Bryophytes and Ferns

Materials

1. Live mosses with sporophytes attached

2. Prepared slide of moss protonema

3. Live Marchantia with archegoniophores and antheridiophores

4. Petri dish with live protonemata (demonstration)

5. Prepared slides of longitudinal sections of archegonial heads of Mnium (or similar moss)

6. Live hornworts (demonstration)

7. Variety of live fern plants, one with expendable fronds that have mature sori

8. Live prothalli (demonstration)

9. Prepared slides (whole mounts) of bisexual prothalli

Some Suggested Learning Goals

1. Understand how the development of gametangia (structures in which sex cells are produced) and zygotes of members of the Plant Kingdom differs from the development of gametangia and zygotes in members of other kingdoms.

2. Understand how the form and structure of bryophytes differs from that of more complex plants.

3. Know what develops or takes place in each phase of the life cycle of a moss.

4. Know what develops or takes place in each phase of the life cycle of a liverwort.

5. Learn how asexual and sexual reproduction of both thalloid and "leafy" liverworts differs from that of mosses.

6. Be able to explain basic differences between the sporophytes of ferns and mosses.

7. Know the life cycle of a typical fern.

8. Understand the nature of a prothallus and a sorus, and the roles they play in a fern life cycle.

Introduction

In algae, fungi, and other relatively primitive organisms that in the past were regarded as plants, the gametes (sex cells) are produced in single-celled gametangia, and the zygote often undergoes meiosis directly. Beginning with the bryophytes (e.g., mosses), however, the gametes are produced in gametangia that are composed of many cells; the zygote, through mitosis, develops into an embryo that, in turn, develops into a diploid sporophyte. Spores are produced by meiosis within a specialized part of the sporophyte.

Alternation of Generations in bryophytes and ferns is marked by the development of distinct, separate gametophyte and sporophyte bodies. In bryophytes, the sporophyte, while a distinct body in itself, is dependent on the gametophyte for most of its nutrition. In ferns, however, both the gametophyte and the sporophyte are photosynthetic and independent of each other.

A. Bryophytes (Phyla Bryophyta, Hepaticophyta, and Anthocerophyta)

Mosses, liverworts, and hornworts are included in these three phyla. Bryophytes differ from higher plants in Jacking xylem and phloem, although some do have specialized cells that can conduct a little water and food in solution. Some species may form extensive low mats consisting of dozens or even hundreds of plants. Because true xylem and phloem are lacking, however, none of the individual plants become very large. They cannot grow or function very long without external moisture; hence their usual association with damp habitats.

Examine the clump of moss provided. The clump consists of small green "leafy" gametophyte plants. ("Leafy" is in quotation marks because unlike true leaves, which are diploid (2n), those of mosses consist of a single layer of haploid (n) cells; moss and liverwort "leaves" also have no internal structure or stomata.) The "leaves" do, however, carry on photosynthesis like the true leaves of more complex plants.

Some of the moss plants may have a thin stalk or seta emerging from the tip. A capsule (sporangium) develops at the free end of each seta. The seta and capsule are diploid (2n) and constitute the sporophyte. Sporocytes (not visible here) within each sporangium undergo meiosis, producing spores. The sporangium is usually partially to completely covered with a "pixie cap" called a calyptra. The calyptra develops from archegonial tissues and is therefore n (haploid). When the calyptra is removed, a second, smaller (2n) cap may be seen covering the free end of the capsule. This smaller cap, the operculum, develops with the sporophyte and eventually pops off, allowing the spores to disperse. Release of the spores is partially controlled by tiny peristome teeth at the rim of the capsule; the peristome teeth, which resemble tiny, cross-ribbed shark's teeth, move in response to changes in humidity.

Turn now to a prepared slide labeled "Moss protonema." A protonema is an algalike body that develops when a moss spore germinates. Notice the chloroplasts present in each cell, and that the transverse walls of the cells usually are not strictly at right angles to the other walls. Note, also, the "buds" that are developing along some of the threads. These buds become new "leafy" gametophyte plants. Some may already have rootlike rhizoids at their bases. Rhizoids are only one cell thick; they may anchor bryophyte plants in the same way true roots do, but like the remainder of a moss gametophyte, they have no xylem or phloem and can absorb water slowly only in very limited amounts. The word rhizoid should not be confused with rhizome, which is a term applied to the horizontal stems of ferns and higher plants.

Next turn to a slide of moss archegonia. Archegonia are female reproductive structures of mosses produced at the tips of female gametophyte plants. Occasionally both male and female reproductive structures are produced on the same plant. Each archegonium loosely resembles a tiny vase with a narrow neck, the enlarged base itself being elevated on a short, relatively wide stalk. Although there are usually several to many archegonia produced at the tip of each plant, the archegonia are not always strictly upright, and they are interspersed among sterile, multicellular hairs, called paraphyses. When microscope slides of moss archegonia are made, very thin longitudinal sections are cut and stained. Parts of the archegonia and paraphyses are often sliced off; because of this you may not have a complete archegonium on your slide. You should, however, be able to see at least one archegonium with its base intact. The cavity within the archegonium base contains an egg.

Turn now to a slide of moss antheridia. These structures, before they are sliced, are shaped like miniature clubs; they contain numerous sperms. Paraphyses usually are present among the antheridia. In nature, a sperm swims down the neck of an archegonium and unites with the egg, forming a zygote. As the zygote divides, it forms an embryo, which is dependent on the gametophyte for its nutrition. The embryo then develops into a sporophyte, consisting of a seta and capsule. Even the mature sporophyte is still largely dependent on the gametophyte for its energy.

Liverworts have nearly all of the reproductive structures found in mosses. Although there are many species of "leafy" liverworts, some of the most common and bestknown forms are thalloid. Thalloid liverworts have flattened bodies that look a little like bright-green foliose lichens. Examine the thalloid liverworts provided. Some, such as Marchantia, have their archegonia and antheridia elevated above the thallus on umbrellalike archegoniophores and disc-shaped antheridiophores. Many thalloid liverworts also reproduce asexually by means of gemmae, which are tiny lens-shaped pieces of vegetation produced within gemmae cups. The gemmae cups are scattered over the surface of the thallus. Each gemma is potentially capable of developing into a new thallus.

Examine the demonstration of hornworts provided. How do the sporophytes of hornworts differ in appearance

from those of liverworts and mosses? Are there any other apparent features that distinguish hornworts from other bryophytes?

B. Ferns (Phylum Pterophyta)

Unlike the bryophytes, the ferns do possess true conducting tissues (xylem and phloem), and the sporophyte is the more conspicuous phase of the life cycle.

Examine the fern plants on display. The leaves or fronds arise from a horizontal stem (rhizome). Notice the small brownish patches on the backs of mature fronds. Remove a small part of a frond that has these patches and examine them with the aid of your dissecting microscope. Each discrete patch is called a sorus and consists of a cluster of sporangia. The sporangia are often partially or wholly covered by a transparent, umbrellalike indusium. Sporocytes within the sporangia undergo meiosis, producing spores. The spores are released through the springlike action of the annulus, which is composed of heavy-walled cells around most of the edge of the sporangium.

Now examine the green heart-shaped prothalli that constitute the gametophytes of ferns. Both living and preserved prothalli may be provided. Some prothalli produce only archegonia, others only antheridia. The prothallus on the microscope slide produces both. Find an antheridium, often located among the rootlike rhizoids. It is circular in outline and contains sperms. Then find an archegonium, which is roughly the same size as an antheridium but has a short neck. Archegonia each contain a single egg; they often tend to be close to the notch of the prothallus at the top. In nature, a sperm unites with the egg in an archegonium, and the zygote develops into a new sporophyte, with which our study of ferns began.

Drawings to Be Submitted

1. Label the following on the drawings of the moss life cycle provided: MALE GAMETOPHYTE, FEMALE GAMETOPHYTE, ANTHERIDIUM, ARCHEGONIUM, SPERM, EGG, ZYGOTE, EMBRYO, DEVELOPING SPOROPHYTE, MATURE SPOROPHYTE, CALYPTRA, CAPSULE, SPOROCYTE, OPERCULUM, PERISTOME, SPORES, PROTONEMA, BUD, and RHIZOIDS. Also indicate where MEIOSIS occurs.

2. Draw portions of sections through the tips of moss gametophytes from prepared microscope slides, using the low power of your compound microscope. Show at least one good ARCHEGONIUM and several PARAPHYSES in the FEMALE GAMETOPHYTE, and at least one or two ANTHERIDIA and several PARAPHYSES in the MALE GAMETOPHYTE. Also label EGG and SPERM(S). Be sure to reread the comments in section A about how parts of structures may be cut off during the manufacture of the slides.

3. Draw HABIT SKETCHES (i.e., how the organisms appear in nature) of a thalloid liverwort and a hornwort.

4. Draw a fern SORUS, with the aid of the highest power of your dissecting microscope. Label SPORANGIA

(and INDUSIUM, if present).

5. Label the following on the drawings of the fern life cycle provided: SPOROPHYTE, FROND, RHIZOME,

ROOTS, SORUS, SPORANGIUM, SPOROCYTES, SPORES, DEVELOPING GAMETOPHYTE, (IMMATURE PROTHALLUS), PROTHALLUS, RHIZOIDS, ANTHERIDIUM, ARCHEGONIUM, EGG, SPERM, ZYGOTE, EMBRYO, DEVELOPING

SPOROPHYTE, and YOUNG SPOROPHYTE. Also indicate where MEIOSIS occurs.

6. Draw a fern prothallus from the prepared slide provided. Label ARCHEGONIUM, ANTHERIDIUM, and

RHIZOIDS. (Use the lowest power of your compound microscope.)

7. Draw a fern frond, showing the position of the SORI.

The Lower Vascular Plant Divisions

The Fern Allies

Introduction

The vascular plants are divided artificially into two major groups, the seedless (or spore-dispersing) vascular plants and the seed plants. There are four major divisions of seedless vascular plants: Psilophyta, Lycophyta, Sphenophyta, and Pterophyta. The first three divisions, often referred to as the "fern allies" , have few living representatives although they are well represented in the fossil record. All of the vascular plants have a dominant sporophyte generation, and a reduced, often, dependent gametophyte stage.

Exercise A

Psilophyta: The Whisk Ferns

The Psilophyta are represented by two living genera, Psilotum and Tmesipteris, both of which have very simple sporophytes.

Examine the living specimens and herbarium specimens of Psilotum . Psilotum is unique among living vascular plants because it lacks both vascular roots and leaves. Only the stem is vascular. The scalelike structures along the stem are called enations.

Note the dichotomous (forking) branching pattern of the aerial portion of the plant body. The below-ground portion of the plant axis is a rhizome (an underground stem) bearing rhizoids for the absorption of water.

Note the three-parted sporangia, which are borne on short side branches. Psilotum is homosporous; that is, it produces only one type of spore. Although Psilotum is homosporous, the gametophytes are bisexual; both archegonia and antheridia are produced on the same gametophyte. The non-photosynthetic gametophytes develop in association with mycorrhizae.

Tmesipteris is an epiphyte that grows on tree ferns and other plants.

Lower Vascular Plants Lab - 2

Exercise B

Lycophyta: The Lycophytes

The living representatives of the Lycophyta are all relatively small plants, with true roots, true stems, and true leaves. The leaves are microphylls, having just one vascular connection or vein. The fossil members of this division, however, include many woody, treelike forms (the Lepidodendrids), which numbered among the dominant plants of the coalforming forests of the Carboniferous period.

Examine the living specimens and herbarium specimens of Lycopodium and Selaginella. Identify the roots, stems, and leaves (microphylls) of these genera. Selaginella species are common in both temperate and tropical rain forests, although it is frequently confused with mosses. Some species of Selaginella, including the "Resurrection plant", are found in very dry habitats. Lycopodium species grow in many wooded areas throughout temperate ecosystems.

Examine the preserved, or herbarium specimens of Isoetes. Although the leaves of Isoetes are much larger than those of Lycopodium and Selaginella, they are still microphylls. In Isoetes, the leaves are attached to a cormlike structure (a fleshy stem). Isoetes is aquatic.

The sporangia of the Lycophyta are borne on leaves which are very similar to the sterile (non sporangia-bearing) leaves of the plant. The sporangium-bearing leaf is called a sporophyll. In Selaginella and in most species of Lycopodium, the sporophylls occur in compact aggregates called cones, or strobili. Examine the strobili on the specimens provided. In Isoetes , sporangia arise at the bases of the leaves, with a single sporangium per leaf.

Lycopodium is homosporous, producing one type of sporangium. Observe the prepared slide of Lycopodium strobilus and locate the sporangia. The gametophytes produce both archegonia and antheridia.

Selaginella and Isoetes are heterosporous. They produce two types of sporangia: megasporangia (female) and microsporangia (male). Spores develop into either female gametophytes (which produce archegonia) or male gametophytes (which produce antheridia). Observe the prepared slide of the Selaginella strobilus. Locate the larger megasporangia and the smaller microsporangia. Compare the strobilus of Selaginella with the strobilus of Lycopodium.

It is exceedingly rare to find gametophytes of the Lycophyta. Most are subterranean and very tiny. However, sporophyte plants develop from the gametophyte structure so that whenever you find a sporophyte plant, you can be assured that a gametophyte was once there. (Once a sporophyte plant becomes established, the gametophyte degenerates.)

Lower Vascular Plants Lab - 3

Exercise C

Sphenophyta: The Horsetails

Although once a very abundant and diverse group of plants, the Sphenophyta today are represented by a single herbaceous genus, Equisetum.

Examine the living and herbarium specimens of Equisetum. The sporophyte of

Equisetum differs from that of the other fern allies in having jointed and ribbed stems with the leaves (microphylls) arranged in whorls at nodes.

Feel the coarse texture of the stems. The stems of Equisetum contain silica. Note that the stems, rather than the leaves of Equisetum are photosynthetic.

Examine the cones or strobili on the specimens provided. The sporangia are borne on umbrella-like structures called sporangiophores, rather than on sporophylls.

Examine the prepared slide of the Equisetum strobilus. Is Equisetum homosporous or heterosporous?

Examine the prepared slide of Equisetum spores. Note that four threadlike structures, called elaters, surround each spore. The elaters, which are sensitive to humidity changes, are used to disperse the spores. When the sporangium breaks open, the sudden change in humidity causes the elaters to uncoil which "whips" the spore out of the sporangium to be carried by air current to a new location. The gametophytes of Equisetum are green, freeliving, and bisexual.

Questions

1. How do moss "leaves" differ from the leaves of more complex plants?

2. What is the difference between a calyptra and an operculum?

3. How is the release of spores controlled in mosses?

4. Where does meiosis take place in mosses?

5. Where, in mosses, are zygotes and embryos formed?______________________________________________

6. In Marchantia, what is the function of archegoniophores and antheridiophores?___________________________

7. What are all the parts of a complete sorus?

8. Where, specifically, are fern antheridia located?_______________________________________________

9. What parts of a fern are 2n?_____________________________________________________________

Where, in a fern, does the switch from 2n to n take place?_________________________________________

Where, in a fern, does the switch from n to 2n take place?_________________________________________

10. In addition to seeds, what do higher plants have that bryophytes lack?________________________________

11. Which phase in the life cycle of a moss consists of a "leafy" plant?

12. In which specific structure of a moss are sperms produced?

13. What is the toothed structure in a moss sporophyte that controls the release of spores from a sporangium?

14. How does a thalloid liverwort differ in appearance from a moss?

15. What is a cluster of fern sporangia called?___________________________________________________

16. Where does meiosis take place in ferns?_____________________________________________________

17. What name is applied to the gametophyte of ferns?______________________________________________

18. Where are fern antheridia produced (i.e., among what structures on the gametophyte)?_____________________

19. What are the differences among rhizoids, roots, and rhizomes?