Alternation of Generations
Let’s back up a minute. Remember the birds and the bees, and how you learned that when two animals mate, they can make another animal? Sperm and eggs and that whole thing? Well plants have sperm and eggs too, but they can also reproduce by creating little mini versions of themselves called spores, which are single, haploid cells that contain the genetic material and instructions necessary to make another plant. Imagine if you could take one tiny cell from your body and put it in the ground and another person appeared. Crazy. Or is it? Plants do it without batting an eye, even potatoes, which actually have eyes. It turns out plants reproduce both sexually (sperm and eggs) and asexually (spores), and both types of reproduction are necessary to complete the cycle. That is, a sexually reproducing plant will make a plant that reproduces asexually, and then the asexually reproducing plant will make a sexually reproducing plant, and the circle goes round and round. Each reproductive event produces a new generation, and they alternate the types of reproduction, thus the name alternation of generations. What’s a generation? You and your siblings are one generation, and your parents are a different (and older) generation, and your grandparents are another, even older, generation.
There are special names for each of the plants we’ve been talking about. A sporophyte is a plant that produces spores (asexual reproduction) and a gametophyte is a plant that produces gametes (sexual reproduction). The word gametes refers to sperm and eggs. You need two gametes to make a new plant; otherwise you might end up with a sperm plant or an egg plant. Wait, is that how we funny purple vegetables? However, you only need one spore to grow into a new plant. All plants have alternation of generations, whether they are flowering or non-flowering, evergreen or deciduous, vascular or non-vascular. If you don’t know what all those words mean yet, don’t worry, we’ll get to them.
Let’s look at ferns as an example of alternation of generations. The fern plants you see out in the woods, or in a garden, or by a river are sporophytes. They produce spores. If you turn over a fern frond (that’s just a fancy word for leaf), sometimes you’ll see little circles all over the underside of the leaf. These are called sori and they are clusters of lots and lots of tiny spores. When the wind blows and releases the spores into the air, they scatter and eventually fall to the ground.
After a few days, a new plant starts growing. This plant is a gametophyte, so it will produce gametes. The fern gametophyte doesn’t grow into a large plant. It’s very tiny and will only last a few days. When the time is right, which is usually after a rain when plenty of water is around, the gametophyte releases its sperm and they swim to find eggs on other gametophytes.
The beginnings of a new plant form when a sperm reaches an egg. The cells start dividing and growing into a new plant, right on top of the gametophyte. Soon the new plant, the sporophyte, gets bigger and bigger and the gametophyte deteriorates. The gametophyte’s job is done now that it has released its gametes, and the new sporophyte grows and grows, starting the cycle over again. A general form of alternation of generations looks like this:
A general form of alternation of generations looks like this:
The forms that the sporophytes, spores, gametophytes and gametes take on vary depending on the type of plant. In bryophytes (mosses and their ilk), the gametophytes are the bigger generation but in other land plants, sporophytes are bigger.
Let’s look at these different types of plants in a little more detail. There are four major types of plants. From earliest in the evolutionary order to latest, they are:
Starting at the bottom of the totem pole, we have bryophytes, which include mosses, liverworts and hornworts. These are non-vascular plants, which means they don’t have a circulatory system. Bryophytes are usually only a few centimeters tall, which is why you probably haven’t gotten to know them very well. But just go check out a few cracks in the sidewalk, and a bryophyte will be happy to make your acquaintance. In bryophytes, gametophytes are the dominant generation and sporophytes grow on top of the gametophytes.
What do bryophytes look like? They’re actually pretty cool looking. Don’t believe us? Check it out yourself, in this old botanical drawing, done in 1904 by Ernst Haeckel:
All of the above plants are bryophytes, with both generations shown. The part on the bottom of each plant is the gametophyte, and the sporophytes are the spindly things growing on top of the gametophytes. Funky, right?
Next up in the evolutionary lineup are ferns. Ferns are tracheophytes, which makes them vascular plants and lets them grow taller than bryophytes because they can circulate water to leaves far from the ground. Ferns do not have seeds: they only have spores. In ferns, gametophytes are tiny and only live for a few days. The sporophyte generation is the dominant generation.
Fern gametophytes are very small and heart-shaped. Here is a picture of one with the sporophyte generation just starting to grow on top:
The fern sporophyte can get quite big:
Image from here.
Gymnosperms are seed plants but they are called "naked seeds" because their seeds don’t have any protective structure surrounding them. Pine trees, spruces, firs, and redwoods are examples of gymnosperms, and are all sporophytes. Not all gymnosperms are what we call "evergreens." Ginkgo trees are gymnosperms, and they lose their leaves during winter. Gymnosperm gametophyes are even smaller than fern gametophytes.
Here is a gymnosperm embryo, in this case an avocado seed, surrounded by gametophyte tissue. Read more about this in the Reproduction section:
Compare the gametophyte to the Gingko biloba sporophyte:
Image from here.
Angiosperms are flowering plants and their bright colors make them favorites in gardens, in bouquets and for corsages. Angiosperms make flowers and fruits, so you can thank them for most of the food we eat in addition to making the garden look nice.
Angiosperm sporophytes can be trees, shrubs, grasses, or forbs. One example of an angiosperm sporophyte is this lily:
The angiosperm gametophyte is so small, you can’t see it without looking into a microscope. And since we don’t have a microscope on hand at the moment, we’ll just skip looking at it. But don’t worry, it’s only a few cells. You’re not missing much.