The corolla of this beautiful and familiar flower (Fig. 6) consists of a narrow tube, somewhat expanded at the upper end (Fig. 7), where the lower lobe forms a platform, on each side of which is a small projecting tooth (Fig. 8, _m_). The upper portion of the corolla is an arched hood (_co_), under which lie four anthers (_a a_), in pairs, while between them, and projecting somewhat downwards, is the pointed pistil (_st_); the tube at the lower part contains honey, and above the honey is a row of hairs running round the tube. [Illustration: Fig. 6--White Dead-nettle.] Now, why has the flower this peculiar form? What regulates the length of the tube? What is the use of the arch? What lesson do the little teeth teach us? What advantage is the honey to the flower? Of what use is the fringe of hairs? Why does the stigma project beyond the anthers? Why is the corolla white, while the rest of the plant is green? [Illustration: Fig. 7.] [Illustration: Fig. 8.] The honey of course serves to attract the Humble Bees by which the flower is fertilised, and to which it is especially adapted; the white colour makes the flower more conspicuous; the lower lip forms the stage on which the Bees may alight; the length of the tube is adapted to that of their proboscis; its narrowness and the fringe of fine hairs exclude small insects which might rob the flower of its honey without performing any service in return; the arched upper lip protects the stamens and pistil, and prevents rain-drops from choking up the tube and washing away the honey; the little teeth are, I believe, of no use to the flower in its present condition, they are the last relics of lobes once much larger, and still remaining so in some allied species, but which in the Dead-nettle, being no longer of any use, are gradually disappearing; the height of the arch has reference to the size of the Bee, being just so much above the alighting stage that the Bee, while sucking the honey, rubs its back against the hood and thus comes in contact first with the stigma and then with the anthers, the pollen-grains from which adhere to the hairs on the Bee's back, and are thus carried off to the next flower which the Bee visits, when some of them are then licked off by the viscid tip of the stigma.[20] [Illustration: Fig. 9.] [Illustration: Fig. 10.] [Illustration: Fig. 11.]