ents ▁form ▁dead ▁cells . ▁The ▁key ▁nut ri ents ▁involved ▁in ▁the ▁majority ▁of ▁the ▁transfer ▁between ▁fung i ▁and ▁orch id ▁plants ▁are ▁carbon , ▁nit ro gen ▁and ▁ph osph orus . ▁ ▁Or ch id ▁my cor rh iz al ▁interactions ▁are ▁unique ▁in ▁the ▁flow ▁of ▁nut ri ents . ▁Typ ically ▁in ▁ar bus cular ▁my cor rh iz al ▁interactions ▁the ▁plants ▁will ▁un id irection ally ▁supply ▁the ▁fung i ▁with ▁carbon ▁in ▁exchange ▁for ▁ph osph orus ▁or ▁nit ro gen ▁or ▁both ▁depending ▁on ▁the ▁environment , ▁but ▁orch id ▁my cor rh iz al ▁nut ri ent ▁transfer ▁is ▁less ▁specific ▁( but ▁no ▁less ▁reg ulated ) ▁and ▁there ▁is ▁often ▁bid irection al ▁flow ▁of ▁carbon ▁between ▁the ▁fung us ▁and ▁plant , ▁as ▁well ▁as ▁flow ▁of ▁nit ro gen ▁and ▁ph osph orus ▁from ▁the ▁fung us ▁to ▁plant . ▁In ▁around ▁ 4 0 0 ▁species ▁of ▁plants ▁there ▁is ▁no ▁flow ▁of ▁carbon ▁from ▁plant ▁and ▁all ▁of ▁the ▁nut ri ents ▁of ▁the ▁plant ▁are ▁supplied ▁by ▁the ▁fung us . ▁However , ▁the ▁net ▁carbon ▁gain ▁by ▁the ▁plant ▁in ▁these ▁interactions ▁is ▁positive ▁in ▁majority ▁of ▁the ▁observed ▁interactions . ▁ ▁Ph osph orus ▁transport ▁ ▁Ph osph orus ▁is ▁a ▁cru cial ▁nut ri ent ▁needed ▁by ▁all ▁plants ▁and ▁often ▁ph osph orus ▁def ic iency ▁in ▁soil ▁will ▁dict ate ▁the ▁formation ▁of ▁a ▁sym bi otic ▁relationship . ▁Ph