Roots of higher plants possess a stele (vascular cylinder) that contains xylem vessels. The stele is surrounded by the cortex where, in some species, lysigenous aerenchyma can be formed by death of the cortical cells. Although many studies have focused on the sizes of xylem or aerenchyma as related respectively to drought or flooding tolerances, possible trade-off between the stele and cortex sizes has received much less attention. Recently, we found that the ratio of cortex to stele area (CSR) is higher in roots of the wetland rice than those of the upland crops, and that roots with a greater CSR can transport more oxygen to the root tips. In this study, we evaluated the root anatomy of 18 wild Poaceae species collected from a range of habitats with various soil water contents. We found that the species growing in higher soil water contents have greater CSR, and the species in soils with lower water contents have lower CSR. These results indicate that wetland and dryland species, respectively, have larger proportions of cortex and stele. Further anatomical analyses revealed that aerenchyma to cortex ratio (ACR) is higher in the species growing in both higher and lower soil water contents, and xylem to stele ratio (XSR) is higher in the species with higher water contents. Finally, we constructed correlation models between CSR, ACR, or XSR and soil water contents of wild Poaceae species. Using these models, we were able to isolate the relevant anatomical features that are essential for successful adaptation to habitats differing in soil water status. From the results of these studies, we will discuss the possible application of these indices into the improvements of crop tolerance to drought or soil flooding.