A Gymnosperm Life Cycle: Reproduction of Plants with 'Naked Seeds'
- Track Progress
- 0:05 Introduction to Gymosperms
- 0:19 Review of Alteration of Generations
- 2:00 The Haploid Stage
- 3:39 The Diploid Stage
- 5:19 Lesson Summary
Some plants, such as pine trees, are able to reproduce with unprotected seeds. We will look at the major structures involved in this form of alternation of generations in gymnosperms.
Introduction to Gymnosperms
Gymnosperms are vascular plants that produce seeds but not flowers. These plants are more advanced than ferns but not quite as complicated as flowering plants. Common examples of gymnosperms include ginkgos and pine trees. These all produce seeds but do not make flowers.
Review of Alternation of Generations
We already know that many plants go through a life cycle that alternates between diploid and haploid, but let's review some basic aspects of this before looking at how gymnosperms go through this process.
Alternation of generations is a life cycle that includes both diploid and haploid multicellular stages. Remember that 'diploid' means 'two sets of chromosomes' and is commonly abbreviated as 2n, where the n stands for 'chromosomes.' In diploid cells, one copy of the chromosomes comes from each parent. For example, in humans, you get one copy of chromosomes from your dad and one copy of chromosomes from your mom.
The same idea is found in plants. Each diploid cell contains one copy of chromosomes from the male parent and one copy of chromosomes from the female parent. 'Haploid' means 'one set of chromosomes' and is commonly abbreviated as n because there is only one copy of the chromosomes.
Let's look at a diagram of the basic idea of alternation of generations. We can see in this diagram that the life cycle is broken into n - on the top - and 2n - on the bottom. Remember that the gametophyte contains haploid cells and that the sporophyte contains diploid cells.
Previously, we also used the Garblinx to illustrate the oddity of this. Remember that the diploid or 2n organism looks like this. However, when it moves into the haploid stage, the Garblinx looks completely different! Two of these haploid organisms will get together and mate in order to produce a new 2n organism that looks similar to our first Garblinx. This Garblinx will then eventually produce a new haploid organism, and so on.
Now that we have reviewed the basic aspects of this life cycle, let's look at how gymnosperms go through the cycle. We will start with the haploid stage as we did previously and then move into the dominant diploid stage.
The Haploid Stage
As we move through the different stages in this alternation of generations, we will refer back to this diagram illustrating how gymnosperms reproduce. It is important to note that the haploid stage is dependent upon the dominant diploid stage for protection and nutrition. This means that the haploid gametophyte is found within the mature sporophyte. Let's look at this all in a bit more detail.
In gymnosperms, two types of spores are produced, so the haploid stage can start as either a male or female spore. The microspore is the small, male spore. This spore will undergo mitosis in order to produce the multicellular male gametophyte. These male gametophytes are the pollen grains that are contained within pollen cones. We can see in this picture that pollen cones are generally small and in groups on pine trees.
The megaspore is the large, female spore. This spore will undergo mitosis in order to produce the multicellular female gametophyte. The female gametophyte is housed in the ovulate cones that look like what you generally think of when you think of pine cones. We can see here that they are larger and generally easier to see than the male pollen cones.
The haploid stage will continue as long as the male and female gametophytes are kept separate. However, at some point, usually in the spring, the tree will release large amounts of pollen. These pollen grains that contain the male gametophyte will travel by air and hopefully land on an ovulate cone. A pollen tube then develops in order for the newly produced male gamete to reach and fertilize the female gamete. Remember that gametophytes produce gametes - in the case of gymnosperms, there will be a male gamete, or sperm, and a female gamete, or egg, produced from different gametophytes.
The Diploid Stage
The diploid - and dominant - stage of life will begin once fertilization occurs. Remember that fertilization is the fusion of an egg and sperm. Both the egg and sperm are haploid, so when they fuse together, we now have one diploid cell. Interestingly enough, pollination normally happens about a full year before fertilization in conifers.
Remember that gymnosperm means 'naked seed.' This means that, while the new embryo has a protective seed coat, it is still very fragile and has a limited food supply. The seeds are released from the ovulate cone once the scales of the pine cone open up. You may have seen individual seeds if you have ever taken a pine cone apart or even accidentally stepped on one. Generally, these small seeds will be spread by the wind. Like with the pollen, hopefully the seed will land in a good location so that it can develop into a mature plant.
This new cell will continue to go through mitosis and grow. After germination, the plant will develop into a small seedling. Eventually, it will become a mature sporophyte and look like the pine trees that you know. There are many different structures in the mature sporophyte, but we'll focus on those involved in reproduction.
The two main structures were previously mentioned when we went over the haploid stage. Most mature gymnosperm sporophytes have both male and female cones. That is, they contain both pollen cones and ovulate cones. The pollen cones contain many microsporangia, which produce the male microspores through meiosis that we started with in the haploid stage. The ovulate cones contain megasporangia, which produce the female megaspores through meiosis that we also started with in the haploid stage.
Gymnosperms are unique plants because they produce naked seeds. This alternation of generations in gymnosperms, such as pine trees, means that there are multicellular stages that are haploid and diploid. We first looked at the haploid stage, which starts as either a male or a female spore that develops into a gametophyte. The male microgametophyte is contained in pollen grains and the female megagametophyte is found in the ovulate cone.
Eventually, the pollen grains are carried by wind to the ovulate cones. There, the egg is fertilized by the sperm, and the conifer moves into the diploid stage of the life cycle. This new diploid seed will be released from the ovulate cone and again carried by the wind. Hopefully, it will develop into a mature sporophyte, which is the structure you think of when you think of pine trees.
The sporophyte contains both male and female sporocytes. The microsporangia - which produce the male spores - are located in pollen cones. The megasporangia - which produce the female spores - are located in ovulate cones. These spores start the haploid stage all over again, continuing the complex life cycle of gymnosperms.
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