This section is from the book "Nature's Garden", by Neltje Blanchan. Also available from Amazon: Nature's Garden; An Aid To Knowledge Of Our Wild Flowers And Their Insect Visitors.
Fine hairs on the erect, leafy, usually single stem of the Downy Yellow Violet (V. pubescens), whose dark veined, bright yellow petals gleam in dry woods in April and May, easily distinguish it from the Smooth Yellow Violet (V. scabrinscula), formerly considered a mere variety in spite of its being an earlier bloomer, a lover of moisture, and well equipped with basal leaves at flowering time, which the downy species is not. Moreover, it bears a paler blossom, more coarsely dentate leaves, often decidedly taper-pointed, and usually several stems together.
Downy Yellow Violet. (Viola pubescens)
Our other common yellow species, the Round-leaved Violet (V. rotundifolia), lifts smaller, pale, brown-veined, and bearded blossoms above a tuffet of broad, shining leaves close to the ground. The veins on the petals serve as pathfinders to the nectary for the bee, and the beard as footholds, while she probes the inverted blossoms. Such violets as have their side petals bearded are most frequently visited by small greenish mason bees (Osmia), with collecting brushes on their abdomen that receive the pollen as it falls. Abundant cleistogamous flowers (see pp. 30 and 108) are borne on the runners late in the season. Bryant, whose botanical lore did not always keep step with his Muse, wrote of the yellow violet as the first spring flower, because he found it "by the snowbank's edges cold," one April day, when the hepaticas about his home at Roslyn, Long Island, had doubtless been in bloom a month.
"Of all her train the hands of Spring First plant thee in the watery mould," he wrote, regardless of the fact that the round-leaved violet's preferences are for dry, wooded, or rocky hillsides. Muller believed that all violets were originally yellow, not white, after they evoluted from the green stage. (Illustration, p. 292.)