How Pollen Works

In most pollen-producing plants, a grain of pollen successfully completes its journey when it travels from the male portion of a plant specimen to the corresponding female portion. Ideally, it finds its way to an entirely different plant to increase outcrossing borne from crosspollination. That's not always a hard and fast requirement, however, although it's important to note that many plant species have ways to prevent a particular plant from pollinating itself. Some are even genetically self-incompatible.

Once a grain of pollen reaches the plant's female portion, in most cases an ovule, one of the lucky sperm (typically out of two) lodged within the pollen will fertilize the egg cell inside. After fertilization occurs, the ovule will gradually develop into a seed, and that seed will transport its embryonic plant to a new home.

Plants that follow this basic reproductive path are known as gymnosperms. Trees that have pinecones and similar reproductive structures, as is the case with most conifers, are examples of gymnosperms. Let's take a closer look at conifers, the most numerous and widespread gymnosperms on Earth today, and pines in particular, since they're some of the most familiar species.

Pinecones generally come in male and female varieties, and they can be all sorts of shapes, textures and sizes, depending on the species. One makes the pollen, and the other receives it. Once a pollen grain arrives at an ovule -- usually adhering with the help of a sticky substance produced by the female pinecone -- it absorbs water, germinates and starts slowly growing a pollen tube in order to place the newly generated sperm inside. Fertilization occurs, and a seed eventually forms. The length of time it takes for the overall process to complete itself varies greatly; in many pine species, the pollination process takes more than a year from start to finish. Once it's finished, the seed is liberated from the cone, to travel on its way.

But although the development of the pollination process was revolutionary, it still had some kinks that could be worked out. On the next page, we'll take a look at the plants who whipped out the evolutionary iron and made the method that much more reliable.

Some plants -- the angiosperms -- evolved to take the pollination process a step further. These are the flowering plants, and not only do they produce seeds, they also flower and produce protective fruits. These reproductive safety nets are also better at luring mobile organisms into assisting them successfully complete their life cycles; in fact, many evolved in tandem with the creatures who pilot the pollination process. In terms of species, angiosperms are the most prolific type; many species of trees and shrubs, along with all manner of fruits, vegetables, grains, cacti and wildflowers are considered angiosperms [source: Raven].

So let's look at how this works in your typical flower and dig down a little deeper into the development of pollen in general. Pollen grains are created through the process of meiosis, during which cells divide and grow in number. The grains of pollen are often located in pollen sacs on the ends of the stamen (the male parts of the flower), which typically surround the carpel (the female parts of the flower). The stamen generally come in two sections: the two-lobed anther, which house the pollen sacks, and the filament, the stalk on which the anther perch. Each grain gradually develops a tough outer wall to shelter it during its journey.

Once it's deposited at its destination, grains of pollen settle on a flower's stigma -- the entrance to the ovary. Like with the gymnosperms, germination and pollen-tube formation follow fertilization, but this time both sperm are used. While one fertilizes the egg cell, the other is tasked with fertilizing another cell that will develop into the endosperm, which is what growing plant embryos consume before and during the sprouting process.

Different flowers grow in different configurations, and while many, in fact the majority of angiosperms, carry both stamen and carpal components, some do not. For those species, male and female reproductive parts can be found on different flowers of the same plant -- similar to how many gymnosperms' pinecones are commonly configured. Or, in some cases, each particular plant specimen may feature only one or the other, varying the process slightly.

Pollen can be carried by wind, rafted by water or shuttled around by any manner of creatures, be they bees, beetles, birds or bats, and deposited on the female reproductive part of another flower. That might sound pretty hit or miss, and it is, which is why plants -- particularly gymnosperms -- produce lots of pollen.

In order for plants to successfully spread their pollen, many coevolved with other creatures to get the job done more frequently and more efficiently. This happened a number of ways. With flowering plants, for example, those with the tastiest pollen were more likely to attract pollinators, so they were the ones that had the best chance of propagating their species. Flowering plants also leverage shape, color and scent to bring in more customers, sometimes in ways that might seem surprising. Many beetle species are attracted to flowers that produce scents we would consider highly unappealing. Some of these plants, among them the common household philodendron, attract beetles by heating up through a chemical reaction. It causes them to produce an odor reminiscent of decomposing organic matter, which the beetles are naturally drawn to. One Sumatran plant, known as the devil's tongue, smells so foul it has reportedly made people pass out. It's pollinator? A species of carrion beetle.

Bright colorful flowers are most likely to attract diurnal creatures, while white or light yellow ones are most likely to be spotted by nocturnal animals. There's also the production of nectar. Many proficient pollinators, such as bees, bats and hummingbirds thrive on nectar, so having nectar cups suited for the pollinator's mouthparts was another important specialization to develop. Lastly, the positioning of plants' sexual parts evolved, too. Those specimens whose arrangement best catered to a potential pollinator's feeding habits were most successful. So stamen that were most likely to be brushed against by a pollinator -- and therefore more likely to be brushed off and carried away -- were the most ideally positioned for evolutionary perseverance.

Plants, pollen and pollinators are obviously of great importance to humans. People surely passed on knowledge of plants throughout our species' long evolution, but some 11,000 years ago, we drastically changed the game [source: Starr]. That's around the time people began domesticating crop plants -- selecting favorite specimens from wild breeds and cultivating them for certain desirable attributes like high yield, pest resistance or heat tolerance. Fast forward to today, and our crop production methods have again leapt forward dramatically from those early beginnings. Now many crops are genetically modified organisms, or GMOs, and our artificial tampering has left lots of people wondering what impact it will have on naturally evolved organisms.

Scientists study whether and under what circumstances GMO crops have the potential to interbreed with conventional crops, as well as related species. One study conducted in Africa, an area where GMOs could have a considerable impact, determined bees there venture close to 4 miles (3 kilometers) away from the nest while foraging [source: Science Daily]. Such range could allow the trangenes of introduced GMO crops to infiltrate wild species. In order to control instances of crosspollination, international bodies such as the European Coexistence Bureau advocate certain isolation measures. These include spatial and temporal steps; in other words, planting crops at certain distances from plants that might be cross-pollinated, as well as timing such plantings so the species flower at different times of the year.

Pollen is also useful stuff to study for other reasons. By taking core samples, scientists who specialize in fields of palynology -- the study of pollens, spores and similar microscopic plant life -- can get a good idea as to what plants were prevalent during different eras of the Earth's history. For example, pollen and other palynomorphs can help determine when agricultural cultivation starts or stops in a certain area, when a stretch of land was wooded or meadowed, or when changes in climate occurred.

On the next page, learn lots more about pollen -- and what to do when it starts you sneezing.