Adrianne has a master's degree in cancer biology and has taught high school and college biology.
There are so many plants in our world. Some grow on the surface of the ground and add beauty to our landscape. Others grow in water and make the underwater scenery pretty. Most plants have a particular climate that they mostly live in. But, there are some plants that are able to live in any climate and are therefore found all over the world. One such group of plants are the liverworts. Liverworts are a group of about 7,000-9,000 species of flat, seedless plants.
Scientists believe that life, in general, started in water and then moved to land. This is the belief for animals as well as plants. Liverworts are believed to be among the earliest of the land plants. Liverworts are relatively small plants that do not grow beyond about 10 cm. You have probably seen liverworts before, especially if you have looked at rocks and other surfaces close to rivers and oceans. They look at lot like moss until you get a closer look.
One interesting thing about liverworts is their life cycle. They spend part of their life as a diploid plant and the other part as a haploid. Diploid means they have two sets of chromosomes and haploid means they have one set of chromosomes. This switching between diploid and haploid is referred to as an alternation of generation life cycle. Let's look a little closer at the full life cycle to see how this comes together.
The life cycle for a liverwort starts with a spore. This spore is called a gametophyte spore since it is haploid and germinates into the gamete-producing form of the liverwort. The gametophyte spore germinates into either a flat thallus or filaments with leaves. The thallus is easily identified by the flat, lobed shape. It resembles the appearance of a lobe of the liver which is how this plant got its name.
The liverwort spends the majority of its lifecycle in the gametophyte stage. In the next step in the lifecycle, the sex organs will grow from the gametophyte. Some liverworts are dioecious, meaning the male and female sex organs are on different plants, and some are monoecious, meaning both sex organs are present on the same plant. Whether it is on the same plant or separate plants, the antheridium, male sex organ, and archegonium, female sex organ, develop upward from the surface of the thallus. The antheridium releases sperm to swim to the eggs in the archegonium.
Fertilization is the next step in the cycle. When the sperm and eggs join together, then this produces the diploid sporophyte. The sporophyte is the diploid spore-producing form of the liverwort. The sporophyte remains in the archegonium for the first part of its development. The sporophyte will develop a foot to anchor itself to the gametophyte, a spore-producing capsule and a seta which is a stalk running between the foot and capsule.
The fully developed sporophyte is ready to make its way out of the archegonium in this next step. The seta on the sporophyte begins to push outward on the walls of the archegonium which causes it to rupture. The seta pushes the capsule up and out of the archegonium. During the same time, mitosis takes place within the capsule to produce spore-producing cells and elaters, which are the cells that will help release the spores. The spore-producing cells undergo meiosis to produce haploid spores.
Now that the haploid spores have been produced, they are ready for release from the capsule. The walls of the capsule are forced open by those elater cells that we mentioned earlier. When the capsule is opened, the spores are released into the air and blown away. These now become the new gametophyte spores that will start the cycle again.
Let's go over the highlights from this lesson. Liverworts are a group of about 7,000-9,000 species of flat, seedless plants. They have an alternation of generation life cycle, meaning switching between diploid and haploid. Diploid means they have two sets of chromosomes and haploid means they have one set of chromosomes.
The lifecycle of a liverwort is as follows:
1. Gametophyte spore (haploid spore that produces gametes).
2. Gametophyte spore germinates into either a thallus, flat, lobed shape leaf or filaments with leaves.
3. Antheridium and archegonium grow from the gametophyte.
4. Antheridium releases sperm.
5. Fertilization occurs producing a sporophyte (the diploid spore-producing form of the liverwort).
6. Sporophyte develops a capsule, foot and seta (stalk connecting the foot and capsule).
7. Seta ruptures the archegonium and pushes capsule out.
8. Mitosis produces spore-forming cells and elaters (cells that help release spores).
9. Spore-producing cells undergo meiosis to produce haploid gametophyte spores.
10. Elaters open capsule to release gametophyte spores and the cycle repeats.
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