The formation of root aerenchyma, the prominent air spaces in the root cortex which are normally induced by waterlogging, has an important role in providing an internal pathway for oxygen transport between roots and the aerial environment. Along this pathway, O2 is supplied to the roots and rhizosphere, while CO2, ethylene, etc. move from the soil to the shoot and the atmosphere. The aerenchyma are formed either as part of normal development, or in response to stress (e.g. hypoxia, nutrient deficiency), by two known mechanisms: schizogeny and lysigeny. Aerenchyma formation increases porosity above the levels that appear in the usual intercellular spaces.