But you are sure to find the female flowers in the immediate neighbourhood of the stems with the male flowers. They are easily recognised by their larger and darker leaves (Fig. 18, _c_); and especially by their little twin pods, green, wrinkled, and pedicellate,[24] which garnish the axil of the leaves. (Fig. 18, _d_.) From this characteristic the female mercury was formerly mistaken for the _male_; and many centuries elapsed before naturalists recognised, what now-a-days seems so simple, that the little pods, joined in couples, and containing each a grain, composed the _fruit_ of a single plant; that every fruit proceeds from an ovary; and that every ovary is a sign of the feminine sex. In the _Historia Naturalis_ of Pliny, who was at once so acute and so credulous an observer, we first meet with the name of _Mercurialis_. "The plant is so denominated," he says,[25] "because it was discovered by Mercury. Its juice, mingled with that of the _hibiscus_ (a species of the _Malvaceæ_) and the purslain, forms a kind of unguent, with which, if you thoroughly rub the hands, they can touch molten lead without being injured." The description which Dioscorides[26] gives of the _Linozosis_, which he also calls _Parthenion_, or _Mercury's Plant_ (Ἑρμου βοτάνιον), applies, in the main, to our Dog Mercury. It is true that its leaves "are not like those of the basilic" (φίλλα ὄμοια ὁκίμῳ); but they resemble in all respects those of the smooth variety of cultivated mint; and, apparently, the basilic of Dioscorides was one of our mints. The fruit of the female, he adds,--evidently meaning the male flowers,--are disposed in clusters. Both species of the Herb or Dog Mercury belong to the family _Euphorbiaceæ_. Our attention must now be directed to another point. It is a fact, that in winters of moderate severity the Mercury continues to infest our gardens and cultivated fields. It only succumbs to a frost equal to six to ten degrees below freezing-point; then its congealed stem totters, and grows black, and its leaves mingle so completely with the soil that it is difficult to discover any vestiges. How singular a contrast! The plants most destructive in our kitchen gardens are frequently the most useful in medicine. There are no drugs more popular than the weeds which we call Herb Mercury, Garden Nightshade, and Dog's-tooth grass. All belong to families whose properties are strongly marked. As already stated, the Mercury ranks among the _Euphorbiaceæ_, remarkable for their acrid and more or less purgative juice. In this family occur the most violent drastics, such as the _Croton tiglium_, whose oil (expressed from the seeds) has long been considered an efficacious medicine. The Garden Nightshade is one of the _Solanaceæ_, and cousin-german of the useful potato; and the Dog's-tooth grass, whose roots compose three-fourths of our possets, is of the same family as our cereals. THE GARDEN NIGHTSHADE. [Illustration: FIG. 19.] [Illustration: FIG. 20.]