Laudator Temporis Acti

Concerning the genus Rudbeckia, see Nathaniel Lord Britton and Addison Brown, An Illustrated Flora of the Northern United States, Canada and the British Possessions, 2nd ed. (New York: Charles Scribner's Sons, 1913; rpt. New York: Dover, 1970), III, 469:

Perennial or biennial (rarely annual), mostly rigid, usually rough or hispid herbs, with alternate undivided lobed or pinnatifid leaves, and large long-peduncled heads of tubular (mostly purple) and radiate (yellow) flowers. Involucre hemispheric, its bracts imbricated in 2-4 series. Receptacle conic or convex, with chaffy concave scales subtending or enveloping the disk-flowers. Ray-flowers neutral, the rays entire or toothed. Disk-flowers perfect, fertile, their corollas 5-lobed. Anthers entire or minutely 2-mucronate at the base. Style-branches tipped with hirsute appendages. Achenes 4-angled, obtuse or truncate at the apex. Pappus coroniform, sometimes of 2-4 short teeth, or none. [In honor of Claus Rudbeck, 1630-1702, Swedish anatomist and botanist.]

About 30 species, natives of North America and Mexico. In addition to the following, some 20 others occur in the southern and western United States. Type species: Rudbeckia hirta L.

But is the genus named after the elder Rudbeck, or his son, who lived from 1660 to 1740? See Linnaeus, letter to the younger Rudbeck (July 29, 1731), quoted in Wilfrid Blunt, The Compleat Naturalist: A Life of Linnaeus (London: W. Collins, 1971; rpt. Francis Lincoln Ltd., 2001), p. 35:

So long as the earth shall survive and as each spring shall see it covered with flowers, the Rudbeckia will preserve your glorious name....I have chosen a noble plant in order to recall your merits and the services you have rendered, a tall one to give an idea of your stature; and I wanted it to be one which branched and flowered and fruited freely, to show that you cultivated not only the sciences but also the humanities. Its rayed flowers will bear witness that you shone among savants like the sun among the stars; its perennial roots will remind us that each year sees you live again through new works. Pride of our gardens, the Rudbeckia will be cultivated throughout Europe and in distant lands where your revered name must long have been known. Accept this plant, not for what it is but for what it will become when it bears your name.

I can't find the original letter online, and I don't even know if it was written in Latin or Swedish, but there is a more complete translation of the letter in A.L.A. Fée, Vie de Linné (Paris: F.G. Levrault, 1832), pp. 85-87.

I also don't know which one of the 30 Rudbeckia species is in my garden, but it's probably not Rudbeckia hirta, as Britton and Brown (p. 470) call this species "biennial or sometimes annual" (p. 470), and the flowers in my garden are definitely perennial. There is a good description of the black-eyed susan in Hal Borland, Sundial of the Seasons (Philadelphia: J.B. Lippincott, 1964), pp. 121-122 (July 18 = Susan's Eyes):

Black-eyed Susan's eyes aren't really black at all. They are a purplish-brown, as anyone can see who takes the trouble to look for more than a passing glance at the Susans which are so profuse now at the roadside and in the meadows. Rudbeckia hirta is the official name, the botanical name, and it is a biennial flower that came out of the West along with commercial clover seed. Once in the East, it took to the soil and climate with vigor and pertinacity. Now its dark-eyed yellow flowers are common from Maine southward, wherever it finds a foothold. And it is particularly adaptable to almost any foothold available.

The yellow petals have a peculiarly rich golden color. They are full of sunlight. Like so many of the Compositae, its petals vary in number—thirteen on this flower, fifteen on another of the same plant, fourteen on the next flower. And they curl and twist, sometimes fray out at the tips for no obvious reason. The flower's sepals, the green "petals" that encase the bud and later form a supporting background for the flower, outnumber the petals, sometimes as much as two to one. And there are countless florets encircling the disk, Susan's eye; they open their tiny blossoms in succession and ring the disk with still another halo of golden yellow, the ripened pollen.

Bees and butterflies love the Susans, and so do most country youngsters. Weeds they certainly are when they invade the garden, but at the roadside they are bright and jaunty and full of the Summer sun. And they don't discourage easily, as many a gardener knows. Cut them or pull them up—they'll be back, as surely as the July sunshine, to which they belong.

Black-Eyed Susan, from my garden (July 14, 2010)