Many plants rely on insects for pollination. They typically attract pollinators with nectar and scent. Interestingly, a variety of specialised mechanisms have been developed to increase the likelihood of successful pollination. Some plants turn their flowers into prison cells, some into gates and levers.... With many different special tricks, all wish to put the most pollen onto their little helpers for reproduction!
Plants such as those from the Aristolochiaceae family adopt a trap-and-release mechanism to increase the retention time of insects, increasing the chance they will be loaded with pollen. They imprison the insects for a short time, matching the pistillate phase and staminate phase of anthesis, to enhance successful cross pollination.
Aristolochiaceae plants featured a trumpet-shape flower with a perianth tube and an inflated base where stamens, pistils and nectar glands grow. The pistils mature first and release a scent that attracts the pollinators. When insects enter the flowers for nectar, they have in fact fallen into a trap! The tubes are covered with hairs that allow only downwards movement, so the insects cannot escape. But the trap is only temporary. Once the stamens mature, the hairs wither and the trapped creatures can leave – carrying full loads of pollen from the stamens!
The absent-minded insects soon forget their imprisonment, and are again attracted by the scent of the pistils from other flowers. With a coating of pollen from the flower they last visited, when they enter another flower, they will accomplish cross pollination without knowing it!
Salvia (Salvia sp.) of the Lamiaceae family has another type of special gear. The corolla are specialised to form upper and lower lips, and the stamens and pistils are hidden in the lower side of the upper lip. The connection between the anthers is elongated to become a lever.
When insects probe for nectar, they trigger the lever, which then causes the stamens to move downwards and deposit pollen on the insects.
Snapdragon (Antirrhinum sp.) of the Plantaginaceae family is more selective in choosing its pollinators. It has a mechanism to select the more effective pollinators, employing with the shape of the flowers and a gate-like gear. The labiate flowers have upper and lower corolla lips which are usually closed, resembling a gate. The code for opening the gate is the weight and size of the visiting insect. The lower lip is opened when the insect is of sufficient weight. However, if the visitor is too large, it cannot get into the flower. Only when the appropriate pollinator can open the gate and enter the flower successfully! The pollinator will then be loaded with pollen from stamens hidden in the flower.
Hammer orchid (Drakaea sp.) is even more picky than Snapdragon, as it specifically selects wasps as its pollinators. The flowers lure the pollinator wasp by mimicking the abdomen of a female wasp with the hammer-shaped lip (labellum), and releasing a chemical much like the wasp's sex pheromone. When a male attempts to mate with the flower and fly off, it will fail, but the momentum swings the stem and the lip, landing the wasp neatly on the stamen to collect its pollen load.