The thick wad of stamens in the middle of the flower is characteristic of St. John’s Worts, most of which also have bright yellow flowers. These plants were growing in a clearing in Highland Park, where they were blooming in the middle of June.
Gray describes the genus and the species:
HYPERICUM [Tourn.] L. ST. JOHN’S-WORT. Sepals 5, usually subequal. Petals 5, oblique, convolute in the bud (except in 6). Stamens frequently united or clustered in 3-5 parcels; no interposed glands. Pod 1-celled or 3-5-celled. Seeds usually cylindrical. Herbs or shrubs, with cymose yellow, flesh-colored, or purplish flowers. (An ancient Greek name of obscure meaning.)
H. perforatum L. (COMMON S.) Stem much branched and corymbed, somewhat 2-edged, producing runners from the base; leaves elliptic- or linear-oblong, with pellucid dots; petals deep yellow, black-dotted along the margin, twice the length of the lanceolate acute sepals; flowers numerous, in open leafy cymes. Fields, etc. June-Sept. A pernicious weed, difficult to extirpate; juice very acrid. (Nat. from Eu.)