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 Pollination Botany
Pollination is a process involving flowers (the pollen source and stigma receiver) and a pollination vector (an agent carrying pollen to the stigma). The study of pollination is an interdisciplinary field involving botany (the plant side) and some other field (the vector side). In some ways, then, it is an ecological discipline. We are focusing here mostly on the plant side, so I am using the title Pollination Botany rather than Pollination Biology or Pollination Ecology.
The gymnosperms used wind as a pollination vector. The pollen grains (endosporic microgametophytes) were carried by wind to the pollination droplets in the micropyles of the naked ovules. Here in angiosperms, the ovules are held inside the folded megasporophyll (the carpel) and so the pollen tube has a greater role. It must grow from the stigma to the micropyle and then penetrate the nucellus (megasporangium) to deliver the sperm cells to the egg and central cell.
Wind pollination could work for the gymnosperms because of where they live. If you have ever been to the taiga (a northern coniferous forest) you can understand how it would work. These vast forests are a nearly monoculture of conifers. There are many individuals of the few species found in these forests. Thus casting pollen to the wind can work as there are plenty of "receivers" downwind.
Wind pollination can work in angiosperms too, where almost-monocultures occur. An example are the grasslands and savannahs. Here huge stands of a relatively small number of species occur. There is plenty of opportunity for stigmas to be down-wind from anthers. Most grasses are wind-pollinated. The pollen is dry (to avoid clumping and precipitation), the male flowers are produced at the top of the plant (to put pollen into the wind), the filaments are thin and shake in the wind (the better to empty out pollen into the wind), the anthers produce vast numbers of pollen grains (it covers your car! the better to assure a stigma gets one), the female flowers are lower on the plant (to catch falling pollen), the stigmas are huge, feathery, and sticky (the better to catch a pollen grain).
Angiosperms that live in, say, a tropical rain forest cannot use wind pollination effectively. In the surrounding acre, there may be several hundred different species present. The next individual of a particular species may be quite some distance away and at some unknown angle with respect to wind direction. The plants of one species are few and far between. What is needed in such a complex biota is a "magic bullet" or "smart bomb" that can visit one flower to pick up pollen, and then seek and find the next individual of that species and carry the pollen to it. Animal vectors are smart enough and agile enough to do this. Not surprisingly, then, plant and animal species have co-evolved to use each other to accomplish feeding and reproductive needs. The plant feeds the pollinator and the pollinator accomplishes the pollen transfer for the plant. This is a kind of mutualism. We will focus here on the plant-side of this mutualism.
Attraction
The plant must first attract the vector. This is usually the function of the perianth. The attraction cues might be visual or olfactory.
Visual cues
Showy petals or sepals with obvious shape, size, and color for the vector's vision are important.
Butterflies and birds are attracted to red and yellow colors, and the stark contrast between a dark red and a bright yellow is important. Such strong markings help "train" the vector to concentrate its efforts on a particular species.
Bees have vision that is shifted toward the blue end of our visible spectrum. They do not see red colors but do see into the near-ultraviolet. Yet, we find bees visiting red flowers...why? Typically the red itself is attractive to other potential pollinators, but there is likely some other color pattern that works for the bee. It may be a pattern in the UV that we cannot see which actually attracts the bee.
Moths and bats fly at night and the limited vision possibilities mean that a white or very pale color is more observable.
Bull's eyes, splotches, and nectar guides are color patterns that form a high-contrast exhibit to make the flowers stand out against a background of green foliage. Again all such things assist a pollinator "see" the flowers and begin to concentrate their visits only on those with certain colors.
Olfactory cues
Some vectors have limited vision but extensive ability to find a flower by its fragrance. Flowers produce volatile chemicals that diffuse and are carried by air movements through an environment. A vector that can recognize this odor and fly up the concentration gradient of this fragrance, can easily find the next flower of a particular species. Flowers have evolved a wide array of odors to assist in attracting vectors. Humans have limited appreciation of and vocabulary for fragrances, but other animals are quite discriminating. Flowers that specialize in attracting flies, for example, are famous for their fetid (manure) aroma.
Shape considerations
The flower has to be designed to accommodate the vector and prevent pollen and nectar robbers from stealing the rewards. The shape can also make a flower more attractive. Flowers pollinated by hovering vectors generally hang down and have long styles and filaments. Flowers pollinated by hungry beetles need to provide lots of structural food (and yet protect the ovules!). A beetle-pollinated flower needs to have easy entrance as beetles are clumsy in flight. Non-hovering insects and birds need perches or landing platforms as part of the flower. The sizes and shapes of the flower parts and their alignments are critical to assure pollination when the vector does visit. A long nectar spur (Habenaneria and moth) protects the reward from robbers and yet allows reward for the actual vector. Moreover, the body alignment while the proboscis is in the nectar spur is critical to the pollination event.
In lecture I go over a few examples that integrate these concepts. Some of the special ones include the Ophrys orchid and pseudocopulation by wasps, the fly bar-room of the Paphiopedilum orchids, the milkweed and its super-glue for butterfly legs, and the spring-trap of mountain laurel that bashes pollen onto the pollinator.
Rewards
The second law of thermodynamics says "there is no free lunch." It applies here too. Just getting the pollinator to land will never do. The vector is usually intelligent enough to avoid the energy waste of behaviors that do not result in some kind of reward. The plant must reward the vector's visit. This will result in training the vector to concentrate its efforts on visiting other flowers of this same species and achieving the pollination goal. The vector's reward is usually one of three things:
Nectar
Somewhere in the flower is some secretory tissue called the nectary. Its location varies from species to species. This tissue is connected to phloem and secretes the sweet liquid known as nectar. This is a carbohydrate rich droplet that is used as an energy source for vectors. Hummingbirds must consume vast quantities of nectar to continue their high-energy method of flight. It is the sole energy source for most butterflies. Bees collect the nectar and evaporate it down to make honey for winter supplies. Nectar is rich in carbohydrate but is a weak source of most other nutrients... trace vitamins and minerals. Thus the person who tells you their kids get not sugar but you watch them pour honey all over their cereal is just plain wrong... honey is essentially sugar with just a little water added.
Pollen
Pollen itself is decent vegetable food. It contains protein, starch, oil, and other nutrients. It is far richer than nectar in vitamins and minerals too. For beetles and bees, the consumption and collection of pollen is critical. It is their basic protein supply. Fortunately, the pollinators are not very careful in cleaning off sticky pollen that collects on their bodies in each visit. Thus they arrive at a second flower with lots of pollen from the first flower they visit. The position of stigma and vector allows this pollen to be removed and stuck to the stigma.
Behavior
Behavior can also be a reward that gets a repeat visit by a vector. The Paphiopedilum orchids provide a bar-room place for an insect to "get high" and that is the only reward. The flies must like the experience and come back for more... achieving pollination. Another example includes the Ophrys orchid sex pheromone used by the female wasps whose mate's virgin attempts at copulation are with the orchid flower that makes the pheromone scent. Thus the flowers attract both sexes, but only one pollinates... the other collects!
There is much more to pollination botany. At big universities you can take a whole course on the subject... complete with labs!
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