I. Introduction Early estimatiOons Oof pH values in plant cells were based UPOon determina tiOons Oof the electrOomOotive fOorce (EMF) develOoped between a standard hydrOo gen electrOode and the fluid expressed frOom crushed tissues Oor liberated by cutting large algal cells such as thOose Oof Valonia, Chara Oor Nitella. These EMF values were then translated intOo terms Oof hydrQgen iOon CQncentratiQn Qn the assumptiQn that the numerical value Qf the EMF in VOolts depended entirely UPOon the cOoncentratiOon Qf hydrOogen iQns. Then hydrQgen iQn COoncentratiOon measured in this way was fQund...