Ferns and their Allies

1. Review fern life cycle.

2. Examine sori.

3. Observe fern archegonia and antheridia on prepared slides. 4. Examine living prothallia.

5. Observe living Salvinia and Azolla.

6. Examine the above-ground tissue, rhizome, sporophylls, strobili, and spores of Lycopodium, a club moss.

7. Examine hydrated and dehydrated Selaginella.

8. Examine Psilotum (whisk fern) and Isoetes (quillwort).

9. Examine living Equisetum and prepared slides of strobili.

10. Compare the structures in common among ferns and fern allies (Table 22-1).

QUESTIONS FOR FURTHER THOUGHT AND STUDY

1 Compare ferns with bryophytes. Which represent the best "engineering job"? Ex­plain.

2. What problems were faced by plants as they moved onto land?

3. What are the distinguishing features of the various "fern allies"? How are the fern allies different from ferns?

4. What are the advantages of vascular tissue in land plants?

By the end of this exercise you should be able to

1. Discuss similarities and differences between ferns and other plants you have stud­ied in lab.

2. Describe the life cycles of ferns and their allies.

3. Describe the distinguishing features of Psilophyta (whisk ferns), Sphenophyta (scouring rush), Lycophyta (club mosses), and Pterophyta (true ferns).

Ferns and their allies include nonflowering plants having a vascular system of fluid­conducting xylem and phloem. The vascular system connects true leaves, roots, and stems.

Ferns and fern allies include the divisions Pterophyta (true ferns), Lycophyta (club mosses), Psilophyta (whisk ferns), and Sphenophyta (horsetails). Biologists often have referred to these divisions as the lower vascular plants and seedless vascular plants. Con­versely, gymnosperms (e.g., a pine tree) and angiosperms (flowering plants) have been referred to as higher vascular plants and seed-forming vascular plants. This separation of "higher" and "lower" vascular plants was invented by early botanists, who believed that all seed-forming plants must be related, and that plants lacking seeds are similarly related. Later investigations of lower vascular plants showed that ferns represent a separate group of seedless plants distinguished by the presence of megaphylls, which are large leaves with several to many veins. The remaining seedless vascular plants possess microphylls (small leaves with one vein) and are only somewhat related to ferns, hence the name "fern allies." Thus the "lower vascular plants" include a diverse group of ferns and fern allies. However, all ferns and fern allies possess sporophylls. Sporophylls are leaf-like structures of the sporophyte generation that bear spores. They may be large and have several to many veins (megaphylls of ferns) or they may be smaller and have one vein (microphylls of fern allies).

DIVISION PTEROPHYTA: FERNS

Ferns inhabit almost all kinds of environments, and possess characteristics of the more advanced seed plants as well as the less advanced bryophytes. Ferns have an independent sporophyte (Fig. 22-1) with well-developed vascular tissue. The diversity of ferns is strik­ing-they range from majestic tree ferns to bizarre staghorn ferns. Tree ferns reach heights up to 16 m. Along with other plants, these ferns once formed forests that were transformed into coal deposits. Today, humans use ferns as decorations, fossil fuels, and in rice cultivation.

Review the fern life-cycle shown in Fig. 22-1.

Fern sporophytes grow indefinitely via underground stems called rhizomes. Examine the fern rhizomes on display. Examine the different ferns available in the lab, and note the different shapes of the leaf-like fronds. Identify the stalk, blade, and pinnae.

Groups of sporangia called sori form on the underside of fern fronds. Sporangia may be protected by a shield-shaped indusium, which is a specialized outgrowth of the frond. Meiosis in the sporangium produces haploid spores, which are the first stage of the gametophyte.

QUESTION 1a. Which parts of the life cycle are haploid?b. Which are diploid?

QUESTION 2a. How many veins are present in each frond? b. What tissues comprise a vein?  c. What is the function of the stalk?

blade? pinnae?

.

 Fig. 22-2 Fern sori.

 

Fig. 22-1 Life cycle of a fern

 

PROCEDURE: EXAMINE SORI

1.        Scrape a sorus into a drop of water and observe it using low power of your micro­scope. You'll note a row of thick-walled cells.along the back of the helmet-shaped sporangium. These cells are termed the annulus.

2.       Place a few drops of acetone on the sporangium while you observe it with a dis­secting microscope.

3.       Watch the sporangium for a few minutes, adding acetone as needed.

4.       Describe what you see.

5.       Examine prepared slides of fern sori and refer to Fig. 22-2.

6.       Diagram and label each structure that you see, and list its function.

 

 

Fern Reproduction

Fern spores germinate and form a thread-like protonema. Subsequent cellular divisions produce an independent, heart-shaped prothallium ("valentine plant").

Rhizoids and reproductive structures occur on the underside of the prothallium. Globe-shaped antheridia form first, followed by archegonia. After producing sperm, an­theridia drop off, leaving sperm to swim to the archegonia. Archegonia are vase-shaped, and are located near the cleft of the heart-shaped prothallium. Observe archegonia and antheridia (see Fig. 22-1) on prepared slides and on a living prothallium.

The zygote develops in the archegonium, and is nutritionally dependent on the ga­metophyte for a short time. Soon thereafter, the sporophyte becomes h:af--like and crushes the prothallium. Fronds of the growing sporophyte break through the soil in a coiled position called a fiddlehead. The fiddlehead then unrolls to display the frond, which is a single leaf. Fiddleheads are considered a culinary delicacy in some parts of the country.

Most terrestrial ferns are homosporous, meaning that they produce one kind of spore which develops into a single kind of gametophyte that produces both antheridia and archegonia. Conversely, aquatic ferns such as Salvinia and Azolla are heterosporous, meaning that they produce two kinds of spores: megaspores and microspores. A megaspore forms a gametophyte with only archegonia, and a microspore forms a gametophyte with only antheridia. Observe living Salvinia and Azolla in the lab.

FERN ALLIES

During the Devonian and Carboniferous periods (300-400 million years ago), fern allies were among the dominant plants on earth. Indeed, most of our coal deposits are made up largely of fossilized fern allies. However, the "giant" fern allies are now extinct, and modern fern allies are represented by their relatively small counterparts.

 

DIVISION LYCOPHYTA (CLUB MOSSES)

Club mosses possess true roots, stems, and leaves. Most asexual reproduction by club mosses occurs via rhizomes. If available, locate the rhizome on a Lycopodium, a club moss. Study the above ground portion of the Lycopodium plant (Fig. 22-3). Lycopodium is ev­ergreen, as are some ferns, most gymnosperms, and some angiosperms.

Sporangia of Lycopodium occur on small modified leaves called sporophylls, which are clustered in strobili (cones) that form at tips of branches. Examine strobili on a living Lycopodium plant. Examine prepared slides of strobili of Lycopodium (Fig. 22-4). Diagram, label, and state the function of each major feature of the strobilus. Examine prepared slides of spores of Lycopodium as well as those available on plants growing in the lab.

 Fig. 22-4 Lycopodium sporophyll and sporangium.                                                                                

Examine some living Selaginella plants (Fig. 22-5). Many species of Selaginella produce two kinds of strobili, which are typically red and yellow. If strobili are present, examine spores derived from these cones. Examine hydrated and dehydrated resurrection (Sela­ginella lepidophylla) plants                                                                         Examine some living Isoetes (quillwort) plants. Compare and contrast the following features of Isoetes with Lycopodium and Selaginella:

 1. shape of aerial portion of plant  

 2. Branching patterns

   3. shape, size, and arrangement of leaves                                                                                                              At the branching points along the stem of Isoetes, an unusual "runnel-like" organ can be observed. These prop-like axes are termed rhizophores, andhave structural features in­termediate between stems and roots. .                          

                      DIVISION PSILOPHYTA: WHISK FERNS There are only two extant representatives of the division Psilophyta: Psilotum and Tmesipteris. Psilotum has a widespread distribution, while Tmesipteris is restricted to the South Pacific. Psilotum lacks leaves and roots, and is homosporous. Examine a prepared slide of a Psilotum sporangium (Fig. 22-7), and examine living Psilotum plants in the lab.                                                                                                                            DIVISION SPHENOPHYTA: SCOURING RUSHEquisetum is the only extant member of Sphenophyta, and is an example of a plant whose vegetative structure identifies the plant better than its reproductive structure. Equisetum is distinguished from other fern allies by its jointed and ribbed stem. Examine the living Equisetum plants.  Feel the ribbed stem of an Equisetum plant. Its rough texture results from siliceous deposits in its epidermal cells. In frontier times, Equisetum was used to clean pots and pans, "sand" wooden floors, and scour plow shears, thus accounting for its common name of "scouring rush." Strobili of Equisetum occur at tips of reproductive stems. Sporangia form atop um­brella-like structures called sporangiophores, which are modified branches. Elaters in sporangia of Equisetum help disperse spores. Examine prepared slides of Equisetum strobili (Fig. 22-8). Diagram, label, and state the function of each major structure comprising the strobilus.                                                             

 

QUESTION 3a. What is the function of an annulus?    b. Are any spores in the sporangium?

QUESTION 4   a. Did the application of acetone cause the spores of the fern to disperse?   b. How is the mechanism for spore dispersal in ferns similar to that of bryophytes?

QUESTION 5   a. Is the prothallium haploid or diploid?   b. Is the prothallium sporophyte or gametophyte?

QUESTION 6  a. What is the adaptive significance of having these structures on the lower surface of the prothallium rather than on the upper surface?

b. What is the adaptive significance of having sperm and egg produced at different times?

QUESTION 7  How Salvinia and Azolla differ from other ferns you've examined today?

QUESTION 8 a. How could a rhizome be involved in asexual reproduction?  b. How is a rhizome different from a rhizoid?  c. Does the rhizome have leaves?

QUESTION 9  a. What is the shape and size of the leaves?  b. What is the significance of this form   c. Is a midvein visible?

QUESTION 10  a. What does the word "evergreen" mean?  b. Is "evergreenness" a good characteristic for classifying plants? Why or why not?

QUESTION 11   How many sporangia occur on each sporophyll?

QUESTION 12a. Is Lycopodium homosporous or heterosporous?

b. Can you see why spores of Lycopodium are sometimes called "vegetable sulfur"?

c. Why are they a good dry lubricant?

d. Which is the dominant part of the Lycopodium life cycle-the sporophyte or gametophyte?QUESTION 13a. Are spores of Selaginella similar in size?

b. What is this condition called?

QUESTION 14

a. What is the functional significance of the difference in the appearance of de- and rehydrated Selaginella?

b. Can you see why these plants are sometimes referred to as "resurrection plants?"

QUESTION 15

a. What type of branching characterizes Psilotum?       b. Are any roots present?         c. Are any leaves present?            d. Where are the sporangia?                                e. Where does photosynthesis occur in Psilotum?

 QUESTION 16a. Where are the leaves?                 b. What part of the plant is photosynthetic?            c. Which part of the life cycle of Equisetum is dominant-the sporophyte or gametophyte?

 QUESTION 17

·       How does strobili formation compare with Lycopodium and Selaginella?

b. How is spore dispersal aided by elaters?