How Plants Arrange for Transportation
Can you imagine a potted plant hailing a cab or hopping a flight to Paris? How about a dandelion commuting to work in a Prius, or an oak tree planning its next vacation in Panama? We don’t normally think of green plants as having transportation needs, and therefore we have little reason to ponder how plants solve those needs. But a surprising thing about plants is that they really do have a serious need to get from one place to another, and they have developed a wide range of ingenious solutions to address this need.
A key point to keep in mind is that there is not just one, but two times in the life cycle of a plant when it faces a crucial need for transportation. The first time is when the plant is a mere little seed, and it is now ready to leave home and strike out on its own. The second time is when the plant is fully grown, mature enough to have sex and make little baby plants of its own. Neither of these essential steps can occur without a good form of transport. And therefore all successful species of plants have worked out an effective way to get from one place to another.
Let’s start with the first stage — when a young seed has ripened and is ready to leave the security of its maternal home. This home may be a fruit, a seed head, a dry pod, a cone, or any similar structure attached to the mother plant. The seed might be large or it might be small. It might be alone in its little structure, or it might share its home with a huge number of sibling seeds. But now the time has come for the seed to seek its fortune in a new location away from Mom — someplace that has soil and water and fresh air and sunshine, a perfect place to take root and begin an independent life. In short, it is ready for “seed dispersal”.
However, unlike a giraffe or a centipede, the seed can’t walk to its new home. It can’t fly or crawl. In fact, most seeds cannot move themselves around at all. Therefore seeds must find a way to hitch a ride. Two common solutions are to sail away on the wind, or to hitchhike with an animal. The latter approach has been especially successful, leading to a wide assortment of variations.
Perhaps the easiest way to hitch a ride with an animal is to offer the critter something to eat. The food could be a small morsel that is swallowed whole, such as a berry that is gulped by a bird. In this case the seed needs to be fairly small, and it needs to be able to survive a journey through the animal’s digestive tract. When the seed gets pooped out later on, it will have arrived at its new home, which could be quite a distance from mama. On the other hand, the treat that tempts the animal could be a very large fruit, which may be dragged around and then broken apart by the dinner guest, with a subsequent scattering of the seeds. Another approach is to produce nutlike seeds that pack a lot of nutrition and can be stored for a long period of time. This package appeals to rodents and birds that store away nuts for later use, cached across the local landscape. The stored supply is typically more than what is needed by the animal, and therefore many of the cached nuts never get eaten, and are free to sprout come spring. In all of these cases — small fruits, large fruits, and nuts — the upshot is a mutually beneficial relationship, where the plant provides the animal with food, and in return the animal provides transport for the plant’s seeds.
Another approach for hitching a ride with an animal is to adhere to the critter as it passes by. This is most easily accomplished with furry creatures — and therefore a variety of seeds have developed a strong tendency to get tangled in fur. These seeds might travel for quite a distance before falling off or getting scratched off. Human clothing makes a good substitute for fur, from the standpoint of these clingy seeds. After you hike in woods or fields, it is not unusual to find a sizeable collection of burs and beggar’s ticks clinging tightly to your pants legs and shoe laces.
Although traveling with beasts is a well-trodden method of seed transportation, it is certainly not the only way to go. The wind is also a popular transport vehicle. Seeds with an effective parachute of fluff, such as dandelions and cottonwoods, may travel quite a distance in a strong wind — although not typically as far as a bird or large mammal might carry a seed. Seeds that are dropped from trees — such as pines and maples — might instead have wings, resulting in a slow, spinning descent to the ground. A winged seed does not travel very far, but often far enough to get out from underneath the overbearing shade of mama.
Wind is not the only fluid capable of moving a seed around. For plants that typically grow next to bodies of water, floating seeds can be a good strategy to find a new home. Mangroves and coconut palms, which both grow along ocean shorelines, are particularly interesting examples. Waves, tides, and currents transport the floating seeds to new territories that provide fresh opportunities.
Up to now, all of this discussion has focused on seeds. But the reality is that certain categories of green plants reproduce using spores instead of seeds. These plants include ferns, mosses, multicellular algae (such as kelp) and several other categories of living things. Needless to say, spores need to get around just as seeds do, and water is one of the most common methods.
A few types of plants have developed mechanical methods for dispersing their seeds, independent of wind, water, or animals. In effect, the seeds reside in spring-loaded capsules. In a plant such as the touch-me-not, parts of the seed pod actually become springs under tension, like that in a mousetrap. When the seeds are mature, the trap will suddenly snap, flinging the seeds far and wide.
Regardless of how a seed arrives at its new home, it faces long odds to survive into adulthood. In fact, only a tiny fraction of seeds will reach this stage of life — becoming a plant that is grown up and ready to make baby plants of its own. So how does transportation play a role in this stage, and what means of transport are typically employed? In short, how do plants go about the business of sexual reproduction?
The obvious issue here is that most plants are rooted for life in a single location. Two plants that are destined to mate must engage in a long-distance relationship. Therefore they need a courier to transport the love-letters. But instead of using envelopes to contain the missives, the messages are packaged into grains of pollen. Each pollen grain contains a male gamete — equivalent to an animal’s sperm cell — waiting for delivery to an expectant ovary. (Yes, the female structures in seed plants are actually called “ovaries”.) Each ovary contains one to many ovules, which are the eggs waiting to be fertilized.
The sneaky thing about plants is that more often than not, a single plant has both male parts and female parts. In other words, it has both pollen-producing bits (stamens) and pollen-receiving bits (pistils, which containing the ovaries). Therefore, for some seeds, the mama and the papa can be the very same plant. This happens when a grain of pollen from a particular plant fertilizes an ovule growing on the very same plant. However, for the long-term survival of the species, it is very helpful if the genes get mixed up a bit — which requires that many of the seeds have a distinct mother and father. To accomplish this, the plant needs a courier that can take pollen from one plant and deliver it to a flower or cone on another plant. As it turns outs, the list of popular couriers is a familiar parade of the usual suspects — animals, wind, and water.
As with seed dispersal, animals are a favorite method of transporting pollen — especially animals that fly, including insects, birds, and bats. A flying animal can easily flit from one plant to another — which is not the case for crawling insects or other small ambling creatures. But as with the transport of seeds, animals need a good reason to serve as pollen couriers. And again, that reason is usually the promise of food. Plants will often bribe the airborne creatures with sweet, fragrant nectar. The nectar provides sugar, a key source of energy for the insects and birds that imbibe it. As the creatures visit flower after flower, sipping nectar at each way station, they accidentally pick up pollen, and inadvertently deposit it at a later stop. For some of the aerial couriers, the pollen itself is also a lure, because pollen contains proteins and other useful nutrients. But collecting pollen is a messy business, and some of it inevitably gets deposited onto just the right spot on another plant.
Regardless of whether the animal is more interested in the nectar or the pollen, the result is a mutually beneficial relationship. The plant provides food to the animal, while the animal transports pollen on behalf of the plant. A plant that needs to attract pollinators will advertise with showy flowers that may be heavily perfumed. The colors and odors both serve as signals that dinner is prepared and served. As it turns out, humans and bees have a similar sense of aesthetics. We enjoy seeing and smelling the same flowers that attract bees — large flowers with bright colors and sweet aromas. On the other hand, our tastes part ways with certain kinds of flies — especially those that are attracted to blossoms that smell like carrion or feces, with colors to match.
Flying pollinators are not limited to bees and flies. Butterflies and moths also serve as pollinators. Night-blooming flowers, especially in the tropics, might be pollinated by bats. And birds can also be important pollinators. In the U.S., we usually think of hummingbirds when we think of birds that sip nectar from flowers. But in certain other parts of world, including places such as Australia and Hawaii, a range of larger birds make their living from visiting flowers. To deal with such big pollinators, some of these plants have developed “brush flowers”. Each flower is simply a large puffy ball of stamens, pointing in all directions. If a bird would like to nab some nectar, then it plunges its head into the cloud of stamens to tap the hidden nectaries — and as a result, its head is thoroughly dusted with pollen. On subsequent visits to other flowers, some of that pollen is brushed onto the awaiting pistils. In contrast, certain other bird-pollinated flowers are shaped like long, slender trumpets — and the birds that visit have long, slender bills to match.
When we humans think of flowers, we invariably picture the blossoms that attract flying creatures. To us, flowers are large and colorful and often scented — characteristics which are useful only for animal pollination. But not all flowers are pollinated by animals. Some species of plants rely on wind pollination instead. The tiny particles of pollen are borne aloft by gusts of air, drifting around until eventually settling down somewhere else. If the grain of pollen is really, really lucky, then that place will be a pistil on a plant of the same species.
A wind-pollinated flower does not benefit from bright colors, large petals, or delightful odors. Such flowers tend to be brown or green in color, quite small, and completely lacking any structures (such as petals) that might interfere with the wind. In some cases, such as oak trees, the male flowers are clustered into dangling appendages, to better catch the wind. Few people who notice such structures are likely to recognize them as flowers.
A plant that depends upon wind pollination needs to make an incredibly large number of pollen grains, because only a miniscule fraction of the grains will actually land on an appropriate pistil. These pollen grains are often extremely small, allowing them to float long distances on the wind. Tiny grains have a second advantage as well — the plant spends much less energy and material on the production of each grain. But these tiny, tiny grains of pollen don’t get along so well with humans; those that land in our noses can cause allergies. In contrast, larger grains of pollen — such as those from insect-pollinated flowers — usually don’t bother us.
For seeds plants, water is seldom an important method for the transport of pollen. But for the many kinds of plants that do not produce seeds — such as ferns, mosses, and kelp — water is usually the critical medium that enables sexual reproduction. For these plants, the male gametes are not enclosed in tough shells the way that pollen grains are. In fact, in many of these plants the male gametes can swim, making them more similar to animal sperm cells than to pollen.
We have now looked at the critical role of transportation at two stages in the life cycle of plants — seed dispersal and pollination — and we have seen that in seed plants, the two most important methods of transport are to hitch a ride with an animal, or to float away on the wind. However, plants are under no obligation to be consistent. For example, dandelions rely on insects to transport pollen, but on wind to transport seeds. Oak trees rely on wind to transport pollen, but on nut-hoarding rodents and birds to move the seeds around.
And now for the final twist on this story. Up to now we have been discussing the habits of wild plants, including their interactions with wild animals. But are humans a part of this story in any important way? Certainly there are times when seeds stick to our clothing, or when weed seeds hitch a ride in soil that we move around. But beyond such examples, there are entire categories of plants that rely on humans for some of their transportation needs — such as our agricultural crops. Corn (maize) is an especially good example, because we always grow it from seed, and because it does not tend to escape and grow wild on its own. Our millions of acres of corn exist only because we take the time and effort to transport and nurture all of those seeds, year after year. So you could say that humans have developed a mutually beneficial relationship with corn — and in fact with all of the agricultural and ornamental plants that we are fond of planting. Therefore humans are indeed a key part of the story about how plants have solved their transportation needs!
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