There doesn’t seem to be much point to a green Poinsettia, but this one is not entirely green: it adds the subtly decorative touch of whitish markings on the upper leaves, no doubt to provide a more prominent target for insect pollinators. It may not be quite the spectacle that the Christmas Poinsettia is, but it’s a handsome plant if we view it with an indulgent eye.
This species, along with its more decorative sister E. pulcherrima, has been in and out of the genus Euphorbia. When botanists are feeling frisky, they split off the genus Poinsettia; then, the next morning, in a fit of remorse, they stuff the genus back into Euphorbia and hope nobody notices. Right now the consensus seems to be that Poinsettia is a subgenus or section of the gigantic genus Euphorbia. This is Gray’s view.
Gray describes the genus, the subgenus, and the species:
EUPHÓRBIA L. SPURGE. Flowers monoecious, included in a cup-shaped 4-6-lobed involucre (flower of older authors) resembling a calyx or corolla, and usually bearing large thick glands (with or without petal-like margins) at its sinuses. Sterile flowers numerous and lining the base of the involucre, each from the axil of a little bract, and consisting merely ol a single stamen Jointed on a pedicel like the filament; anther-cells globular, separate. Fertile flower solitary in the middle of the involucre, soon protruded on a long pedicel, consisting of a 3-lobed and 3-celled ovary with no calyx (or a mere vestige). Styles 3, each 2-cleft; the stigmas therefore 6. Pod separating into three 1-seeded carpels, which split, plastically into 2 valves. Seed often caruncled (ours only in §§ 5 and 6). — Plants (ours essentially herbaceous) with a milky acrid juice. Peduncles terminal, often umbellate-clustered; in the first section mostly appearing lateral, but not really axillary. (Named for Euphorbus, physician to King Juba.)
POINSÉTTIA (Graham) Baill. Involucres in terminal clusters, 4-5-lobed, with few (or often solitary) cup-shaped glands; erect annuals, with entire, dentate, or sinuate leaves, all or only the upper ones opposite, the uppermost often colored, especially at base; stipules reduced to small glands.
E. dentàta Michx. Erect or ascending, hairy, 2.5-12 dm. high; leaves ovate, lanceolate, or linear, petioled, coarsely toothed, 4-8 cm. long, only the lowest alternate, the upper often paler at base; involucres almost sessile, with 5 oblong dentate lobes, and one or sometimes more short-stalked glands; seeds ovoid-globular, slightly tubercled. — Rich soil, Pa. to Wyo. and Tex. July-Sept.