The Manual of the Botany of the Northern United States / Including the District East of the Mississippi and North of North Carolina and Tennessee

by Asa Gray

[+][+] Corolla more or less 2-lipped.

[++] Stamens distant and straight, often divergent, never convergent nor curved.

[=] Stamens 2, with or without rudiments of the upper pair.

9. Cunila. Calyx very hairy in the throat, equally 5-toothed. Corolla small.

[=][=] Stamens 4, calyx 10–13-nerved, and hairy in the throat (except n. 10).

10. Hyssopus. Calyx tubular, 15-nerved, equally 5-toothed. Stamens exserted.

11. Pycnanthemum. Calyx ovate or short-tubular, equally 5-toothed or somewhat 2-lipped. Flowers in dense heads or clusters.

12. Origanum. Calyx ovate-bell-shaped, 5-toothed. Spikes with large colored bracts.

13. Thymus. Calyx ovate, nodding in fruit, 2-lipped. Bracts minute; leaves very small.

[++][++] Stamens (often 2 only in n. 16) ascending or arcuate, often more or less converging (or ascending parallel under the erect upper lip in n. 14 and 15).