<P> Aerenchyma is a spongy tissue that forms spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and the root . The channels of air - filled cavities (see image to right) provide a low - resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant above the water and the submerged tissues . Aerenchyma is also widespread in aquatic and wetland plants which must grow in hypoxic soils . </P> <P> When soil is flooded, hypoxia develops, as soil microorganisms consume oxygen faster than diffusion occurs . The presence of hypoxic soils is one of the defining characteristics of wetlands . Many wetland plants possess aerenchyma, and in some, such as water - lilies, there is mass flow of atmospheric air through leaves and rhizomes . There are many other chemical consequences of hypoxia . For example, nitrification is inhibited as low oxygen occurs and toxic compounds are formed, as anaerobic bacteria use nitrate, manganese, and sulfate as alternative electron acceptors . The reduction - oxidation potential of the rhizhosphere decreases and metal ions such as iron and manganese precipitate . Aerenchyma is a modification of the parenchyma . </P>

Aerenchyma is present in which category of plants