Please bring your textbooks to lab this week!
There are three phyla of nonvascular plants, the Phylum Hepatophyta (liverworts), Phylum Anthocerophyta (hornworts), and Phylum Bryophyta (mosses). The plants of these phyla often are called nonvascular cryptogams, because they do not have pollen or seeds. Today, we will study only the Hepatophyta and Bryophyta.
Phylum Hepatophyta (the liverworts)
As you study liverworts, you might want to refer often to the life cycle of a typical liverwort, on pp. 354-355 of your textbook. There also are some excellent pictures on pp. 348-353. If you have the Photographic Atlas, there are many excellent pictures in Chapter 6. Keep in mind that the gametophyte is the dominant phase of all liverwort life cycles.
- Living Gametophyte whole-mount
- Take a small piece (2 inches) of the living gametophyte (sometimes called a thallus) back
to your table to work with. First, examine it using a dissecting microscope. Note the following
- gametophyte thallus
- The basic body, lacking any obvious structures like stems or leaves
- air pores
- Little holes on the upper surface of the thallus
- Thin 'roots' extending from the lower surface of the thallus
- Thicker 'roots' on the lower surface of the thallus
- Gemma Cups
- Small 'dishes' on the upper surface of the thallus that contain gemmae. In Lunularia they are crescent shaped; in Marchantia they are round.
- Disc-shaped green objects inside the gemma cup
- Living Gametophyte cross-section
- Now make a cross section of the thallus. Lunularia and Conocephalum are thicker
than Marchantia, so they are a little easier to work with, if available. This is a tricky skill to
master, so be patient! To make a clean section, we'll be using the double-sided razors. They are sharper
than the single sided razors, but they dull quickly. Get a half-razor from the front desk, and be sure to
dispose of it in the sharps bin at the end of the lab.
- Start by cleaning off your sample, clearing away any soil and rinsing the thallus in water to remove any small grit or sand.
- Place the sample on a clean microscope slide, and wet it with a small drop of water.
- Place the slide under the dissecting microscope, and carefully cut several sections with the half-razor while viewing at the lowest magnification.
- Remove your sample, leaving only the sections on the slide. If you need to, add another drop of water, then place a cover slip on the slide.
- Move your slide to the compound microscope, and look for the following structures:
- Air pores
- Dome-shaped openings along the upper surface of the thallus.
- Chlorophyllose filaments
- A layer of cells just below the upper surface with lots of chloroplasts in them.
- Storage parenchyma
- A thick layer of cells below the chlorophyllose cells. They are bigger than the chlorophyllose cells, contain few or no chloroplasts, and may contain oil bodies for storing energy.
- Thin 'roots', only one cell thick and usually with many small pegs projecting into the cells from the cell wall.
- Thicker 'roots', two or more cells thick, and lacking pegs.
- Cross section of gametophyte (slides in separate slide box, on side counter)
- Compare what you see in your fresh sections with the prepared slides. You should be able to see all of the features listed above.
The thalloid liverworts have a waxy cuticle on the surface of the thallus to reduce water-loss. The cuticle also seals off the thallus from gas exchange. The air pores allow gas to pass into the chlorophyllose cells below - why is this important? Liverworts, like all bryophytes, do not have true roots. They absorb mineral nutrients through all parts of the thallus (at least, all parts without cuticle). The rhizoids and scales serve only as anchors, keeping the thallus attached to the substrate. They are not specialized for absorbing nutrients like true roots.
Liverworts, like all bryophytes, display alternation of generations. The leafy green plant that we see is the gametophyte. The actual gametangia form in specialized structures called antheridia (males) and archegonia (females). The eggs are formed inside the archegonia. After they are fertilized, they develop into an embryo, still protected within the archegonia. The embryo, and the sporophyte that it develops into, are nutritionally dependent on the gametophyte. This means that the sporophyte cannot produce its own food, but must rely entirely on resources provided by the gametophyte. However, the sporophyte and gametophyte are still two separate individuals - there are no direct connections between them, not even plasmodesmata. The gametophyte exudes sugars across a placenta, and it is absorbed through the foot of the sporophyte.
The sperm are transferred from the antheridia to the archegonia by a combination of splash dispersal and free swimming. Splash dispersal requires that a raindrop lands on the antheridia, and in the subsequent splash carries away the mature sperm. Hopefully, some of the sperm are splashed onto a female gametophyte, and can then swim to the archegonia. In contrast, spores are dispersed by the wind, and are incapable of independent locomotion.
- Preserved sporangia and gametangia
- On the side counter, examine preserved antheridiophores and archegoniophores under the dissecting scope provided. Note the following: archegonial rays, antheridial disk. Note that all the structures you see are haploid.
- Marchantia antheridia (slide #1, slide box C)
- Note antheridiophore, antheridial disk, antheridia, sperm, rhizoids. Note that all structures are haploid.
- Marchantia archegonia (slide #2, slide box C)
- Note archegoniophore, archegonial rays, archegonium (neck, venter, egg) . Note that all structures are haploid.
- Marchantia sporophyte (slide #3, slide box C)
- In addition to the structures noted above, note the sporophyte (foot, seta, sporangium), spores, elaters, calyptra. Consult the diagram on pp. 354-355, and note which structures are diploid and which are haploid.
You should know the function of sperm, eggs, and spores. How do the other structures contribute to the role of these three components?
- Cross section, gemma cup -- on display under compound microscope, side counter.
- (the slide label may say "cupule" = an archaic term for gemma cup)
- Gemma are vegetative propagules. How do you think they are dispersed?
- How else could a liverwort gametophyte reproduce asexually - what advantage could having an unspecialized thallus-type body have when it comes to asexual reproduction?
(see Fig. 16-16, p. 356, textbook)
There are many more species of leafy liverworts than of thalloid liverworts. The gametophytes of leafy liverworts often look like mosses, except that:
- They have two kinds of leaves: large leaves arranged in two parallel rows, and smaller leaves along the lower surface of the stems
- Their leaves do not have midribs
- The larger leaves have a small lobe at the base
Leafy liverworts are common on tree bark, rocks and especially in wet places, such as on rocks near waterfalls.
- Living Gametophyte
- There are a variety of leafy liverworts on display. You need to be able to distinguish them from true mosses. Be sure you've examined representative specimens under the dissecting microscope, so that you have seen and understood the distinguishing characteristics listed above.
- Use the forceps to put a single liverwort leaf flat on a glass slide, and examine it with the compound microscope. Carefully shift the focus up and down: can you tell how thick the leaf is - how many cell layers are there? Is there a midrib?
Phylum Bryophyta (the mosses)
Moss gametophytes usually have small leaves that do not overlap like those of leafy liverworts. When moss spores germinate, they form filamentous structures called protonemata. After a time of growth, the protonemal filaments form multicellular budlike structures that grow into mature, leafy moss gametophytes. Refer often to the moss life cycle diagram on pp. 362-363 of your textbook, and to pictures on pp. 358-366.
- Living Moss Protonemata
- Make a temporary water mount from the protonema culture. Note: protonema cells, chloroplasts, rhizoids.
- These are mature cultures, so you should be able to find some buds, where the stem and
leaf structure typical of moss gametophytes has developed.
- How can you distinguish moss protonemata from algae?
- Are chloroplasts present in the rhizoids?
- Permanent Slide, Moss Protonemata (slide #7, slide box C)
- Note: protonemata and buds.
- Are any rhizoids visible?
- Living moss gametophytes whole-mount
- Examine some of the representative moss specimens under the dissecting microscope. Can you tell them apart from the leafy liverworts? Moss leaves are usually spirally arranged around the stem, and are all the same size and shape. Make a slide of a single moss leaf and view it under the compound microscope. How thick is it? Is the whole leaf the same thickness? Does it have a midrib?
- Living moss stem cross-section
- Using the same technique you used to make a cross-section of the thalloid liverwort, try to make a section of the moss stem. Pick a species with thick stems, and use the forceps to remove the leaves before you start, then use the half-razor to make the sections directly on the glass slide, under the dissecting microscope.
- If you can make a clean section, you should see:
- the dense layers of cells along the outer edge of the stem
- the larger cells inside the epidermis
- central strand
- a layer of smaller cells in the center of the stem
- The inner-most cells of the central strand are water-conducting hydroids, surrounded by sugar-conducting leptoids. These cells are elongated, but are not lignified like the true vascular tissues in the vascular plants.
- Permanent slide, Mnium stem (slide box on side counter)
- Compare what you see in your fresh preparations with the prepared slide. You should be able to see all the structures listed above. In addition, you may see leaf traces. These are branches of the central strand that connect to the midrib of the leaves. Not all mosses have leaf traces.
- Permanent Slides of Moss Gametangia: Mnium antheridia (slide #8, slide box C)
- Note antheridia, and sperm.
- Note that all structures are haploid.
- Mnium archegonia (slide #9, slide box C)
- Notice that these archegonia are erect, not pendulous [hanging down] as in Marchantia.
- Note archegonium, archegoniophore, neck, venter, egg.
- Polytrichum capsule (slides 10 and 11, slide box C)
- The capsule is the meiosporangium of mosses. Note spores, columella, tapetum, operculum, and peristome region. (The common name of Polytrichum is hairycap moss.)
- Which structures are haploid and which structures are diploid?
- What the the function of each structure?
- Sporophytes of living mosses
- Re-examine the living moss specimens. Do any of the gametophytes have sporophytes? How many of the structures that you saw on the slides can you identify on the whole plant? You should be able to recognise at least the seta and capsule. Some of the specimens may still have a calyptra, which develops from the venter and covers the capsule before the spores are released. If there are specimens available with mature capsules, examine them under the dissecting microscope and see if you can find the operculum and the peristome teeth.
- Which structures are haploid and which are diploid?
A moss that is not a moss !! . . .
The Spanish Moss on display is not a moss. It is a vascular plant that reproduces by flowers, fruits and seeds. It is related to pineapple! (Pineapples, Spanish moss, and their relatives are called bromeliads.) Sometimes bromeliads are called air plants -- and what does that mean? Also, many bromeliads are epiphytes -- and what does that mean?