The apoplast consists of the cell wall continuum and the intercellular spaces. Flooding of the apoplast is the major causative factor of the hyperhydricity (HH) syndrome. Flooding reduces gas exchange by cells. This leads to anaerobiosis and to accumulation within cells of gases like ethylene. The flooding is caused by the ample availability of water in tissue culture and occurs more on liquid medium and Gelritesolidified medium than on agar-solidified medium. Gelrite causes more HH since medium solidified with Gelrite liquefies, probably because of chelating compounds excreted by plant tissues. Root pressure causes translocation of water from the roots to the apoplast. At the same time, the water that has entered the apoplast cannot easily be removed in vitro by transpiration due to the very high relative humidity in the headspace. When the stomata are forced to close by addition of salicylic acid, transpiration is even more impaired. This results in an increase of HH.