Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
BIOLOGY—102
Professor: Dr. Thomas R. Sawicki
Text: Biology 8th ed. (Campbell and Reece Chapter 29)
A weed is a plant whose virtues have not yet been discovered. -- Ralph Waldo Emerson
Lecture # 4 Plant Diversity I: How Plants Colonized Land
Domain Eukarya
Kingdom Plantae
Algae
Phylum Rhodophyta
Phylum Chlorophyta
Class Charophycaea
red algae
green algae
Chara
Seedless nonvascular plants
Phylum Hepatophyta
liverworts
Phylum Anthocerophyta
hornworts
Phylum Bryophyta
bryophytes
(true mosses)
Seedless vascular plants
Phylum Lycophyta
Phylum Psilophyta
Phylum Sphenophyta
Phylum Pterophyta
club mosses
whisk ferns
horse tails
ferns
Domain Eukarya
Kingdom Plantae
Plants are photosynthetic organisms with their photosynthetic apparatus located in plastids. These
plastids (e.g., chloroplasts) were acquired by primary endosymbiosis and as such have two bounding
membranes. The kingdom can be viewed as comprising four broad non-taxonomic groupings of plants:
some are algae (recall brown algae are now classified within kingdom Chromista), seedless
nonvascular plants, seedless vascular plants, and seed plants. Be sure to be able to associate each
phylum in the kingdom with its corresponding non-taxonomic group as detailed in the test boxes on page
one. For instance, ferns are classified within phylum Pterophyta and are grouped with the seedless
vascular plants. In this lecture we will cover the algae, seedless nonvascular plant and the seedless
vascular plants. We will cover the seed plants in lecture five.
The first algae evolved more than a billion years ago when a heterotrophic protist and a cyanobacterium
entered a mutualistic relationship. The cyanobacterial endosymbiont eventually became so dependent
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
upon its host that it became an organelle (figures 25.9 and 28.2). Page 576 in your text provides an
excellent description of the evolution of protists via both primary and secondary endosymbiosis.
Red and green algae are aquatic organisms. The aqueous environment provides a means for algae to
float up toward the sun and hence they do not need to fight gravity to grow toward light in the same
manner of terrestrial plants. Additionally, the fact that they are surrounded by an aqueous medium means
they do not have a need to move water and nutrients throughout their body, unlike terrestrial plants that
must move water and minerals from within the ground up to their leaves. A great deal of data indicates
that terrestrial/land plants (mosses and their allies, the ferns and their allies as well as the seed plants)
and the red and green algae share a common ancestor. The cladogram depicted in figure 28.3 shows
this ancient link, which is supported by a variety of lines of evidence.
Traditionally, red and green algae have been considered protists and not plants—this is how your book
classifies them. For this course I have placed the red and green algae into kingdom Plantae. This may
or may not be how the taxonomy eventually works out but it seems the simplest way to classify these
organisms in order to show their close phylogenetic relationship to “true plants” and not add a lot of new
and confusing terminology. We will separate out red and green algae from “true plants” by habitat—algae
are aquatic and “true plants” are found in terrestrial environments. I am defining “true plants” in this
contest as the seedless nonvascular, seedless vascular and seed plants. Now, to be clear, there are
some “true plants” that live in the aquatic environments, however, this is a secondary recolonization. That
is to say they evolved to live in the sea or lake from a most recent common ancestor that lived on land—
much like whales did in the animal kingdom. In addition “true plants” or terrestrial plants have much more
complicated tissues and organs than “algal plants”—mostly reflecting the fact that terrestrial plants must
adapt to gravity and living in a non-aqueous environment.
Phylum Rhodophyta: red algae
There are approximately 6000 described species of red algae. Almost all known species are found in
marine habitats. Only about 100 species of red algae are found in fresh water environments. The
common name “red algae” is due to their redish coloration which is derived from their accessory
carotenoid pigments, specifically a pigment called phycoerythrin. Most species of red algae are red, but
not all; in general, species adapted to shallow water use less phycoerythrin and thus appear green. The
deeper the plant is found the greater the phycoerythrin content and thus the darker its coloration—deep
water species can even appear black. Can you think of a logical explanation as to why this would be the
case?
Like plants, the cell walls of red algae have a framework of the polysaccharide cellulose; however, the cell
walls of red algae are predominately composed of polysaccharides of the hexose sugur galactose—an
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
isomer of glucose. Polysaccahides composed of galactose result in molecules we call agars and
carageenans. The helical structure of these polysaccharides facilitates the absorption of water and thus
both of these substances are called mucilages—mucilage is a gum-like or jelly-like substance produced
by the cell walls of algae, probably for protection. Humans use agars and carageenans as thickening
agents. For instance, carageenans are found in ice cream and other food stuffs and agars are used as
nutrient media for scientific studies. For example, microbiologists use agars to gel beef extract or blood
on a Petri dish. Bacteria then feed off of these media. Carrageenan and agar are chemically very similar
but carageenan requires higher concentrations to form gels. Chondrus crispus, a species of red algae
commonly called Irish moss, is found in the sublittoral zone of rocky shorelines in the North Atlantic and
is harvested for its carrageenans. The littoral zone marks the region between the high and low tide lines
within a body of water, especially a sea. The name carrageenan is derived from the Irish village of
Carragheen where Irish moss is found in abundance. Agars are derived from other red algal species.
Other species of red algae (especially those associated with tropical coral reefs) embed calcium
carbonate into their cell walls and are called coralline algae (see picture on lecture PowerPoint).
Coralline algae play a significant role in the make up of many coral reefs. In general, red algae are very
commonly found in tropical regions.
Some species of red algae are edible. Palmaria palmata, known
commonly as dulse, has an edible thallus (figure 28.19). Recall that the thallus of an alga consists of the
holdfast, stipe and blade. I have eaten dulse, and as I recall, the name was deceiving! Not all red algae
are thalloid, some are encrusting (see picture on lecture PowerPoint).
Red algae are also known for their very complex life cycles. Most have three distinct multicellular phases.
Why they have such complicated life cycles is not known. Red algae do not have flagellated gametes
and depend on water current and random chance for their gametes to fuse forming zygotes. One
hypothesis is that there may be a selective advantage to having a life cycle that takes advantage of
fertilization when it occurs. In this view, one of the multicellular stages may be an amplification stage,
producing huge amounts of spores from each zygote.
Phylum Chlorophyta: green algae
There are approximately 7,000 described species of chlorophytes. Most are either unicellular,
filamentous (long thin cylindrical cells joined end to end), colonial (a group of cells living together and
dependent upon one another, these dependencies can include physical connection and the functioning as
a single unit) and even multicellular (organisms consisting of more than one cell, and having
differentiated cells that perform specialized functions) species. Green algae are very common in fresh
water environments and can even be found in damp soils. As you will learn some species are found
living in a mutualistic relationship with fungus and form organisms we call lichens. There are many
species of green algae that live in marine habitats as either phytoplankton or seaweed. Green algae can
even be found in snow. Figure 28.20 shows an example of watermelon snow. In the spring and summer,
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
snow turns red due to the blooms of the chlorphyte Chlamydomonas nivalism, and its carotenoid
pigments.
Chlamydomonas is a commonly studied unicellular chlorophyte. At maturity, Chlamydomonas is a
haploid biflagellated organism, with a single cup shaped chloroplast and a red “eye spot” (figure 28.22).
Chlamydomonas can reproduce either sexually or asexually. Figure 28.22 goes over the life cycle of a
typical Chlamydomonas species. When reproduction is asexual, a haploid cell begins to divide
mitotically, producing up to 16 daughter cells, which break out of the parental cell wall as zoospores.
Sexual reproduction is often a response to adverse environmental conditions. Sexual reproduction
results in the mitotic division of a parental cell into gametes. The gametes are designated as either + or –
and not sperm or egg because they are isomorphic—they look exactly the same. Isomorphic gametes
are called isogametes. After syngamy the zygote produces a tough cell wall—by which it survives the
adverse environmental conditions—and is called the zygospore. When environmental conditions
improve, the zygote undergoes meiosis releasing four haploid cells.
Spirogyra is a classic example of filamentous green algae. On the lecture PowerPoint you will find a
picture of the filamentous green alga Spirogyra, its genus name is derived from the unique, spiral shape
of its chloroplast. Notice how the cells line up one after another to form a long chain or filament.
The best known colonial chlorophyte is Volvox (see pictures on lecture PowerPoint). Up to a few
thousand biflagellated, haploid, photosynthetic cells form a hollow, spherical structure with their flagella
facing outward; the cells have light detectors and the colony can use phototaxis. These cells, which are
evolved from a Chlamydomonas like ancestor, are embedded in a gelatinous matrix. Each cell is
connected to surrounding cells by extensions of their cytoplasm.
Also within the spherical grouping of cells are nonflagellated reproductive cells. Reproduction can be
both asexual as well as sexual. Asexual reproduction occurs when the reproductive cells begin to divide
mitotically until a flat plate of cells forms; this plate will break free and form a spherical structure within the
parental colony. Initially, the flagella of the daughter colony cells are facing inward. Eventually the
daughter colony will flip itself inside out and the colony will use enzymes to digest a hole through the
parental colony.
Sexual reproduction can also occur within Volvox. Some of the reproductive cells will divide to form
gametes. Since most species of Volvox are dioecous, there are male and female colonies. The gametes
produced will thus either be sperm or egg. Sperm from male Volvox colonies swim out in search of eggs.
After syngamy the daughter colony begins to divide within the mother colony and eventually will break
free in a similar manner as described for their asexual reproduction.
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
Chlorophytes can also have a multicellular thallus. One of the best known examples of a multicellular
chlrophyte is commonly called sea lettuce: the genus Ulva (figure 28.21 a). Like the brown algae we
studied in kingdom Chromista, multicellular green algae undergo an alternation of generations between a
haploid gametophyte and a diploid sporophyte. On the lecture PowerPoint I have a diagram of the life
cycle of the multicellular green alga Ulva. You will notice that the gametophyte and the sporophyte are
morphologically very similar—thus these generations are called isomorphic. Also notice that the
gametes look like Chlamydomonas cells!
Class Charophyceae
This group of chlorophytes is extremely important because of their evolutionary relationship to terrestrial
plants. A classic example of a charophycean is the genus Chara, commonly called stoneworts (figure
29.3). Figure 29.4 show that charophyceans and land plants share a common ancestor. There is a great
deal of morphological, biochemical and genetic data to support this theory including:
Rosette cellulose synthesizing complexes (figure 29.2)—in non charophycean algae, there are linear
arrays of proteins embedded in their cell membranes that produce the cellulose for their cell walls;
however in both land plants and charophyceans, the proteins which produce cellulose are arranged in a
very specific, ring-shaped conformation.
Peroxisome enzymes—Plants, of course, live on dry land and charophyceans often live in very shallow
freshwater environments. In both locations the organisms are exposed to high light levels and the
potential for photorespiration is very high. Both groups have special enzymes in their peroxisomes to
help deal with photorespiration. No other photosynthetic eukaryote has these enzymes.
Sperm ultrastructure in charophyceans and in those species of terrestrial plants that have sperm is very
similar.
Production of a phragmoplast—the cell plate which forms between dividing cells in plants and certain
charophyceans such as the genus Chara is the product of a specialized system of Golgi derived vesicles
and cytoskeletal elements called a phragmoplast (see figure 12.9b). You may recall from Bio 101 that
the cell plate—a combination of cell wall and cell membrane material that separates plant daughter cells
along the region of the metaphase plate—is formed from Golgi derived vesicles. These vesicles are
moved to the location of the metaphase plate via motor molecules that ride microtubules and this entire
structure is called a phragmoplast.
Genetic data—over the past few years there has been an international collaboration to genetically study
many plant and algal groups. This study has been called “Deep Green.” Genetic data confirms what the
biochemical and morphological data suggests, the Charophyceae and terrestrial plants share a most
recent common ancestor.
How did plants colonize the land?
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
The earliest fossils of terrestrial plants are seen approximately 475 million years ago during the
Ordovician period (see figures 25.1 and 29.6). Molecular clock studies have indicated that plants evolved
either from 426-490 million years ago (matching the fossil data) or in a separate study, as much as 700
million years ago. Further genetic study and paleobotanical studies looking for older fossils will eventually
create a better understanding for when plants first evolved.
As noted above, it is becoming clear that plants evolved from a charophycean-like ancestor. Figure 29.5
shows some of the characteristics that all land plants share. Your book considers them all derived traits
of terrestrial plants. I would disagree. As you examine the figure, based on what you currently know, ask
yourself if all these characters are found ONLY within the terrestrial plants.
1. Apical meristems—a region at the tip of each shoot and root of a plant in which cell divisions are
continually occurring to produce new stem and root tissue respectively.
2. Alternation of generations—two distinct multicellular stages in the life cycle of a plant. The
gametophyte is haploid and produces gametes; the sporophyte is diploid and produces spores.
As we will see, apomorphic plants have a highly reduced gametophytic generation, whereas
plesiomorphic plants have a reduced sporophytic generation.
3. Sporopollenin walled spores produced in the sporangia—sporangia (sporangium singular)
are multicellular structures, found in the sporophytic generation, in which haploid spores are
produced via meiosis. These spores will surround themselves with a tough wall made of the
polymer sporopollenin. What is important here is that the wall protects against desiccation
(drying out) and allows the spore to be blown by the wind. It should be noted that charophyceans
use sporopollenin to protect their zygotes from desiccation when water levels in their pond drop—
although their spores are not surrounded by sporopollenin (see page 601). I would suggest you
look up the term preadaptation—why did I ask you to look this word up here?
4. Multicellular gametangia—gametangia are structures within which gametes are produced via
mitosis. Eggs are produced within archegonia and sperm within antheridia.
5. Land plants are sometimes called embryophytes—because the growing embryo is retained
within the tissues of the female parent. Nutrients are sometimes delivered to the embryo from the
female plant via specialized cells called placental transfer cells.
Examine the text boxes that begin this lecture. You will note I have separated the plants into the Algae,
Seedless Nonvascular, Seedless Vascular and Seed plants. Your book does something very similar in
table 29.1 and figure 29.7 on page 605. Plant vascular tissue includes xylem and phloem. These
tissues are produced from cells that form tubes through which water and minerals (xylem tissue) and
photosynthates (phloem tissue) flow. I would also like to introduce you to the term bryophytes. All
seedless nonvascular plants used to be classified within a single phylum—Bryophyta. However, the
evolutionary relationship of the “bryophytes” is currently unclear (see the dotted lines in figure 29.7). In
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
this class we will assume that each group of seedless nonvascular plants form a separate clade denoted
by their own phylum: Hepatophyta, Anthocerophyta and Byophyta. Thus, phylum Bryophyta as used in
this class will represent the “true mosses.” However, commonly, any seedless nonvascular plant is often
called a bryophyte or a moss—even if they are not technically classified within the Bryophyta. An
examination of figure 29.7 may help with your understanding of these terms and concepts.
Phylum Hepatophyta: liverworts
Liverworts derive both their common and scientific names from the fact that in medieval Europe, their
shape was inferred to have medicinal properties beneficial to treat liver diseases (Hepatophyta is derived
from the latin hepaticus meaning liver). To the best of my knowledge there is no medicinal benefit from
these plants. Most liverworts live in moist tropical environments with high rainfall. Some are aquatic and
some live in moist temperate forests.
Liverworts can be divided into two major groups morphologically: leafy or thaloid (figure 29.9).
Marchantia, which is a species of liverwort that can actually be found in temperate climates, is a thalloid
liverwort. The gametophyte forms a flat, green structure that is sheet-like in appearance. Plagiochila has
a leaf-like structure. Each “leaf” is one cell layer thick.
Liverworts often have separate male and female gametophytes. The male gametophytes produce
antheridia, which produce sperm, and the female gametophytes produce archegonia, which produce
eggs. In figure 29.9 examine the antheridiophores (structures that bear antheridia) and
archegoniophores (structures that bear archegonia). Also notice that after an egg is fertilized in a given
archegonium, the zygote grows into a small sporophyte. The sporophyte remains attached to and
dependent on the female gametophyte and is composed of three distinct parts, the foot, seta and
sporangium or capsule. Thus the sporophyte produces a sporangium and inside the sporangium
spores will form via meiosis.
Reproduction can also occur asexually. On thalloid gametophytes, mitotic reproduction results in lens
shaped structures called gemmules. The gemmules simply bud off and are blown by the wind.
Phylum Anthocerophyta: hornworts
Hornworts derive their name from the shape of their sporophyte. It looks like a horn (figure 29.9) or a
blade of grass. Like liverworts, the gametophytes of hornworts tend to grow horizontally. Also, like most
liverworts the sporophyte remains photosynthetic. Many species of hornworts have a mutualistic
relationship with nitrogen fixing bacteria, which live in their tissues. On the lecture PowerPoint I have a
close-up picture of a hornwort as well as a diagram of their lifecycle. Note that the sporangium is found in
the center of the sporophyte.
Phylum Bryophyta: bryophytes (true mosses)
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
Unlike liverworts and hornworts, the gametophytes of true mosses tend to grow more vertically. Their
sporophyte does not tend to photosynthesize and is thus completely dependent upon the gametophyte for
nutrients. Figure 29.9 shows a picture of Polytrichum, a common moss genus.
Figure 29.8 goes over the life cycle of Polytrichum. This life cycle is fairly similar to the liverworts and
hornworts. Notice that there is both a female and male gametophyte. The male gametophyte produces
antheridia, which produces sperm, and the female gametophyte produces archegonia, which produces
ova. Notice also that the sperm must have a moist environment in order for syngamy to occur. The
sperm swim toward the ova—thus the sperm are flagellated. This fact is true for all seedless nonvascular
plants. This should make sense; plesiomorphic land plants would not logically be fully capable of living in
completely dry habitats and should retain plesiomorphic characteristics such as flagellated sperm. They
do not have roots and are dependent on a humid/moist environment for many aspects of their life-cycle,
including reproduction.
After fertilization, the zygote begins to divide mitotically to form the sporophyte. You should be familiar
with the morphological structure of a mature bryophyte sporophyte, in general this is the structure of all
sporophytes of seedless nonvascular plants. The Foot is a region of the sporophyte where it attaches to
the gametophyte. The Seta/stalk is a region of elongation. The seta moves the sporangium up and
away from the gametophyte. The sporangium/capsule is the region at the tip of the sporophyte, within
which certain cells will begin to divide meiotically to form spores. These spores will be covered by
sporopollenin to protect them from desiccation; mature spores will then be released from the
sporangium and blown by wind currents—these facts are generally true for most seedless vascular
plants. In certain species of bryophytes, the sporangium is capped by a structure called a calyptra. The
calyptra is a cap that is shed when the sporangium is ready to release its spores. The calyptra is
surrounded by a peristome, which is a tooth-like structure at the tip of the sporangium that help to
regulate the dispersal of spores (see SEM in lower left hand corner of figure 29.8).
.
If a spore lands in a favorable area it will bud out in a filamentous structure called a protonema (figure
29.8). The protonema appears very much like a filamentous green algal species. The haploid male and
female gametophytes will develop from the protonema. The gametophyte will produce either archegonia
or antheridia depending on whether it is a female or a male (figure 29.8). Rhizoids also develop from
the protonema. Rhizoids are long, tubular cells that look root-like; however, the primary job of rhizoids in
seedless nonvascular plants is not the transfer of water and minerals—as it is in roots of vascular
plants—but just anchorage, which of course is also something roots do.
One particular genus of moss is noteworthy. Sphagnum, also called “peat moss” is found living in cool
moist climates. It produces phenolic compounds that increase soil acidity and lower the soil’s oxygen
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
content. As a result, bacterial decomposition is greatly inhibited, resulting in fantastic fossilized remains
(see figure 29.11 b). Peat is also important in sequestering CO2—because the biomass is largely not
oxidized; however, greatly increased harvesting of peat moss for energy and other uses is negating their
ecological benefits. It should also be noted, that vast regions of frozen peat in Siberia and Alaska are
thawing. Very slow microbial decomposition in these bogs has resulted in the accumulation of many
billions of tons of methane. This methane is now being released into the atmosphere. Methane is 20
times more potent a greenhouse gas than CO2. The implications of these melting boggs are dire.
Seedless vascular plants
For about 70 million years nonvascular plants dominated the landscape of earth. Then about 420 million
years ago the first vascular plants arrived. Figure 29.12 shows a reconstruction of a forbearer of modern
vascular plants. This species shows two derived characters not seen in earlier nonvascular plants:
1. The sporophytic generation has a branching structure—called dichotomous branching. This
allowed for multiple sporangia to form.
2. The sporophytic generation is showing signs of increased size and independence; although in
these early plants the fossil evidence suggests that the gametophyte and sporophyte were
codominant. Continued reduction in the gametophytic generation is a characteristic that will
become more and more evident as we move to the more derived plants.
The evolution of multiple sporangia was probably due to the onset of herbivory by primitive insects.
Those plants that happened to have multiple sporangia had a greater chance of producing spores in the
event of herbivory.
Vascular tissue—xylem and phloem—enabled the sporophyte to become independent of the
gametophyte. Xylem is a tissue that transports water and dissolved mineral nutrients in vascular plants.
Phloem is a tissue that transports sugars, amino acids and other organics. We will discuss these tissues
in greater detail in lecture five. But for now it is important to understand that by being able to efficiently
transport these vital chemicals, the sporophyte was able to become independent from the gametophyte.
Another development that aided the independence of the sporophyte was the evolution of roots. Roots
are the part of vascular plants that grows beneath the soil surface in response to gravity and water. It
anchors the plant in the soil and absorbs water and mineral salts, which are then transported to the rest of
the plants via xylem tissue.
Whether roots evolved once from an ancestral plant and was then inherited by all modern plants, i.e., a
homologous character, or was inherited independently due to convergence is not clear. What is known
is that root tissues of modern plants closely resemble stem tissue of early vascular plants, hinting at how
they evolved.
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
All vascular plants have leaves. Leaves are thought to have evolved as small outgrowths of stems,
which allowed for increased surface area and thus greater photosynthetic activity. There are two types of
leaves, microphylls and megaphylls (phyll is Greak for leaf but don’t confuse it with phil, which is Greek
for loving.) Figure 29.14 shows the difference between the two types. Today, only two plesiomorphic
phyla have microphylls—the phyla Lycophyta and Psilophyta. As the name suggests, microphylls are
very small leaves. They are usually spine shaped and are supported by a single vein—a vein consists of
vascular tissue—xylem and phloem. We will discuss the tissues and organs of vascular plants in detail
in lecture five.
All other vascular plants show some type of megaphyll. Megaphylls are so named because they are
larger than microphylls. Megaphylls have branched vascular systems. A leading hypothesis on how
megaphylls evolved is that individual microphylls fused (figure 29.14). Note that the first microphylls
evolved 410 million years ago, but the first megaphylls were not seen until 370 million years ago.
The next major evolutionary advance made by vascular plants was the emergence of sporophylls.
These are leaves that produce spores within sporangia! In lycophytes, sphenophytes and gymnosperms
(e.g., conifers), the sporophylls congregate into clusters to form a strobilus, plural strobili. This name is
derived from the Greek strobilos, which means cone. Thus, a pine cone is nothing more than a bunch of
modified leaves that produce spores in sporangia!
The spores produced in the sporangia of sporophylls can either be homosporous or heterosporous
(page 613 has a box describing the difference). Many seedless vascular plants, e.g., ferns are
homosporous—they produce only one type of spore which will have a bisexual gametophyte. Most seed
plants are heterosporous. They will produce two types of spores—megaspores and microspores.
Megaspores develop into the female gametophytes and microspores develop into the male
gametophytes.
Now let’s take a look at the four phyla that make up the extant seedless vascular plants.
Phylum lycophyta: club moss
There are approximately 1000 extant, described species of lycophytes. Their short leaves—microphylls—
give them a mossy appearance. They often produce strobili that look like clubs and so they are often
given the common name of club moss; however, to be very clear, they are seedless vascular plants and
as such are NOT mosses—remember, common names are often not scientifically accuratae.
Lycophytes dominated the landscape of the Carboniferous Period some 359 to 299 million years ago.
Much of the coal we burn today is the remains of lycophytes. During this period they grew into large
trees; today, however, they are bushy plants and many grow as epiphytes—a plant that grows on
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another plant, using it as a support, but it does not parasitize the plant it grows on. Epiphytic lycophytes
are found in tropical forests. Other lycophytes are found growing on the forest floor in more temperate
regions.
Lycophyte sporophytes produce vertical stems with small leaves—microphylls. Lycophyte sporophytes
also produce horizontal stems. From these horizontal stems dichotomously branching roots are derived.
There are three groups of lycophytes (figure 29.15):
Club mosses—named for their club-shaped strobili.
Quilworts—named for the pointy shape of their leaf.
Spike mosses—they have small delicate leaves and strobili and are usually smaller than club mosses.
They grow more horizontally to the ground whereas club mosses grow up a bit more vertically.
Club mosses are homosporous but quilworts and spike mosses are heterosporous.
Phylum Psilophyta: whisk ferns
There are only 129 extant species of psilophytans. They are usually epiphytic and are commonly found in
the south pacific. These plants exhibit a very primitive morphology including dichotomously branched
stems, no roots and scale-like microphylls.
Another plesiomorphic character is that each stem will have a yellow knob which is made of three fused
sporangia (figure 29.15). All psilophytans are homosporous.
Phylum Sphenophyta: horse tails
There are only 15 extant species of sphenophytans! Figure 29.15 shows a picture of Equisetum arvense.
The name of the phylum and the genus is descriptive for the bushy, horse tail like shape of their stems.
The stems of these species are jointed. Thus they are sometimes called arthrophytes—arthro is Greek
for joint. At each joint on the stem are leaves that exist in a whorl. Some species have stems containing
photosynthetic leaves and are called vegetative stems with completely separate stems with a strobilus at
the end called fertile stems. Remember, each strobilus is composed of multiple sporophylls that
produce multiple sporangia. These sporangia produce spores that will form gametophytes. In horsetails
the spores are homosporous.
Horsetails were often used as scouring rushes to scrub pots and pans!
Phylum Pterophyta: ferns
There are about 11,000 species of ferns. Most are tropical and adapted to live in humid environments.
Ferns produce horizontal, large leaves called fronds. A frond starts as a coiled tip called a fiddlehead—
you will see this in lab. Some ferns in the tropics grow into tree like structures about 20 feet tall. Others
are vine-like and still others are epiphytic. Almost all species are homosporous. Figure 29.13 shows the
life cycle of a typical fern.
Bio 102 Notes: Dr. Thomas R. Sawicki
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Lecture # 4: Plant Diversity I: How Plants Colonized Land; Campbell and Reece Chapter 29
The spore produces a gametophyte called a prothallus. The prothallus is very small, only a few
millimeters across and looks heart-shaped. The gametophyte produces both archegonia and antheridia.
The prothallus is usually located on or just below the forest floor. Once syngamy occurs (usually on a
separate prothallus from which the sperm were produced) the sporophyte begins to grow. The
sporophyte turns into a fiddlehead—it is called a fiddlehead because of the shape it takes. The
fiddlehead unfurls into a frond—sporophyll—which will produce sporangia in structures called sori on
its underside. The sporangia release their spores and the cycle is complete.
Figure 29.16 shows a reconstruction of a Carboniferous forest dominated by seedless vascular plants.
The Carboniferous Period existed from 360 to 299 million years ago. The massive radiation and
domination of these plants during the Carboniferous period resulted in a huge drop in atmospheric CO 2
and an onset of global cooling that included glaciations. But during the Carboniferous, shallow tropical
forests were found across the globe. Today’s massive coal seems are derived from these plants. Of
course, we are putting all the carbon sequestered by these plants over millions of years back into the
atmosphere in a matter of centuries! Hopefully this will give you an appreciation of why scientists are so
worried about the consequences of current global climate change.
Bio 102 Notes: Dr. Thomas R. Sawicki
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