bio

Aerenchyma

Plant tissue shot through with large air channels, used to conduct oxygen internally. It is a feature of land plants living in water or anoxic mud: roots still need to respire, but waterlogged sediment cannot supply oxygen, so the plant plumbs air down from above. What matters for classification is that algae have no such tissue and do not need it — a thin body immersed in water gets all the dissolved gas it needs by diffusion. So aerenchyma in a submerged organism is the trace of a lineage that used to live on land.