Crash Course: Biology
The plants & the bees: Plant reproduction
- A couple of weeks ago I thought about a strategy for reproduction that the very first plants came up with called alternation of generations. The strategy that nonvascular plants still use today. Hopefully this is coming back to you. A plant can take two different forms that alternate back and forth between generations. The first form, the sporophyte, has diploid cells, two sets of chromosomes and the second form, the gametophyte, has haploid cells, just one set of chromosome. Well a lot can happen in 470 million years. Today, vascular plants still use the basic alternation of generations model but they've tricked it out so that it works a whole lot different than it did back in the Ordovician swamps where planthood got its start. Compared with their small, damp, nonvascular brethren vascular plants, with all their cones and flowers and other flashy accessories to look like a bunch of drag queens at a Carmen Miranda conference and Samba dance-off. Which might seem like overkill but we rely on these crazy kooks and their upstart reproductive strategies for-- pretty much for our everything, the food we eat, the air we breathe, the bouquets that we send to our wives and girlfriends when they're mad at us. Basically, what I'm saying is, that we need vascular plants to have sex. (upbeat music) So as you'll recall, the alternation of generations in nonvascular plants is pretty straightforward. A gametophyte produces either sperm or eggs which find each other if it's wet enough for the sperm to swim to the nearest egg. Once the egg is fertilized, the gametophyte creates the sporophyte, which is the little capsule on a stalk that has a bunch of spores in it. The spores are released into the air, they land in a moist place, germinate, and bam a new gametophyte generation is born. But nonvascular plants are what you call gametophyte dominant. What you're looking at when you look at a moss or hornwort or a liverwort is the gametophyte. It's the form that has only one set of chromosomes. For them, the sporophytes are tiny and tucked away inside the gametophytes, which they rely on for food, water, and protection. But vascular plants, it's the opposite. They're sporophyte dominant. When you look at a fern or a pine tree or a morning glory you're looking at the sporophyte generation. And the gametophytes are the teeny, tiny sex making materials that it has stashed away in special parts. So yes, all vascular plants are sporophyte dominant but that does not mean that they all reproduce in the same way. No sir! The simplest form of vascular plants are the ferns which reproduce a lot like nonvascular plants in that they have spores that grow on the underside of the frond, or fern leaf, which are released into the wild, blue yonder to find a nice soggy patch of ground to germinate on. The spore then makes a tiny gametophyte which is only a few centimeters wide and has both male and female reproductive organs on the underside of its leaves. If it's moist enough, the sperm on the boy side of the gametophyte will find the egg on the girl side and it will create a sporophyte, which is what we recognize as a fern. There's a lot of fossil evidence to suggest that at one point there probably were ferns that produced seeds and that all the fancy pants vascular plants that do have seeds and flowers evolved from them. But those seed-bearing ferns are all extinct now, so we can just gaze longingly at their fossils and wonder what their alternation of generations looked like. But there are other groups of plants that are more complex than ferns and what they all have in common is that they reproduce by creating pollen, which contains the male gametophyte and the female gametophytes, or ovules, which are fertilized by the pollen. The complete fertilized cell grows into a seed, which ripens and can produce a complete adult plant. So reiterating, in your more advanced vascular plants, that's how the alternation of generation works. The sporophyte generation grows from a seed and produces tiny gametophytes, either pollen or ovules. They then combine to form another seed, which produces another sporophyte. This evolutionary change from spores to seeds was a big deal and it began with the gymnosperms. They're single-serving, plant-making packages. Cut out the middle man by allowing an adult plant to grow immediately from a seed, rather than having to wait for a spore to go through that intermediate gametophyte stage. It also means, in most cases, that there doesn't have to be water present in order to reproduce. Today, gymnosperms include conifers, ginkgos, and tropical palm-like plants called cycads and none of them produce flowers because they evolved before flowers were invented. Instead, their reproductive structures are cones and you've seen a few of these in your day. In fact, their name gymnosperm means naked seed and that comes from the fact that their ovules develop exposed on the surface of the cone's scales. Now what we think of as cones are the spiky, woody things that Boy Scouts are throwing at each other at camp, right? But those things are actually female cones which house the ovules. The male cones are smaller and kind of spongy and their job is to crank out pollen. All this pollen is carried on the wind and some of it might find its way to a female cone where it fertilizes the ovule located at the base of each of the scales of the female cone. As the fertilized embryo matures, inside the cone, it makes a seed, containing enough nutrients to sustain it for a while after it germinates. The seed has a tough, shiny casing to protect it from the elements and once it's matured, the scales of the female cone just peel back and the seed falls to the ground and makes a new tree. But some gymnosperms have evolved a need for special conditions in order to reproduce. Take the lodgepole pine. It's a super tough tree that evolved in a pretty dry climate where there's lots of lightning storms that regularly start fires that burn through a forest every few years. Not only do lodgepoles have no problem withstanding a good low-intensity forest fire, their female cones are serotinous, so they will only open and drop their seeds when exposed to extreme heat. Now this sounds kind of crazy, but really it's super smart because the lodgepoles have evolved to take advantage of forest fires. They know that the forest fire will probably get rid of a lot of pesky underbrush that would crowd out their babies and maybe even it would kill some adult lodgepole pines so they just wait for the competition to be removed before they expose their seeds. So now I'm fixing to pull out the big guns, the angiosperms, because the angiosperms are the winners of the all-invitational plant division of things that live on Earth, at least for the past 140 million years or so. They're rookies really but they know what they're doing. For starters, they have seeds like gymnosperms but they also have flowers and flowers are awesome because they don't have to rely on the wind to carry their pollen to another flower like gymnosperms do with their cones. For the most part, flowers put animals to work, toting their pollen from one flower to another. In fact, angiosperms and flying insects probably evolved together, or co-evolved. The flowers providing food for the insects in the form of nectar and the insects providing transportation for the pollen to another flower's female reproductive parts. This my friends is what we call mutualism, the interaction of two organisms which mutually benefits both. Angiosperms reproduce by making flowers that contain the gametophytes. In this case, the sporophyte is made up of the stem and the roots and the leaves and even the flowers. All of the other parts of the plant except the pollen and the ovum, which are the actual gametophytes. Some flowers contain both male and female gametophytes. These are called perfect flowers, no pressure other flowers. Other flowers have both male and female sex organs on the same plant but in different flowers and some have male and female flowers on entirely different plants. There are no rules with angiosperms, they're just winging it. To see how flowers work, let's take a look at a perfect flower as an example because a lot of the garden flowers you see have both male and female reproductive parts. Starting from the bottom up, a flower as sepals, which look like leaves or petals, but they're usually green tissue that covered the flower when it was a little bud. The petals are usually colored to attract a certain kind of pollinator like a flag. The male parts of flowers consists of an anther which produces the pollen and sits on the end of a long filament attached to the base of the flower. This whole male reproductive set up like this is called a stamen. Now when it comes to lady parts, in contrast to gymnosperms, angiosperms don't leave their eggs hanging out all exposed. They lock their ovules down in an ovary at the bottom of a vase-like structure which also has a neck called a style and opening at the top called a stigma. Now all that's left is to get the male gametes packaged up and their gametophyte, the pollen, and have them carried to the female gametophyte, the ovule, to fertilize them. This is pollination and flowers do it by luring animals with smells, colors and food and in return, the animals mix and match the pollen with different individual flowers. Bees are the most famously successful at this but lots of other insects do it too as well as birds, like hummingbirds and even some bats. So no matter who does it, after fertilization happens, the ovule starts to swell and the ovule wall starts to toughen up because it's going to become a seed. The ovary meanwhile starts to grow around it and become the fruit. Now there are a bunch of different types of fruit. Fruit is defined as anything that the ovary, the protection around the seed, turns into. So anything that contains a seed is a fruit. And that's a lot of different things including many, many things we think of as not a fruit. To test your fruit skills, how about a round of fruit and not a fruit? So which one of these is the fruit and which one of them is the not a fruit? One, a sandspur you get while walking around the beach or a carrot? Answer, a sandspur, the little annoying thing that attaches to your pants is actually the swollen up ovary of a flower. A carrot is the root of plant. A stalk of celery or a piece of dandelion fluff? Fluff fluff. That little piece of fluff is attached to a dry little fruit that contains the seed. Celery is the actual stalk of a celery plant. A strawberry or a zucchini? The zucchini. A strawberry is actually the swollen end of the stem of the strawberry flower so it doesn't contain the seed, it actually has the seeds on the outside. It's one of the hard little things on the outside of the strawberry, those are the fruit. Some people argue about this because what seems more fruity than a strawberry But zucchinis? They're definitely fruits because they contain seeds. Fruits are important to angiosperms because they like to get their seeds as far away from themselves as possible so they're not competing with their own offspring. So some fruits can be carried away by the wind while others move around by being totally delicious so they can be eaten by an elephant and pooped out in an elephant turd far, far away. So that's the steamy sex lives of vascular plants. Mmm, wow that is good.
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