A Gymnosperm Life Cycle: Reproduction of Plants with 'Naked Seeds'

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  • 0:05 Introduction to Gymosperms
  • 0:19 Review of Alteration…
  • 2:00 The Haploid Stage
  • 3:39 The Diploid Stage
  • 5:19 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

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 produce seeds but not flowers, such as the examples shown here.
Seed-producing plants

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.

Diagram of alternation of generations
diagram 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.

Image of diploid organism

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.

Organisms look different in the diploid and haploid stages.
comparison of diploid and haploid

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 our basic 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.

In the spring, male pollen cones release pollen that lands on an ovulating female cone.
Haploid stage

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 above 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.

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