Aerenchyma are the parenchymatous cells that contain air cavities and help in floating and gaseous exchange.
Mechanical strength is provided by sclerenchymatous cells, which are long and tapering at both ends and have narrow lumen. They are dead tissues and act as skeleton in plants. They are hardest plant tissue.
Chlorenchyma are parenchymatous cells which contain chlorophyll and help in photosynthesis.