Test scheduled for 3 December
Considering the enormous amount of diversity among the plant kingdom, it would not be unreasonable to expect a similar large range of methods of reproduction. In one sense this is true, but the more important observation is the large degree of similarity in the life cycles of plants ranging from the simple mosses to the modern flowering plants. In this unit we will study the life cycles and reproductive processes of the mosses, ferns, and a few other "lower" plants. Keep in mind the general similarities among these plants as well as their unique differences. In Unit 15 we study the life cycles of the seed bearing plants, the conifers and flowering plants.
Learning Objectives: The successful student will be able to ...
- describe the life cycle of a simple green algae, moss, and fern.
- relate these life cycles to the alternating haploid and diploid stages of the cycle.
- describe the important similarities and differences in the life cycles of these three groups.
- name the major taxonomic group to which these organisms belong and their presumed evolutionary relationships to one another.
- relate changes in the life cycles of these groups to their habitat and size.
Lesson One: Chlamydomonas, Life Cycle Of A Simple Alga.
Before we begin our study of plant life cycles, we will study the life cycle of one of the simplest eukaryotic organisms, Chlamydomonas. This is a single celled, green algae in the kingdom Protista and phylum Chlorophyta. The similarity of their genome and biochemistry of their photosynthetic metabolism has led scientist to conclude that modern multicellular plants evolved from this group. Don't make the mistake of thinking that Chlamydomonas is the ancestor of modern plant; this species is simply a member of the large group of green algae some of which evolved into plants.
Go to this page by Professor Taggart at Michigan State University and read the details of the Chlamydomonas life cycle. Notice that this alga can reproduce sexually and asexually. Understand how these two different modes of reproduction are tied to the environmental conditions. Be sure you can describe this life cycle in detail including the timing of meiosis, the relationship between the haploid and diploid phases of the cycle, and the differences between the isogametes and zoospores ("iso" means "looks the same" and "zoo" means motile).
Homework, Due 17 November. Email the answers to the following questions using your Fontbonne account.
- List some of the reasons why algae in the phylum Chlorophyta are thought to be evolutionary precursors to the higher plants.
- Another green algae is Volvox. Describe how this species is similar and how it is different from Chlamydomonas. Start here with your research.
- Same question for Spirogyra and Euglena.
Image of Chlamydomonas from EMBO
Image of moss from Seeds Aside
Lesson Two: Life Cycle of Mosses.
The mosses are among the most primitive land plants. They belong to the phylum Bryophyta and are characterized by their lack of vascular tissue (xylem and phloem) and true roots, dependence on moisture for reproduction, and generally small size.
Kimball has a brief section illustrating the life cycle of mosses. Notice the similarity to the alternating diploid (sporophyte) and haploid (gametophyte) phases of the life cycle. As with Chlamydomonas, be sure you understand the relationship between these two stages and the events of meiosis, gamete and spore production, and fertilization. Here is another good review from the Tree of Life project including a good discussion of the importance of mosses in the evolution of land plants (in more detail than you need). Here is a nice animation of the life cycle of a typical moss.
Homework, Due 22 November. Email the answers to the following questions using your Fontbonne account.
- In animals gametes are produced by meiosis. Is this true of the green algae and mosses? Explain.
- The life cycle of mosses is usually described as "alternating generations." What does this term refer to and would is also apply to Chlamydomonas?
- Does Chlamydomonas have distinct female and male genders? How about the mosses? Explain.
- How is the reproduction of moss dependent on moisture and why does this explain where we usually find them? Explain and give examples.
- How does this dependence on water link the evolution of mosses to the green algae?
Lesson Three: Life Cycle of Ferns.
Ferns are vascular plants from the phylum Pteridopsida. They can grow to much larger size than mosses and are found in a much wider range of habitats than mosses. They have an extensive fossil record going back more than 300 million years and were a major feature of the Carboniferous era that produced much of the Earth's coal deposits. Go to this section of Kimball and study the life cycle of a typical fern. Notice the similarity of the alternating generation life cycle of ferns to that of mosses. Appreciate the difference in size between the large sporophyte stage and small gametophyte stage. This difference will carry through in the so called higher plants (conifers and flowering plants). This section of the Online Biology text gives a good summary of ferns and their life cycle.
Homework, Due 30 November. Email the answers to the following questions using your Fontbonne account.
- Explain why mosses are far more restricted in their habitats than are ferns. Include a discussion of their anatomical structures and mode of reproduction.
- Explain why ferns are rarely found in dessert biomes that support a large variety of conifer and flowering plants. Hint: think what major feature evolved in the conifers and flowering plants that ferns (and mosses) lack.
- Why can ferns grow to a much taller hight than mosses?
- We have just studied a few examples of these mosses and ferns. There are a number of other groups related to these examples. Pick one other example of a non-vascular plant and one other example of a non-seed producing vascular plant. Describe their appearance, their typical habitat, and their life cycle. This section of the Online Biology text will be a good place to start your research.
Autumn ferns, Acadia National Park, Maine from WallpaperWeb.org
