THE Foxglove and the White Dead Nettle, while differing in many respects, both have their five petals completely united to form a tubular corolla. In the former, this is bell-shaped, inclined downwards, and has a projecting lower lip for the convenience of humble-bees, which are its chief visitors. The essential organs are pressed against the roof of the tube within, so that while they do not hamper the entrance of the insect, they are certain to rub against its hairy back when it is probing for nectar at the far end. As the anthers mature before the stigma, there is a good chance that cross-will be effected. Should this fail to
occur, however, the flower’s own pollen is generally applied to its stigma at the time when the wilting corolla falls to the ground. Pollination by one means or another is thus practically certain; which explains the fact that almost every flower of the Foxglove’s long spike manages to set abundant, even in localities where humble-bees are scarce.
In the case of the White Dead Nettle—also a humble-bee flower—the tubular corolla stands erect, and, besides a lip, is furnished with a hood for the protection of the essential organs. When the bee alights on the lip, and thrusts its head and shoulders into the tube to reach the nectar, its back rubs first against the stigma, which thus receives any pollen brought from a neighbouring flower. A fraction of a second later it takes up a fresh supply of pollen from the anthers, which will be carried to the next flower visited. Since the White Dead Nettle’s stigma is receptive at the same time that the pollen is being shed, self-may readily occur through the agency of insects. But because the are frequently visited, and the bees pass rapidly from one to another, there is at least an even chance that cross-pollination will be effected.
In the case of the Salvia, the Dead Nettle’s near relative, the risk of self-pollination is negligible. The stamens are hinged, and the visiting bee forces them to bend down until the anthers come into contact with its back. This happens when the flower first opens. In older flowers that have matured and shed their pollen, the style of the pistil lengthens and bends down, so that the stigma grazes the bee’s back at the exact spot touched by the anthers of a younger flower.