynthesis. They have to get all their nutrients, energy, water, and carbon from the host plant they are attached to. So holoparasitic plants don't use carbon dioxide and don't release oxygen into the air.
The Plants Formerly Known as Saprophytes
The other type of plants that don't use CO2 are the parasitic plants we use to call saprophytes. These plants are now called
myco-heterophytes. They have a fascinating connection to the trees that feed them. A fungus, called a mycorrhizal fungus, connects the myco-heterophytes to the tree and transfers the nutrients from the host plant to the parasitic plant. The host plant does all the photosynthesis work to make food, then "shares" some of it, unwillingly no doubt, with the parasitic plant. The fungus feeds the parasite. I assume the fungus feeds itself also.
Dr G
http://www.ftexploring.com/askdrg/askdrg6.html
