Thin-sepal Monkeyflower

Erythranthe hymenophylla, synonym Mimulus hymenophyllus, is a species of flowering plant in the lopseed family known by the common names thinsepal monkeyflower and membrane-leaf monkeyflower. It is native to Hells Canyon on the border between Oregon and Idaho in the United States. It has also been reported from Montana.

This plant is a perennial herb producing masses of thin, fragile, winged stems up to 25 centimeters long. They are coated in glandular hairs with a slimy exudate. The leaves are oppositely arranged in pairs. They are lance-shaped to oval with pointed tips and are very thin, nearly membrane-like. Flowers are borne in pairs in the leaf axils. Each has a tube-shaped calyx of green sepals. The corolla is funnel-shaped and variable in size, up to 2.8 centimeters in length. It is yellow with some red or purple dots inside, and it has a beard of yellow hairs on the lower lip. Flowering occurs in April through September.