| | 1. Plant terrestrial (or becoming so), of seasonally wet soil, lake margins, temporary streams, vernal pools; generally < 1500 m |
| |
| | 2. Translucent membrane covering < 75% of sporangium |
| I. howellii |
| | 2' Translucent membrane covering > 75% of sporangium |
| |
| | 3. Plant of wet soil; leaf generally > 8 cm, > 1 mm wide at middle, rigid, almost brittle |
| I. nuttallii |
| | 3' Plant of vernal pools; leaf generally < 8 cm, < 1 mm wide at middle, soft, flexible |
| I. orcuttii |
| | 1' Plant underwater in persistent lakes or pools; > 1500 m |
| |
| I. echinospora |
| | 4' Female spore ridged or tubercled |
| |
| | 5. Leaf abruptly tapered to tip; female spore 0.3–0.5 mm wide |
| I. bolanderi |
| | 5' Leaf gradually tapered to tip; female spore 0.5–0.7 mm wide |
| I. occidentalis
|