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?