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?