Orchids differ from other plants because they pass through a prolonged seedling.  they acquire fixed carbon heterotrophically through their mycorrhizal fungal partner. Orhcid mycirrhizas are morphologically distinct distinct consisting of intracelleular hyphae that forms a interface between plant and fungi. There is possibility acholroflous fungi need Co2 for formation of acids which may start some metabolic pathway, otherwise there is no need Co2 may occur if photosynthesis is not occurring in the plant.

yes orchids get their carbon from their mycorrhizal symbionts whcih may get it from other plants (e.g. ectomycorrhizal). The association between orchids and mycorrhizal fungi may in some instances be considered parasitic, althouhg it can be argued that orchid roots provided a safe place for the fungal partner.

land plants and mycorrhizas evolved together; why do fungi still associate with orchids? basically the orchid tricks the fungi into the association, the fungi have not evolved a means to avoid orchid roots. Note that plants whcih are non-mycorrhizal (e.g. Brassicaceae) have evolved mechanisms to stop mycorrhizal fungi.

Dear colleagues,

I can add that orchids are not unique in its cheating. There are plenty of other mycoheterotrophic plants including non-flowering ones which lack photosynthesis and parasitize mycorrhizas to obtain its carbon from green mycorrhizal plants via fungus. The reasons for fungi not to avoid cheating from plants can be different. Some fungal genotypes are less resistant to cheaters (and monotropoid cheater plants are known to reduce sexual reproduction in "their" fungi to avoid appearing of resistant genotypes). Besides parasitic plant hosts fungi and can be useful for fungus in something else besides nutrition. And finally mycorrhiza is a wide net with multiple partners involved and cheated at one relationship can be a cheater in another one