Orchid
Orchids’ life cycles are not drastically different from those of other flowering plants. After a pollinator deposits pollinia on a stigma, the pollen tubes germinate and extend vertically down the column to fertilize an ovule. As a result, the pollinia inside the stigma, as well as the stigma itself, may expand. About six weeks later or later still, the pollen tubes have reached the ovules, and one or more of the ovules have been fertilized, resulting in zygotes that will eventually develop into embryos. Upon germination, most angiosperms receive nutrition from the sperm that formed the zygote and the endosperm that will support the developing plant until it can produce its food. There is no endosperm in an orchid seed. A simple, dry seed coat encloses a pro-embryo composed of a small mass of undifferentiated cells. Due to its diminutive size and little weight, this device can float on air currents and travel great distances before coming to rest. There is a direct correlation between the high density of seeds in an orchid’s fruit and the low probability that any given seed would land in a good location when carried by the wind.
The labellum, which serves as a landing platform for pollinating insects, and the consolidation of the flower’s stamens and pistil into a single column represent the pinnacle of evolution in flowers’ adaptation to pollination by insects. Once this is accomplished, a wide variety of specializations can be made to attract particular pollinators.
The median plane, just opposite the labellum, serves as the focal point for pollen deposition. With its back turned toward the flower’s center, the anther deposits pollen on the optimal side for receipt and deposition. Orchids also tend to drop pollen in clusters of two to eight, called pollinia, which is another indicator of their meticulous nature.
Orchids as a group use nectar as the principal attractant, whereas pollen, sought by pollinators among more primitive plant species as a protein-rich meal, has been withdrawn. This is related to the precision of pollination mechanisms, but it also means that orchids can no longer serve as the sole source of nutrition for the pollinator and that additional flowers must be present in the biosphere to keep visitors coming back.
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Nectar is given in tubular nectaries (Brassavola, Angraecum, Comparettia, and other genera), in grooves on the labellum (e.g., Listera and Epipactis), and at the base of the column and lip (e.g., Dendrobium and Scaphyglottis) (e.g., Dendrobium and Scaphyglottis). Many orchid species lack nectar but have instead evolved misleading attractants to entice pollinators.
Many orchids attract pollinators using pseudopollen, a powdery mass simulating pollen that is found on the labellum of several species of Maxillaria and Polystachya. Occasionally the grains are detached outgrowths called papillae, and sometimes they are dissolved in multicellular hairs that are loaded with starch.
Flowers that attract bees often bloom during the day and have pleasant aromas, vivid colors (excluding red, which bees regard as black or grey), a landing platform, nectar guides in the shape of colored lines flowing into the depths of the flower, and secret nectaries. The column forms the upper side of a tunnel formed by the lower sections of the orchid lip. The bee enters the tube to get at the nectary, and when the bee backs out some of the stigmatic fluid may be smeared on its back. The pollinia connect to the sticky substance as the bee moves back, and the bee then carries the pollinia to the next bloom. The pollinia of more complex orchids may be linked to a sticky pad called the viscidium, which separates from the stigma and clings to the pollinator.
Some orchid species rely on trickery to attract the bees needed for pollination. Pseudoantagonism may be at work in the fact that male Centris bees pollinate the female-only flowers of the huge genus Oncidium. The male bee may view the blossom as a threat and attempt to push it away from his territory. The pollinia are affixed to the bee’s head as it strikes the flower.
Orchids that rely on male euglossine bees for pollination offer some of the most intriguing and bizarre examples of trickery, traps, and manipulation of pollinators (species of the bee tribe Euglossini). Male euglossine are attracted to the scent of the flower, which is the basis for the syndrome of flowers pollinated by male euglossine. Despite its name, the male euglossine bee does not feed on the orchid’s nectar. Male euglossine bees get their nutrition from nectar-producing flowers; however, when they come across orchid blossoms that don’t generate nectar, they rub the lip surface with their front feet, collecting the resulting odor in specialized tarsal brushes. The enlarged hind tibiae (leg segments) then serve as a vector for the odor to be transferred to the rest of the bee. The orchid manipulates the bee by releasing an odor that travels to the insect’s rear tibiae.
In the genus Coryanthes, orchids use a different strategy to influence pollination. The blooms are monstrously huge and possibly even ugly. When the flower opens, the sepals and petals fold back like sails to show a peculiarly formed lip. The hypocrite, located at the top, is globular or hood-shaped; the mesophile, located in the middle, is elongate and sometimes fluted; and the epic hike, located at the bottom, is bucket-shaped. Two faucet-like organs at the foot of the column leak water into the epic hike in the final hours before the flower opens and for a short period afterward. A potent odor is released by the hypocrite, where male euglossine bees scratch, which attracts females. The bees periodically land in the water-filled bucket as they strive to launch into the air to carry the smell to their hind legs. The vertical and waxy edges of the bucket ensure that the bee will not be able to fly out. The bee can only escape through a tiny tunnel created by the column’s and the lip’s epichile’s apexes. The pollinarium is transferred to the bee’s thorax as it struggles to emerge from the tunnel. If the initial polling has been removed and the stigmatic cleft has opened, the pollinia might be implanted in the stigma of another flower during a subsequent visit.
Moths, which typically only fly at night, are drawn to brightly colored, fragrant flowers that bloom during the day. Moths sit motionless on the flower’s petals as they sip nectar. Flowers that are pollinated by moths typically have a long, thin nectar tube that is filled to the brim with nectar. Horizontal or hanging flowers that provide a sweet or musky scent are the norm. Butterflies, on the other hand, only fly during the day and are drawn to brightly colored flowers, regardless of whether or not they have a pleasant scent. Since they can’t hover in mid-air, butterflies typically settle on the blossom they’ve come to admire. Flowers stand upright to serve as landing pads. Sometimes the base is little more than a clump of flowers standing tall and tight. Because of their keen color vision, butterflies tend to favor blooms that are predominantly red, orange, or yellow in hue. In most cases, nectar is plentiful and can be found at the end of lengthy tubes. Most orchids are pollinated by insects, and in these cases, the nectar tube is set up to direct the tongue or beak in such a way that the pollinia are properly attached to the pollinator.
A case in point is when the English naturalist Charles Darwin predicted that a moth with a 25-centimeter-long proboscis would be discovered on the island of Madagascar as the pollinator of the orchid Angraecum sesquipedale because a moth would need a long tongue to reach the nectar hidden in the very long nectary at the base of the lip. This kind of moth has been discovered, and it has been watched pollinating the beautiful orchids.
Bird-pollinated flowers tend to be vivid in hue, with reds, blues, and yellows being the most common. They frequently have a lengthy nectary and a tubular shape, and nectar is always present. Although birds have a limited sense of smell, they are drawn to the vivid colors of flowers that are pollinated by them. Orchids that are pollinated by birds are mostly similar to other bird flowers, although there are notable exceptions. Like the orchid Epidendrum Secundum, many Western Hemisphere orchids appear to have evolved to bird pollination as an extension of butterfly pollination. Orchid blossoms that have already been acclimated to butterflies are not drastically altered in appearance under these conditions. While bee-pollinated orchids haven’t altered much throughout time, hummingbird-pollinated orchids have undergone significant transformations. There are several striking parallels between the bird-pollinated plant genera Cochlioda, Sophronitis, Elleanthus, Isochilus, Comparettia, Hexisea, and Meiracyllium. All are brightly colored tubes with a callus or hump on the lip inside that presses the bird’s beak against the column.
