Draba glabella Pursh.
Habitat: Calcareous outcrops, cliffs, and talus, often adjacent to water.
Range: Circumboreal south to Maine, New York, and Vermont.
Aids to Identification: Smooth whitlow-grass appears very similar to rock whitlow-grass, Draba arabisans. Both grow in tufts and form mats in similar habitats and have lanceolate-spatulate basal leaves 2-10 mm wide. The leaves are covered with stellate pubescence on both sides. Both have white, 4-petaled flowers in elongate clusters on the stems. Draba glabella can be distinguished from D. arabisans by its stalked stellate hairs on the underside of the leaves (opposed to sessile stellate hairs in D. arabisans). Also, the fruits are usually flat in D. glabella whereas they are twisted in D. arabisans.
Ecological characteristics: Found at only one site in Maine on exposed rock near the shores of a large lake.
Phenology: Flowering June through August.
Synonyms: Draba arabisans Michx. var. canadensis (Burnet) Fern. & Knowlt.; Draba canadensis Burnet; Draba daurica DC.; Draba glabella Pursh var. megasperma (Fern. & Knowlt.) Fern.; Draba glabella var. orthocarpa (Fern. & Knowlt.) Fern.; Draba megasperma Fern. & Knowlt.
Known Distribution in Maine: This rare plant has been documented from a total of 1 town in the following county: Piscataquis.
Reason(s) for rarity: At southern edge of range, calcareous habitat is naturally scarce in Maine.
Conservation considerations: The population in Maine is not easily accessible. The only threats would be natural threats such as rock falls or ice scour.