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Reading: Seedless Plants

  • Page ID
    7683
  • Introduction

    Plants (kingdom Plantae) are autotrophs; they make their own organic nutrients. The term “organic” refers to compounds that contain carbon. Organic nutrients such as sugars are made by photosynthesis.


    Plants are adapted to living on land. For example, the above-ground parts of most plants are covered by a waxy layer called a cuticle to prevent water loss. Aquatic plants are secondarily adapted to living in water.

    Some evidence that suggests that plants evolved from the green algae is:

    • they both use chlorophyll a, chlorophyll b, and carotenoid pigments during photosynthesis.
    • the primary food reserve of both is starch.
    • they both have cellulose cell walls.

    Genetic and morphological evidence indicates that plants evolved from a group of green algae called charophyceans. Many charophyceans inhabit shallow freshwater environments. Natural selection may have favored individuals capable of surviving occasional drying in these environments and this gave rise to land plants.

    These traits occur in plants but not charophyceans. Some evolved independently in other algae.

    • Apical meristems
    • Alternation of generations
    • Spores with protective walls
    • Spores produced in sporangia
    • Gametes are produced in multicellular structures called gametangia; Antheridia produce sperm; Archegonia produce eggs
    • Multicellular dependent embryos
    • Many have a cuticle that waterproofs and offers some protection

    Alternation of Generations

    The basic alternation of generations life cycle is illustrated below.

    Spores produce gametophytes. The gametophytes contain antheridium and archegonium and produce sperm and eggs. The eggs undergo fertilization and become a sporophyte. The Sporophyte contains sporangia. the sporophyte undergoes meiosis and produces spores. The cycle continues from generation to generation. Generation N is the gametophyte and 2N is the sporophyte.

    The diploid plant that produces spores is called a sporophyte. The haploid plant that produces gametes is called a gametophyte.

    Some protists also have an alternation of generations life cycle but the structures that produce gametes in protists are usually single cells. Plants produce gametes in multicullar structures that have an outer protective layer. Sperm are produced in structures called antheridia (sing. antheridium), eggs are produced in archegonia (sing. archegonium),. As in protists and fungi, spores of plants are produced insporangia (sing. sporangium).

    A dependent sporophyte is a sporophyte that is small and grows attached to the gametophyte. It obtains nutrients from the gametophyte. An independent sporophyte grows separately from the gametophyte. Similarly, a dependent gametophyte is small and grows attached to the sporophyte while an independent gametophyte grows separately from the sporophyte.

    The evolutionary trend in plants has been from plants with a dominant gametophyte and reduced, dependent sporophyte (ex. Mosses) to plants with a dominant, independent sporophyte and a reduced, dependent gametophyte (ex. Seed plants).

    Classification

    Evolutionary relationships among the plants are shown below.

    An evolutionary tree starting with two branches: Charophyceans and plants. Plants branches off into Bryophytes (nonvascular) and vascular plants. Vascular plants branches off into Seedless vascular plants and seed plants. Seed plants branches off into gymnosperms and angiosperms.

    We will study the following phyla of plants.

    Characteristics Classification
    Bryophytes (no vascular tissue)

    Liverworts (Phylum Hepatophyta)

    Mosses (Phylum Bryophyta)

    Hornworts (Phylum Anthocerophyta

    Seedless vascular plants Club mosses, Spike Mosses, Quillworts (Phylum Lycophyta)Horsetails, Whisk Ferns, Ferns (Phylum Pterophyta)
    Gymnosperms (vascular, naked seeds)

    Conifers (Phylum Coniferophyta)

    Cycads (Phylum Cycadophyta)

    Ginkgos (Phylum Ginkgophyta)

    Gnetophytes (Phylum Gnetophyta)

    Angiosperms (vascular, protected seeds)

    Flowering Plants (Phylum Anthophyta)Monocots

    Eudicots

    Bryophytes

    Phylum: Bryophyta (Mosses)

    1. Observe different kinds of moss on display and note the body form of the gametophyte.

      Figure 1. Moss growing on a rock.
      Figure 1. Moss growing on a rock.

    2. Obtain live sporulating moss and identify the sporophyte and gametophyte generations.

      Figure 2. Moss gametophytes and sporophytes

      Figure 2. Moss gametophytes and sporophytes

    3. Draw the life cycle of a typical bryophyte such as moss. Your drawing should contain the following terms:
      1. 2N
      2. N
      3. sporophyte
      4. sporangium
      5. meiosis
      6. spores
      7. protonema
      8. gametophyte
      9. antheridium
      10. sperm
      11. archegonium
      12. egg
      13. fertilization
    4. Observe a slide showing the antheridial head of Mnium (a moss). Begin using the scanning (4X) objective and then switch to the low power objective (10X).
    5. What is produced in this structure (the antheridium)?
    6. Show where the antheridium occurs on the live moss plant. Indicate where this structure occurs in the life cycle diagram that you prepared (above).

    Figure 5. Mnium (a moss) antheridial head

    Figure 3. Mnium (a moss) antheridial head

    Figure 4. Mnium (a moss) antheridial head x40

    Figure 4. Mnium (a moss) antheridial head x40

    Figure 5. Mnium (a moss) antheridial head x100

    Figure 5. Mnium (a moss) antheridial head x100

    1. Observe a slide showing the archegonial head of Mnium (a moss). Begin using the scanning (4X) objective and then switch to the low power objective (10X).
    2. What is produced in this structure?
    3. Show where the archegonium occurs on the live moss plant. Indicate where this structure occurs in the life cycle diagram that you prepared (above).

    Figure 6. Moss archegonial head x 40

    Figure 6. Moss archegonial head x 40

    Figure 7. Moss archegonial head x 100

    Figure 7. Moss archegonial head x 100

    1. After the egg is fertilized, it grows and produces a sporophyte. A capsule containing a sporangium is found at the tip of the mature sporophyte. Refer back to figure 2 to view a sporophyte.
    2. Use a dissecting microscope to view a longitudinal section (cut lengthwise).
    3. What is produced within this structure (the capsule)?
    4. Be sure that you can identify the sporophyte and the sporangium on the live moss plant. Indicate where these structures occur in the life cycle diagram that you prepared (above).

    Figure 8. Moss capsule containing spores

    Figure 8. Moss capsule containing spores

    Figure 9. Moss capsule x40

    Figure 9. Moss capsule x40

    1. How are moss spores dispersed to new locations?

    Phylum: Hepatophyta (Liverworts)

    Observe live Marchantia (a liverwort). Do the plants contain gemma cups? What is the function of gemma cups?

    Figure 10. Marchantia (live)

    Figure 10. Marchantia (live)

    Seedless Vascular Plants

    Phylum: Pterophyta

    Ferns

    Figure 11. Fern gametophyte

    Figure 11. Fern gametophyte

    1. Draw the life cycle of a fern. Your drawing should contain the following terms:
      1. 2N
      2. N
      3. sporophyte
      4. sorus
      5. sporangium
      6. meiosis
      7. spores
      8. gametophyte
      9. antheridium
      10. sperm
      11. archegonium
      12. egg
      13. fertilization
    2. Observe sori on the underside of a fern leaf. Are sporangia visible? Indicate where this structure occurs in the life cycle diagram that you prepared (above).Figure 12. Fern showing sori on underside of leaf

      Figure 12. Fern showing sori on underside of leaf

      Figure 13. Fern sorus x40

      Figure 13. Fern sorus x40

    3. View a slide of a fern gametophyte showing antheridia. What reproductive cells are produced by gametophytes? Indicate where the gametophyte occurs in the life cycle diagram that you prepared.

      Figure 14. Fern prothallium (gametophyte)

      Figure 14. Fern prothallium (gametophyte)

    Horsetails

    Observe a live horsetail if available. Find the a strobilus. What reproductive structures are contained within the strobilus?

    Figure 15. Horsetails

    Figure 15. Horsetails

    Figure 16. Horsetail showing strobilus

    Figure 16. Horsetail showing strobilus

    Phylum: Lycophyta

    Members of this phylum have horizontal stems, upright stems, and small, spike-shaped leaves called microphylls.

    Club Mosses

    Observe a specimen of live club mosses such as Lycopodium. Find rhizomes. Identify microphylls. Do the specimens have any strobili? (Be sure to look up these words if you do not understand them.)

    Figure 17. Club moss (lycopodium)

    Figure 17. Club moss (lycopodium)

    Spike Mosses

    Observe a specimen of a spike moss such as Selaginella. Note the structure of the microphylls.

    Figure 18. Spike moss (selaginella)

    Figure 18. Spike moss (selaginella)


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