Here is a plant that most people probably recognize by its leaves, rather than by its flowers. Redwood sorrel (Oxalis oregana) is a small plant with leaves that form the classic shamrock shape. It grows in dense carpets on the floor of shady, wet, forests on the west side of the Cascades, as in the photo below.
Oxalis oregana (for some reason, it seems most appropriate to me to use the scientific name of this plant) is most common in healthy, old-growth forests.
The white or pinkish flowers are about 0.5 to 1.0 inches in diameter (1.2 to 2.0 cm).
This plant is adapted to life in the shade. The three heart-shaped leaflets are positioned to capture the scant sunlight that makes it to the forest floor, like little solar panels. The leaflets fold down in heavy rain or bright sunlight.
When out hiking in the forest, I often grab a few Oxalis leaves and pop them in my mouth. They are edible and have a nice, tangy taste. But take care not to eat very much of this plant– the tangy taste comes from Oxalic acid in the leaves, which can be toxic if too much is ingested.
Oxalic acid gets its name from the plant, and not the other way around. The acid was first isolated from a species of Oxalis.