Ninety-one plant species from wetland (WL), intermediate (INT) and non-wetland (NWL) habitats were grown in flooded and drained soils and responses to flooding were assessed in relation to root anatomy and fractional root porosity (FRP).

Flooding response and tolerance were related to FRP. Rooting depth increased with FRP in accordance with diffusion model predictions and emphasized the ventilating efficiency of aerenchyma. Major determinants of FRP were cortical cell configurations, closeness of cell packing, the relative proportions of configuration types, porous: non-porous tissue ratios, aerenchyma development and the type and degree of secondary growth. A classification of cortical types based on cell and aerenchyma patterns is presented.

Aerenchyma, both schizogenous and lysigenous, developed preferentially where pre-aerenchymatous cortical cell configurations (in TS) were radial and particularly, cubic and radial typical of WL and INT species. Aerenchyma rarely formed from hexagonal non-radial arrays which occur chiefly in NWL and in the outer cortical zones of WL and INT plants. The ventilating potential in non-aerenchymatous tissue was shown to be greater for cubic (fractional porosity, FP, max. 0–2146) than for hexagonal arrays (FP max. 0˙0931); closer packing greatly accentuated the differences. It is suggested that cubic: hexagonal zonal ratios in roots may reflect a balance between respiratory and mechanical needs.

In a majority of WL and INT species, shoot weights were unaffected by or increased with flooding and maximum rooting depths usually much exceeded 50 mm. Cubic packing raised the FRPs, as did aerenchyma which was often much greater under flooding. In the dicotyledonous species, a suppression of secondary growth in some, and a highly porous phelloderm in others, helped maintain high FRP. A minority of species were anatomically and responsively similar to NWL plants; survival under flooding was attributed to shallow rooting.

Under flooding, the FRP of almost all NWL species was < 0˙055 due to hexagonal packing, a lack of aerenchyma and, in dicotyledonous plants, secondary growth with scanty phelloderm. Shoot weights were reduced in 50% of cases, rooting depths were <50 mm, and some species died. Some species were exceptional in having cubic and radial packings; a lack of aerenchyma was associated with continuing meristematic activity in the primary cortex.