Of all the British Orchids, the Listera ovata, or Twayblade (fig. 85), has the most curious structure. It grows in woods and pastures, has a creeping root, oval leaves, a downy stem, and yellowish green flowers. Fig. 85 represents a lateral view of a blossom, with all the sepals and petals removed, except the labellum. In this plant the rostellum (_r_) is large, thin, convex in front, concave behind, and arches over (_s_) the stigmatic surface. When the flower is full blown, the anther cells (_a_) are already open, and the naked and friable pollen grains united by a few threads, which form the pointed tips of the pollinia, rest upon the concave back of the rostellum. The labellum, which is contracted at the base, is exceedingly long, hanging down like a narrow ribbon. It is divided half-way up, and furrowed along the middle, from the bifurcation close up to the base of the stigmatic surface (_s_). The borders of the furrow are globular, and secrete much nectar. The rostellum is internally divided into a series of longitudinal cells, or chambers, which contain and expel viscid matter with violence on the slightest touch, and the viscid matter sets hard in two or three seconds, and soon assumes a purplish brown tint. So exquisitely sensitive is the rostellum, that a touch from the thinnest human hair suffices to cause the explosion. As the pointed tops of the loose pollinia lie on the crest of the rostellum, they are always caught by the exploded drop. This never fails. So rapid is the explosion, and so viscid the fluid, that it is difficult to touch the rostellum with a needle quick enough not to catch the pollinia already attached to the partially hardened drop, and consequently the slightest touch of any small insect which enters the flower, suffices to explode the rostellum, and the pollinia which attach themselves to its proboscis are carried by it to the next flower to adhere to the viscid stigma and fertilize it. Mr. Darwin has seen two Hymenopterous insects retreat from one of these plants with bright yellow pollinia on their heads, and Mr. C. K. Sprengel saw an insect of that kind leave pollen upon a stigma. The action and structure of Neottia Nidus-avis is almost identically the same as that of Listera ovata.