Step 1: Edaphic (substratum-based) plant categories describe where a plant physically roots and grows: oxylophytes root in acidic soils, psammophytes root in sand dunes or sandy soil, and chasmophytes root in cracks and crevices of rocks.
Step 2: Sciophyte, by contrast, describes a plant adapted to low light or shaded conditions, a classification tied to illumination, not to the soil or rock a plant grows on.
Because three of the four terms describe substratum and one describes light exposure, the odd one out is Sciophytes.
