Of the _Mercurialis perennis_ Mr Sowerby writes:--"This plant was formerly used in medicine, but has long been abandoned as a remedy. It is extremely acrid, and even poisonous, though recommended in some old books as a good pot-herb, probably from being confounded with the annual species. When steeped in water, the leaves give out a fine blue colour resembling indigo. This colouring matter is turned red by acids, and destroyed by alkalis, but is otherwise permanent; it might possibly prove valuable as a dye, if any means could be discovered of fixing it, and the herb has been introduced into this work with the view of drawing the attention of chemists to the subject; no experiments seem to have been lately made upon it." [Illustration: FIG. 18.] Let us now advise you how to distinguish our medicinal plant from the "ill weeds" with which it loves to associate. Its ovate, rough, irregularly-dentated, and petiolated leaves would not give it a sufficiently marked character, had it no other features peculiar to itself. But observe the yellowish-green _glomerules_, arranged, like millet, on a long frail spike. (Fig. 18, _a_.) They exhale, as your nose will inform you, a peculiar aroma, like that of spiced bread: no other plant but our Dog Mercury is gifted with this odour. Now, bring your magnifying-glass to bear upon it; with the point of a knife or a feather open one of the grains which form the _glomerules_ of the spike; out of it will leap, as if impelled by an invisible spring, a large number of stamens, easily distinguished by their elastic thread-like anthers, covered with tiny yellow beads. Each greenish grain is a _flower_; the calyx, which also serves as the corolla, is represented by three little petals, forming the external envelope of the little flower. (Fig. 18, _b_.) But something essential is still wanting; in the centre of the stamens you do not find any pistil. Why is so important an organ wanting? Because our little rounded flowers, with their spice-bread odour, have but one sex, are _unisexual_; they are male flowers, since they are furnished only with stamens. In vain do you hunt on the same stem for their companions, the female flowers. You will find them only upon _other_ stems, distinct from those which bear the male flowers. The Dog Mercury, then, is a plant whose two sexes are lodged in two different houses, οἴκοι--is, in fact, a _dioecious_ species.