SPECIAL MECHANISMS OF ENTOMOPHILOUS FLOWERS Pollination of Sage.—Such a plant as the sage (Salvia pra tensis) has a typical humble-bee flower. The bee alights on the platform formed by the lower lip of the sympetalous corolla and pushes its head down the tube to reach the nectar at the bottom. Each of the two stamens is of special shape; the connective is very large and two-armed, and is hinged to the short filament. The longer arm bears a half anther while the short arm is sterile, the whole stamen having a lever mechanism. The bee, in probing for the honey, comes in contact with the short arm of the lever and in pressing this down brings the half anther at the end of the longer arm down upon its back where the pollen becomes de posited. The flower is protandrous and in a later stage the style elongates and is brought into the same position as occupied by the back of the bee when in contact with the anther. Cross pollina tion is thus brought about when the bee passes from a younger to an older flower.
Papilionaceous Type.—The Leguminosae (Pea family) show a very interesting series of pollination mechanisms. In this familiar type of flower to which the pea and gorse belong, the essential parts of the flower are enclosed in the keel. The nectar is secreted by the inner sides of the lower part of the staminal tube; one of the ten stamens is usually free and at its base are two openings leading to the nectar. The nectar is thus not only carefully con cealed but is also at a considerable depth. Cleverness and length of proboscis are thus required so that as might be expected these flowers are bee flowers. An insect visiting the flower alights on the wings, thus depresses them and, as they are joined to the keel, this is depressed also. The stigma and stamens are thus forced out, the stigma usually first so that it has the chance of brushing off pollen from the under-side of the bee and thus being cross pollinated.
There are four different types. (I) Flowers in which the stamens and stigma return within the keel so that repeated visits are possible ; examples are the clovers, melilot (Melilotius) and laburnum. (2) Flowers that are explosive, since the style and
stamens are confined under tension in the keel and when it is de pressed they are released with suddenness, thus scattering pollen on the undersurface of the bee. Only one insect visit is thus ef fective. Examples are broom (Genista), gorse (Ulex), lucerne (Medicago). (3) Flowers which display a piston mechanism—the pollen is shed early and the heads of the five outer stamens act as a piston so that the weight of the bee on the keel squeezes a narrow ribbon of pollen through the pore at the apex of the keel. A further pressure causes a protrusion of the stigma which is thus brought in contact with the bee. Examples are lupin (Lupinus), rest harrow (Ononis) and bird's foot trefoil (Lotus corniculatus). (4) Flowers which show a brush mechanism, for the pollen is again shed early and the style, which is provided with a brush of hairs, sweeps the pollen in small portions out of the tip of the keel. Flowers of this type usually allow of repeated insect visits.
Pinch Trap Flowers.—This type of flower is found in the Asclepiadaceae. The pollen is massed together into pollinia and pairs of these are attached to a sort of clip in which the leg of the insect (bees, wasps, etc.) becomes caught. The pollinia are thus carried away to another flower and are likely to come in contact with its stigmatic surface. Pitfall flowers are shown by the cuckoo pint (Arum maculatum), a common British plant, and by Aristo lochia and Asarum. In A. maculatum there is a spathe surrounding a spadix which bears a fringe of stiff downwardly projecting hairs at the top. These hairs allow the entry of insects but not their return; the insects are thus trapped for a time in the spathe and pollinate the pistillate flowers ; later the hairs wither and exit becomes possible. Flies are the common visitors.