Bryophytes: primitive land plants
These are the plants that were present soon after land was colonized, over 400
million years ago. A few plants living today are closely related to those ancient plants, and
we often call them “living fossils”. Two major lineages of plants evolved in these early
times. Plants without conductive tissues occurred, whose descendents are the mosses and
liverworts—or bryophytes. Plants with conductive tissues (xylem and phloem), called
vascular plants, also appeared at this time. Both of these groups of plants had life cycles,
involving two generations. One generation, the gametophyte, produced sexual organs and
male and female gametes, or small moving spermatozoa and larger immobile eggs. A
separate generation, the sporophyte, produced haploid spores after meiosis. This life cycle
was derived from their algal ancestor. Here we describe those bryophytes and ferns that you
will encounter on the FIU campus, and also common in South Florida.
In a generalized life cycle, gametophytes, which are haploid (N), alternate with
sporophytes, which are diploid (2N). Antheridia (male) and archegonia (female) are
the sex organs (gametangia) produced by the gametophyte; they produce the sperm and
eggs, respectively. The sperm and egg fuse during syngamy (fertilization) to produce
the first diploid cell of the sporophyte generation, the zygote. Meiosis occurs within
the sporangia, which are the spore-producing organs of the sporophyte. The resulting
spores are haploid and are the first cells of the gametophyte generation.
Bryophytes
The bryophytes, division or phylum Bryophyta, are land plants that are nonvascular (no real conducting tissues) with a life cycle that is different from that of vascular plants.
The green leafy structures are the sexually-reproducing generation (gametophyte). The capsulelike structure sticking above is the spore-producing generation (sporophyte), that is mainly
dependent upon the gametophyte for its nutrition. The bryophytes are split into three classes: (1)
the mosses; (2) the liverworts; and (3) the hornworts. We will observe the first two groups, by far
the most important, from plants growing on campus and in the Wertheim Conservatory.
Phylum Bryophyta: Mosses
These small plants grow in very damp and protected places in south Florida, but
we don’t know very much about the plants living here. You will see moss gametophytes
growing on the protected trunks of some palms and trees on campus, such as the date palms
outside of the Conservatory. You will also see mosses growing on the rocks by the waterfall in
the conservatory. Mosses are dioecious, meaning that they have male and female plants. At
certain times of the year these mosses will produce tiny sporophytes. Moss sporophytes
consists of capsules located atop stalks, or seta, that extend upward the moss gametophyte. A
sporophyte is attached to the gametophyte be a structure called a foot (see figure next page).
Prior to this generation, the tiny plants produced sexual organs (archegonia and antheridia) at
the tips of the shoots. The archegonia produce single eggs and the antheridia produce many
motile swimming sperm. Mosses can also reproduces asexually by fragmentation.
Procedure – examine mosses
1.
Observe the living moss on display.
Procedure – water absorption by
moss
1.
2.
Make a wetmount of one leaflet of
Polytrichum and examine it with low
magnification.
Questions
1.
How many cells thick is the leaflet?
2.
Is there a midrib vein?
3.
Are stomata or pores visible on the
leaf surface?
4.
How does the symmetry of a moss
gametophyte compare with that of a
liverwort gametophyte? (come back
to this question and answer after
observing liverworts)
2.
3.
4.
Weigh 3g of Sphagnum moss and 3g
of paper towel
Add the moss and towel to separate
beakers each containing 100mL of
water.
After several minutes, remove the
materials from the beaker.
Measure the amount of water left in
each beaker by pouring the water into
a 100 mL graduated cylinder.
Remember that 1 mL of water weight
1g. Record your data
Questions
1.
How many times its own weight did
the moss absorb?
2.
How does this compare with the
paper towel?
3.
Why is Sphagnum often used to ship
items that must be kept moist?
Phylum Hepaticophyta: Liverworts
Liverworts have a similar life cycle to that of mosses, only their spores are different. We will
examine Marchantia, in which the gametophytic thallus grows as a large, flat photosynthetic
structure on the surface of the ground.
Asexual reproduction: Liverworts can reproduce asexually via fragmentation. In this process
the older, central portions of the thallus die, leaving the growing tips isolated to form individual
plants. In another form of asexual reproduction, structures called gemmae cups occur on the
dorsal surface of some thalli near the midrib (see figure below). Gammae cups represent another
means of asexual reporduction by liverworts. Inside the gemmae cups are lens-shaped
outgrowths called gemmae (sing. gemma), which are splashed out of the cups by falling drops of
rain. If a gemma lands in an adequate environment, it can produce a new gametophyte plant.
Sexual reproduction: Many species of Marchantia are dioecious, meaning that they have
separate male and female plants. Gametes from each plant are produced in specialized sex
organs born on upright stalks (see pictures below). Archegoniophores are specialized stalks on
femal plants that bear archegonia. Eack flask-shaped archegonium consists of a neck and a
venter, which contains the egg. Antheridiophores are specializes stalks on male plants that bear
anteridia. Sperm form in antheridia. Flagellated sperm are released and washed from the
antheridia during wet conditions and eventually fertilize the egg, which is located in the venter.
The zygote remains in the venter and grows into a sporophyte.
A
B
Marchantia. (a) A thallus bearing upright male reproductive
structures called antheridiophores. (b) A thallus bearing upright
female reproductive structures called archegoniophores.
Detail of Gemmae cups “splash cups”
Procedure – examine live Marchantia
1.
Examine live Marchantia a using the naked eye or a dissecting scope. Be sure you can
identify the archegoniophores, antheridiophores, and gemmae cups.
Questions
1.
How do the positions of the archegonium and antheridium relate to their reproductive
function?
2.
When would it be advantageous for the Marchantia to reproduce asexually through
gemmae cups instead of through sexual reproduction?
Seedless Vascular Plants
These early land plants had specialized vascular tissue (xylem and phloem) in both roots and
stems that conducted water and nutrients up, and translocated sugars and some amino acids
down, the roots and stems. They had life cycles similar to the mosses and liverworts, but
reversed in the relative importance of the two generations. In their life cycles the dominant
green generation is the sporophyte, and the smaller less conspicuous generation is the
gametophyte. All of the groups of seedless vascular plants seen below share very similar life
cycles, the principal differences being in the structure and arrangement of the sporangia and
the structure of the gametophytes. Sporangia is where spores are produced by meiosis.
They are formed on sporophylles, which are leaf like structures of the sporophyte that bear
spores. Sporophylls may be large megaphylls as on true ferns, or smaller micrphylls such as
those on whisk ferns, scouring rush, and club mosses. We illustrate the life cycle of a
typical fern as illustrative of all of these plants.
We describe the major groups of these primitive vascular plants below, including plants that
you will see on campus and in south Florida. By far the most important group of these
plants is the ferns, but the other groups have wide distributions and are representative of
groups that were once very important during the early history of land plants.
Division Polypodiophyta: the Ferns
This is by far the most important and diverse group of all of the primitive
land plants. Ferns grow from the arctic to the equator, although they are most diverse in
tropical forests, particularly at higher altitudes. Ferns may be extremely tiny, almost the size
of mosses, and extremely tall, as tree ferns 15 m high. There are over 12,000 species worldwide, and some 150 species in Florida. All ferns share a general morphology. They produce
leaves, called fronds, and underground stems, called rhizomes. Since these are sporophytes,
the sporangia are produced in clusters, called sori, in the undersurface of their fronds. The
nature and distribution of these sori help in identifying ferns. Usually the sori are protected
by an indusium, which is a specialized outgrowth of the frond. The sporangia are produced
on a stalk and include thickened cells, forming an annulus, that help them spring open and
disperse the spores when mature and dry.
Procedure – examine sori
1. Scrape a sorus into a drop of water and use the low power on your microscope to observe the
sorus. Note the row of thick-walled cells along the back of the helmet-shaped
sporantium. These cells are the annulus. Alternatively, you can observe sori on the back
of a fern leaflet using the dissecting scope.
Questions
1. What is the function of the annulus?
2. Are there any spores in the sporangium?
Fern reproduction
Fern spores germinate and form a
threadlike protonema. Subsequent
cellular divisions produce an independent,
heart-shaped prothallium. Rhizoids and
male and female reproductive structures
occur on the underside of the prothallium.
However, a prothallium rarely fertilizes
itself because the antheridia and
archegonia mature at different times.
Globe-shaped antheridia form first,
followed by archegonia. After producing
sperm, the 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.
The zygote develops in the
archegonium and is nutritionally depend
on ton the gametophyte for a short time.
Soon after, the sporophyte becomes
leaflike 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 world.
Procedure – examine prothallium
1.
Observe archegonia and antheridia
on living prothallium.
2.
Also observe archegonia and
antheridia on prepared slides.
Questions
1.
Is the prothallium haploid or
diploid?
2.
Is the prothallium sporophyte or
gametophyte?
3.
What is the adaptive significance
of having the archegonia and
antheridia on the lower surface of
the prothallium rather than on the
upper surface?
4.
What is the adaptive significance
of having sperm and egg produced
at different times?
Pleopeltis polypodioides--Resurrection
fern. This small fern grows as an
epiphyte on trees in south Florida and
south into the Caribbean region. It is
particularly common on our native live
oaks. The rhizomes grow into the bark
and produce the small green fronds.
Notice the circular sori on their
undersurface. A close look with a
magnifying lens will reveal the
individual sporangia. These plants are
called resurrection ferns because a long
dry period causes the fronds to dry up,
but subsequent rains cause them to
quickly resurrect themselves, and
become green and healthy again. This
is an amazing process that is poorly
understood. You can see these ferns on
live oaks on campus and on the trunks
of the date palms growing by the
Conservatory.
Phymatodes scolopendria--wart fern. This
handsome fern is native to southeast Asia, but
is commonly cultivated in south Florida and
has escaped from cultivation as well. It grows
from a thick and hairy horizontal rhizome
which produces the erect and lobed fronds at
intervals. These fronds produce large circular
sori on their undersurfaces, which make an
impression on the upper surface as well.
These ferns are actually related to the
resurrection fern. Some of these plants have
established on the date palms near the
Conservatory, but are most common in the
atrium of OE, just outside of the laboratory
where you meet inside.
Thelypteris palustris--Marsh fern. This
is a ferns with rather delicate and soft
fronds produced from a dense web of
rhizomes. The small sori are protected
by an indusium on the undersurface of
the fronds. This grows in damp areas in
South Florida, and often is found in wet
and disturbed areas. It grows in the OE
Atrium and in the Ecosystem Preserve as
well.
Nephrolepis biserrata--Boston Fern. A
strange name for this fern. It is a tropical
fern, not native to New England at all, but
was sold as an ornamental plant even in the
19th century when it got its name. It grows
rather tall (1 m) and thick. It has a wide
distribution in the tropics, including south
Florida. Horticultural varieties have been
selected and it is sold in nurseries as well. It
grows in the OE Atrium and in other areas on
campus. It is probably the most commonly
cultivated fern in Miami. A related species
grows as an epiphyte on the date palm trunks
near the Conservatory. It has conspicuous
sori with indusia on the undersurface. It is
called “biserrata” because the teeth on the
margins of the pinnae have teeth themselves.
Adiantum cappillus-veneris--maiden
hair. This beautiful and delicate fern
(where it gets its name) shows up
anywhere conditions are very moist and
protected. So, it has established itself
all over the rocks on the waterfall in the
Conservatory. You will see it in similar
places at Fairchild Tropical Garden and
elsewhere in Miami. The frond is
compound with a black and wiry midrib, and its pinnae (leaflets) protect the
sori with an infolding of the blade
margin. This is a character of the
genus.
Blechnum serrulatum--saw fern. This is a
common and rather weedy fern. It is native to
south Florida and elsewhere in the Caribbean.
It grows from a horizontal rhizome, and the
margins of the pinnae have fine teeth (like a
saw blade). The sori are produced by the
midrib of each frond, and protected by leaf
tissue growing over it. This grows in the
Ecosystem Preserve and in the OE Atrium by
your laboratory.
Sphaeropteris cooperi--Australian Tree
Fern. This is native to the rainforests of
Queensland, in the north of Australia, and
is the only commonly cultivated tree fern in
South Florida. You can find it planted
commercially, but in protected areas. There
is a fine specimen growing in the
Conservatory. The “trunk” is actually a
dense web of individual vertically growing
roots.
Angiopteris evecta—Angiopteris. This
is a most spectacular and primitive fern,
native to tropical rainforests in southeast
Asia. It has distinctive simple sporangia
in rows under the pinnae of its huge
fronds. These arise from a massive
rhizome. This is the spectacular fern that
arches over the pond in the Conservatory.
It also grows in the Conservatory at
Fairchild Tropical Garden.
Ferns that are Not Ferns. Many plants are called ferns because they have fine, lacy and
highly dissected leaves. However, they reproduce by seeds and not by spores and
independently living gametophytes. The artillery fern (Pilea microphylla) is a tiny plant in
the nettle family, known for expelling its seeds some distance when the fruits ripen. It grows
as a weed in any wet-humid place. We constantly remove it from the Conservatory. The
asparagus fern (Asparagus sprengeri) is a wiry and thorny vine that shows up in hedges. Its
lacy foliage is pretty, but the plant is a pain and difficult to remove. The fern-leaf tree
(Filicium decipiens, growing by Duplicating and near the Tower Dorm) has attractively
divided foliage but is obviously not a fern.
Division Lycopodophyta: The Club Moss
These plants do not grow outside in south Florida (although a couple of species grow in the
scrub lands north of Lake Okeechobee), and the ones you see have been planted inside of the
Conservatory. These plants are called club mosses because their sporangia are protected by
special leaves (bracts) to form cone-like structures called strobili at the tips of the branches.
Few of these plants grow very large and none are economically very important. There are
two living families, the Lycopodiaceae and the Selaginellaceae. These differ in details of
their life cycles and in their leaf arrangements. Look at the plants closely to see these
differences.
Selaginella
These are sometimes called spike mosses. There are at least three species in
the Conservatory. All share a flattened arrangement of two sizes of small leaves on their
horizontal stems. They produce strobili on the tips of their branches (but you probably
won’t see them on our plants). The life cycle of these plants represents an evolutionary
transition in origin of seed plants. They produce both megaspores and microspores (in the
same or different strobili depending on the species), and the gametophyte generation is
retained inside of the spore walls, the microspores producing sperm and the megaspores
producing eggs. These are all tropical rainforest plants; those growing at higher latitudes (as
Central Florida) are extremely drought-tolerant. Some desert plants can dry up completely
and be resurrected when immersed in water. Two of the plants in the conservatory, the
straggling Selaginella willdenowii and the smaller Selaginella uncinata, produce an
iridescent blue leaf color in very shady spots. This is probably an adaptation to low light
conditions in the understory of tropical rainforests. These are commonly called peacock
ferns (but they are really not ferns).
Division Psilotophyta: The Whisk Ferns
You will see Psilotum nudum, a cosmopolitan tropical plant, at the base of
palms on campus. We didn’t plant them; they arrived from spores on their own. We once
thought that these plants represented the most primitive of vascular plants. They have no
roots, no real leaves, and look similar to fossils of the most ancient of land plants. We now
know that these plants are actually highly reduced descendants of the ferns
Lycopodium
The ground pines grow in
forests in temperate and tropical regions.
All have a spiral arrangement of their
small leaves with strobili at the tips of
their branches. Again, these plants do not
grow in south Florida and you can only
observe them in our Conservatory or in
the conservatory at Fairchild Tropical
Garden. They grow as both epiphytes
and terrestrial plants. Observe the
Lycopodium plants growing near the
waterfall.
Lycopodium cernuum: a, mature plant,
x3/4; b, strobilus, x71/2; c, sporophyll,
abaxial surface, x50
Lepidodendron
These plants once grew in
the coal swamps of the Carboniferous era,
some 300 million years ago. Pressure and
temperature turned them into the coal
deposits of West Virginia, Pennsylvania and
other states. Fossils are commonly found
of all parts, particularly the stems
(Lepidodendron), leaves (Lepidophyllum),
and strobili (Lepidostrobus). From these
form genera, we know what the intact
plant was like. Many of them were large
trees. Observe the fossils of
Lepidodendron and Lepidostrobus adjacent
to the Lycopodium plants in the
conservatory. Compare them to diagrams
of the fossils and a rendering of the entire
plant below.
Division Equisetophyta: The Horsetails
This was once a very important group of plants that dominated the coal
swamps of the Carboniferous along with Lepidodendron. Today it is only represented by a
single genus of very few species, Equisetum. This plant is commonly called the horsetail
(some of the species branch and look vaguely like tails) or the scouring rush (it contains
silica in its stems and campers sometimes using them to clean their pans). It is rarely seen in
south Florida, although a small clump occurs in the north end of the Ecosystem Preserve.
Again, we have planted some examples in the conservatory.
Equisetum
Notice the general
appearance of the plant. Vertical stems
arise from an underground rhizome.
These stems produce tiny scale-like
leaves at jointed nodes on the stem.
Briefly and early in the year the stems
produce a club-shaped strobilus at their
tip. This strobilus consists of hexagonal
discs (sporangiophores) under which the
sporangia are produced. Compare the
plants you see to the diagram below.
Calamites
This was a treelike plant in the
coal swamps of
the
Carboniferous.
Thus it has left
numerous
fossils that can
be found in the
slag heaps of
coal mines,
along with
those of
Lepidodendron.
Observe the
fossil
impressions of
this plant
adjacent to the
living
Equisetum,
near the
waterfall in the
Conservatory.
Compare these
fossils to the
diagram and
rendering of
what the intact
plant probably
looked like.