Topics in Depth
The Theme of Plant Reproduction in Plant Biology
Bryophytes are primitive plants that don’t have seeds or vascular systems. Because they lack a mechanism for moving water through their bodies, bryophytes are pretty small. When was the last time you saw a tall moss? Exactly. Mosses, liverworts and other bryophytes are so tiny that it is easy to ignore them most of the time, even if they are under your feet. Mosses actually are underfoot a lot—check out the cracks in the sidewalk and you’ll probably find moss there. If you’re lucky, you might get to see a gametophyte wearing a sporophyte like a hat. Now that would be an interesting fashion statement.
Gametophytes don’t "wear" sporophytes, exactly; they don’t put them on in the morning and change out of them at night. What really happens is that the sporophyte grows out of the top of the gametophyte, so it looks like a headdress of some kind.
In this image of stagshorn moss, the green leafy part is the gametophyte and the tall spindle on top is the sporophyte.
The first thing bryophytes need to reproduce is water. Since they usually live in places that are moist at least some of the time, this isn’t really a problem for bryophytes. However, they still wait until a rainy period to reproduce, because they need water to carry sperm to the eggs.
Bryophyte reproduction happens in two ways, like with other plants. Remember all that alternation of generations stuff? Asexual reproduction occurs when a sporophyte releases spores, and sexual reproduction happens when gametes fuse and form a zygote.
When a bryophyte spore settles somewhere, it grows into a gametophyte. Gametophytes are green and leafy, but small. A gametophyte’s reproductive organs are called antheridia (male) and archegonia (female). The antheridia and archegonia look like little umbrellas sticking up from the plant. Antheridia make sperm and archegonia make eggs. As Rihanna says, when it’s raining more than ever, the eggs stay under the umbrella; meanwhile the sperm take a free ride on the rainwater and seek out eggs.
In the liverwort Marchantia polymorpha, the antheridia look like umbrellas:
After sperm and egg join in fertilization, the zygote grows into a sporophyte. This is still happening on the gametophyte, which makes the sporophyte generation completely dependent on the gametophyte generation.
Sporophytes make spores in the plant’s spore factory, called a sporangium. The spores are then released from a capsule on top of the sporophyte.
Ferns are seedless vascular plants. Instead of seeds, they grow from spores. Spores grow into gametophytes, which in ferns are very tiny and short-lived. The gametophytes release sperm to fertilize eggs, and fertilization happens right on top of the gametophyte. The sporophyte grows into a new fern plant, and produces spores to complete the life cycle.
A few things make this simple story a little more complicated:
Spores are produced in a sporangium (plural = sporangia). This is the spore factory, and that word will come back to haunt you. Sporophytes are diploid (2N), but since they produce spores through meiosis, spores are haploid (N). Sporangia occur on the underside of fern fronds in little clusters called sori (singular = sorus). If you lift up a fern frond, you will often see little dots in neat lines. These are sori, which will release spores when the time is right. Spores are carried by the wind (dispersed) to new locations.
Image from here.
The densely packed brown dots are sori, clusters of spores underneath fern leaves.
The fern’s sperm are produced in the sperm factory, or antheridium. Meanwhile, the female gametes, eggs, are housed in an archegonium. Gametophytes and gametes are haploid (N).
Sperm have flagella, which are whip-like tails that allow them to swim. In order to swim, they need water. This means ferns can only successfully reproduce in wet places, or after a rain. After fertilization, the sporophyte grows up on top of the gametophyte. A mature sporophyte makes spores, and the life cycle starts over again.
Gymnosperm means "naked seed," a name that reflects the fact that gymnosperms have no fruits to protect their seeds. However, the presence of seeds isn’t the only thing that separates gymnosperms from ferns; a lot of changes happened along the way in the evolution of seed plants. These changes include:
- the seed
- reduced gametophytes
- pollen grains
First, the seed. What is a seed? A seed consists of an embryo and its food supply. A seed has the potential to grow into a new plant, but isn’t considered a separate plant until it disperses and germinates. Because a seed has a protective coating in the form of a seed coat, it can lie dormant for months or years until conditions are right for it to germinate. Spores don’t enjoy the same relaxed schedule as seeds because they are usually made of only one cell, with no protective coating.
An example of a gymnosperm seed is the pinyon pine (below). Those round things in the middle are the seeds, with the remnants of a cone surrounding them.
(Public domain image from National Park Service, retrieved from)
Reduced gametophytes: We know gametophytes are essential for plant life cycles. In ferns, the gametophytes lived on their own, were photosynthetic, and were small compared to the sporophyte. In gymnosperms, the gametophytes are even smaller, only visible through a microscope. They aren’t photosynthetic and don’t live on their own: instead, gametophytes live on the sporophyte. There are also two kinds of gametophytes, bringing us to the next subject…
Heterospory: In seed plants, there is no one-size-fits-all spore. Instead, there are megaspores, which grow into female gametophytes, and microspores, which grow into male gametophytes. Each sporangium makes only one kind of spore, so the sporangia are either megasporangia or microsporangia. Where do we find these sporangia? On pinecones! And because not every gymnosperm is a pine, we also find sporangia on cones of other species, such as firs, redwoods, and spruces.
You probably have spent much more of your life thinking about ice cream cones and sno-cones than plant cones. Pinecones aren’t as tasty as frozen sweets, but have very important jobs to do, so we can’t blame them too much. The technical name for plant cones is strobili (sounds like a type of pasta, doesn’t it?), and they are the site of all the sporangia. Male cones (microstrobili) hold microsporangia, and female cones hold megasporangia. Male cones are temporary structures that exist only long enough to make and release pollen, but female cones can grow for years while the seeds they hold develop.
Here is a picture of male pine cones. See how small they are?
Ovules: As we already established, a megasporangium produces a megaspore, and they are stuck on a spore-bearing leaf (a sporophyll) on the sporophyte. A layer of sporophyte tissue surrounds and protects the megasporangium and megaspore. This layer of tissue is called an integument. The integument, the megasporangium, and the megaspore together make up the ovule. An ovule is where a female gametophyte develops and produces eggs. The integument will later give rise to the seed coat that protects the seed.
Pollen grains: Once the microsporangium produces a microspore, that microspore develops into a pollen grain that consists of a male gametophyte and a tough pollen wall that protects the gametophyte while the pollen grain disperses. When a pollen grain lands in an appropriate place (i.e. an ovule of the same species), the pollen grain germinates and a pollen tube grows. Sperm travel down this pollen tube into the ovule.
If you get seasonal allergies, you can probably blame your runny nose and itchy eyes on the wind-pollinated plants that release their pollen into the air and straight into your nasal passages. Just tell those plants that they would be better off finding animal pollinators and to leave you alone! Gymnosperms are wind pollinated, and some angiosperms, such as grasses, have evolved wind pollination even though their angiosperm ancestor was not wind-pollinated.
Why on earth would they do this? Wouldn’t it be better to have a direct pollen transfer from a specific animal rather than making lots of extra pollen and hoping it lands on another plant of the same species? It turns out pollen is cheap to make, energetically speaking, especially compared to sugary nectar and showy flowers. So some plants have become successful by letting the wind do the work. And if you think about where grasses live, in fields or open prairies, it seems likely that the pollen will land on the right spot.
OK so where is all this pollen going? And what exactly is pollen, anyway? Remembering the Alternation of Generations section, we learned that there are two parts of a plant’s life cycle, the gametophyte generation and the sporophyte generation. These in turn, bear gametes and spores. Pollen contains sperm, which is a gamete, and the sperm will fertilize an ovule, which is the plant’s egg. Put the two gametes together and…voila! We get a zygote that develops into a sporophyte. All the angiosperms and gymnosperms are sporophytes when visible to the naked eye. We’ve accounted for the gametes and the sporophytes, but where are the spores and the gametophytes?
Gametophyte spores come in two types: microspores and megaspores.
Microspores are the male spores, which develop into microgametophytes. In both gymnosperms and angiosperms, the male gametophyte is contained in the pollen grain. So pollen is really just an immature male gametophyte, which will mature when the pollen lands on a stigma and starts growing a pollen tube.
Megaspores are female spores. A megaspore divides and grows into a megagameophyte, also known as the female gametophyte. This happens on the sporophyll, which in a pine tree is just one scale of the pine cone. The gametophytes are tiny. The female gametophyte produces eggs.
Flowering plants, or angiosperms, are the dominant plants on Earth today. They evolved 220 million years after the first seed plants appeared, but quickly dominated plant life because of some major improvements:
- Flowers, which attract pollinators and improve the plant’s mating success
- Fruits, which protect the seed and help with seed dispersal
Flowering plants are masters of deception. You wouldn’t know it, with their perky stalks and brightly colored flowers or their tasty fruits, but there is a lot going on behind their innocent smiles—plants are manipulating us. We aren’t the only ones they dupe, either. Lots of animals are falling for their little tricks. Believe it or not, plants are tricking animals into helping them mate and carrying their offspring around for them. And you thought a rose is a rose is a rose.
Even though angiosperms would have us believe that flowers exist for the sole purpose of sending them to loved ones on birthdays and Mother’s Day, we know that plants have another motive: reproduction.
Just like animals and the other plants in this section, angiosperms reproduce with a sperm and an egg. But unfortunately for angiosperms, they don’t get to enjoy romantic walks on the beach and candlelit dinners before the sperm and egg meet. Most can’t just release their precious pollen to the wind and hope it lands on another plant of the same species. Angiosperms are stuck relying on a middleman for their reproductive success: a pollinator.
The pollen carries the genetic material that gets transferred from one plant to another, but pollen doesn’t have legs so it can’t walk around looking for mates. Instead, the pollen is picked up, usually by a flying animal such as a bee, bird or bat (the pollinator), and gets transferred from plant to plant as the pollinator looks for nectar, which is a sugary liquid flowers secrete. In the process of drinking delicious sweet nectar from flowers, the pollinator inadvertently carries pollen from one plant to another. Once the pollen arrives it fertilizes the plant’s ovary and a seed begins to develop.
In the image below, a honeybee extracts nectar from a flower. Yummy!
Plants that rely on animal pollinators are more successful than plants that use wind or water to transfer genetic material: this is why so many angiosperms have evolved and prospered. Even though angiosperms have fancy fruits and flowers, they still have gametophytes, gametes, spores and sporophytes. The angiosperm life cycle looks like this:
If you can ignore the flower for the moment, you’ll notice that on the sides of the diagram, a spore is just developing into a gametophyte, which makes gametes. This is similar to what we’ve seen before. On the right side is the microspore, which is the male side of things, and on the left side is the megaspore, holding down the female camp. When egg and sperm meet at the bottom, the ovule is fertilized and a seed and fruit form.
Now for the flower. Let’s look at the structure of a flower to figure out where the pollen starts and where it ends up. Don’t be scared of all the terms! They can’t hurt you!
The four main parts of a flower are the:
Sepals help protect the flower bud before it opens, since they are often thicker than petals and can enclose the bud completely. Sepals are often, but not always, green. In orchids, the sepals look just like the petals and can have complicated multi-color patterns.
Petals are the eye-catching part of the flower, and come in every color of the rainbow. Petals attract insects and other pollinators, and sometimes have hidden messages on them. Seriously. When viewed under ultraviolet light (which birds and bees can see usually, but humans can’t), some petals show nectar guides that point pollinators to the nectar.
Sometimes they look like landing strips for bees, and they often change color after the flower has been pollinated. This way, bees don’t waste their time looking for nectar in places where it has already been taken, and the flowers that haven’t been pollinated yet have a better chance of flagging down their pollinator.
Image from here.
The carpels, which are collectively called the pistil, are the female reproductive parts. Carpels are made up of ovaries, which contain reproductive cells called ovules. The ovules will develop into seeds after fertilization. On top of the ovary is a stem-like thing called the style, and the stigma, which is sticky and receives the pollen. One way to remember where the pollen lands is that the stigma is sticky.
The stamens are the male reproductive parts and contain pollen. The pollen is contained on the tip of the stamen, which is called the anther; it sticks up like an antler on a deer or moose. The filament is the stalk that holds the anther up. A pollen grain is actually more than meets the eye. In the anther, microspores divide and develop into male gametophytes, but the male gametophyte is only two cells. One of these two cells, the generative cell, produces two sperm. The other cell, the tube cell, produces the pollen tube that sperm travel down to reach the female gametophyte. The generative cell and the tube cell together make up a pollen grain.
Plant reproduction is pretty complicated, with a lot of steps, so it’s easy to understand why plants would enlist the help of other unsuspecting organisms. After all, plants don’t have legs or wings or any means of getting up and moving around. Makes it hard to go on a date, huh?
Pollen is found on the anthers.
Once the pollen grain lands on (or is brought to) a receptive stigma, a pollen tube starts to form from the tube cell. A pollen tube is a pathway that forms to transfer the sperm to the ovule; it is just one cell that grows longer and longer. The generative cell produces two sperm, which travel down the pollen tube. Fertilization isn’t complete until the two gametes (sperm and egg) fuse.
Flowering plants do a few things that are really strange. The female gametophyte only has seven cells in it, but the one in the middle has two nuclei. When the pollen grain arrives, two sperm travel down the pollen tube toward the egg. One sperm fertilizes the egg, forming a zygote, and the other sperm fuses with the two nuclei in the central cell of the female gametophyte, forming a triploid cell. This process is called double fertilization. The triploid cell will develop into the endosperm, which provides nourishment for the embryo.
Fruits, in addition to being tasty and full of vitamins, are vital to plant reproduction. All flowering plants make fruits, though these aren’t all fruits you would want to eat. A fruit, in the botanical sense of the word, is an enlarged ovary. So yes, that orange or apple you just ate is an ovary. However, in the case of the apple and many other tasty fruits, the edible part is more than just the ovary; often we eat the receptacle too.
While ovaries are developing into fruits, the ovule is developing into a seed. The zygote quickly becomes an embryo, and cotyledons start to form. In monocots, only one cotyledon forms and in eudicots, two cotyledons form. Cotyledons, sometimes called seed leaves, are important storage organs, and in eudicots will accompany the seedling when it emerges aboveground.
What happened to that weird triploid cell that resulted from double fertilization? It became endosperm. Endosperm is the plant’s way of storing food in the seed that the embryo will use when it germinates. Endosperm begins as a liquid; the triploid cell divides into more and more cells, but eventually turns solid as those cells develop cell walls. And guess what? You’ve probably eaten endosperm. Coconut milk is liquid endosperm, as is the juice that squirts out when you bite into a fresh corncob. Both of these have solid counterparts: coconut meat and the white part of popcorn. As the seed matures, cotyledons absorb nutrients from the endosperm.
In case the weather turns cold, a seed never leaves home without a coat. The seed coat is a hard protective layer that forms from the integuments of the ovule, and usually has to be opened in some way before the seed can germinate.
When you eat an orange, apple, watermelon, or cherries, what do you do with the seeds? Spit them out? That’s exactly what the plant wants you to do. They make a nice tasty fruit for us (or other animals) to eat, and then they make a hard seed that of course we are going to spit out, dispersing the plant’s seeds for them. The plant’s offspring are getting a free ride to a new place, which is good for the plant. If the plant and all its offspring are too close together, they could crowd each other out for sunlight, water and nutrients. Plus, a whole bunch of the same plant crowded together make it easier for pathogens to attack the whole lot; if the plants spread out a bit they are less likely to come under attack. Synthesis:
|Need water for fertilization?||Dispersal Mechanism|
Trickery by certain plants, such as orchids, is so common that there is even a name for what happens when a bee attempts to mate with a flower: pseudocopulation (basically, fake mating). The male bee is fooled into thinking that the flower is a female bee, and so he goes to mate with her. He inevitably picks up pollen from one flower and transfers it to the next flower he tries to mate with. When he leaves, the bee leaves behind fertilized flowers but not fertilized female bees.
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