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Introduction to the Diversity of Plants
Name __________________________
Please include the questions in your submission.
10 Points Each
1.
Define the term plant.
2.
List the types of plants that are found in each of the following groups:
Nonvascular Land Plants
Vascular Nonseed Plants
Gymnosperms (Define Gymnosperm and give examples.)
Angiosperms (Define Angiosperm and give examples)
3.
Discuss the importance of plants to us under the following headings:
Oxygen and carbon dioxide interactions
Sources of food
Sources of commercial products
Sources of energy (list the sources)
4.
What is autotrophic nutrition?
Compare it to heterotrophic nutrition.
5.
Define photosynthesis and write its equation.
Plants store their carbohydrate food in the form of what molecule?
6.
What kind of cells do plants have? Define this term.
What structure surrounds and protects the plant cell?
Name the molecule that composes this structure.
7.
What are chloroplasts?
What is the function of chlorophyll?
8.
Explain the concept of Alternation of Generations.
Define the stages that alternate in the plant life cycle. Define the terms haploid
and diploid and include these terms in your definition of the stages.
Explain how the stages alternate from one stage to the other during the life cycle.
(How do we get from a gametophyte to a sporophyte?)
9.
Compare plant cells and animal cells by completing the following table. Include 6
characteristics.
Comparison of Plant Cells and Animal Cells
Characteristic
Plant Cells
Animal Cells
.
10.
Discuss the changes in structure that have evolved to adapt plants to live on
land. Include the changes that have reduced the loss of water, permit gaseous
exchange on land, and are responsible for transport.
Chapter 1
Lecture Outline
What is Plant
Biology?
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
©McGraw-Hill Education.
Outline
• The Relationship of Humans to Their Environment
– The Effect Humans Have on the Environment
– Human and Animal Dependence on Plants
• Botany as a Science
– Scientific Method
• Diversification of Plant Study (Botanical Disciplines)
©McGraw-Hill Education.
The Relationship of Humans to Their Environment
The Effect Humans Have on the Environment
We have had major impacts on the environment:
– Drained wetlands
– Cleared natural vegetation
– Dumped wastes and pollution
– Used pesticides and herbicides
©McGraw-Hill Education.
The Effect Humans Have on the Environment (2)
• We must reduce our environmental impact:
– Change agricultural practices
– Render pollutants harmless
– Recycle
– Replace pesticides with biological pest controls
– Conserve water and energy
– Preserve habitats and species
©McGraw-Hill Education.
Human and Animal Dependence on Plants
• Plants convert the sun’s energy into energy that is usable to
plants and to animals.
• In the process, plants produce oxygen and remove carbon
dioxide in the air we breathe.
©McGraw-Hill Education.
Human and Animal Dependence on Plants (2)
• Plants are the
sources of
multiple products
of human society:
– Food
– Perfumes
– Dyes
Copyright © McGraw-Hill Companies Permission Required for Reproduction or
Display
©McGraw-Hill Education.
Human and Animal Dependence on Plants (3)
• Plants are the
sources of
multiple
products of
human society:
– Beverages
– Lumber
– Paper
Coffee
©McGraw-Hill Education.
Human and Animal Dependence on Plants (4)
❖ Plants are the sources of
multiple products of human
society:
• Clothing
• Medicines
• Coal and oil
• Alternate energy
sources
Cotton plants
©McGraw-Hill Education.
Botany as a Science
• Botany is the study of plants.
• At first, interest in plants was practical.
– Centered around the production of food, fibers, fuel, and medicine
• Eventually, an intellectual interest arose.
– Led to plant study becoming a science
• Science involves the observation, recording, organization, and
classification of information.
©McGraw-Hill Education.
Scientific Method
• The scientific method describes the procedures of developing
and testing hypotheses.
• Hypothesis – Tentative, unproven explanation of an
observation
©McGraw-Hill Education.
Scientific Method (2)
• Experiment – Test to determine if a hypothesis is correct
– Must be repeatable
– Variables – Aspects of the experiment that can be changed or held
constant
– Good experiments consist of two parts:
©McGraw-Hill Education.
1.
Variable changed
2.
Variable held constant = Control
Scientific Method (3)
• Data – Results from the experiment
• Principle – Useful generalization derived from experimental
data
• Theory – Grouping of related principles
©McGraw-Hill Education.
Diversification of Plant Study
(Botanical Disciplines)
• Plant Anatomy
– Internal structure of plants
• Plant Physiology
– Plant function
• Plant Taxonomy
– Describing, naming and classifying plants
– Plant Systematics
•
©McGraw-Hill Education.
Developing methods for classifying and
naming plants
Cross-section of
Magnolia wood
Diversification of Plant Study
(Botanical Disciplines) (2)
• Plant Geography
– Plant distributions
• Plant Ecology
– Interaction between plants and
their environments
• Plant Morphology
– Form and structure of plants
©McGraw-Hill Education.
Diversification of Plant Study
(Botanical Disciplines) (3)
❖ Genetics
• Science of Heredity
– Potential development of better agricultural, medicinal, and other useful
plants
❖ Cell Biology
• Cell structure and function
❖ Economic Botany and Ethnobotany
•
Practical uses of plants and plant products
•
Still vast amounts of botanical information yet to be discovered
©McGraw-Hill Education.
Review
• The Relationship of Humans to Their Environment
– The Effect Humans Have on the Environment
– Human and Animal Dependence on Plants
• Botany as a Science
– Scientific Method
• Diversification of Plant Study (Botanical Disciplines)
©McGraw-Hill Education.
©McGraw-Hill Education.
UNIT I:
DIVERSITY OF THE PLANT
KINGDOM
CHAPTER 1
INTRODUCTION TO THE PLANT KINGDOM , NONVASCULAR
LAND PLANTS
OBJECTIVES
1) To learn the economic and ecological importance of plants.
2) To learn the characteristics of plants and determine how plants are distinguished from other
groups of living organisms.
3) To understand how plants are classified on the basis of structural, physiological and genetic
characteristics and recognize that the classification of a plant reflects its evolutionary
relatedness to other similar organisms with which it is placed.
4) To study the diversity of the plant kingdom by examining the structure, function, and life
cycles of representative plants.
5) To understand the concept of alternation of generations; to examine the relationship between
the gametophyte and sporophyte in plant life cycles and to see how this relationship changed
as plants evolved from lower plants to higher plants.
6) To learn the characteristics of nonvascular plants.
7) To investigate the life cycles of nonvascular plants.
8) To explore the evolution of nonvascular plants, focusing on the structural, functional, and
reproductive adaptations that enabled plants to evolve from aquatic plants to land plants.
9) To consider the economic importance of nonvascular plants.
1
INTRODUCTION TO THE PLANT KINGDOM
DEFINITION
Plants are eukaryotic, photosynthetic organisms that contain chlorophyll a and b, have cell walls
containing cellulose, and store food as starch within plastids.
CHARACTERISTICS OF PLANTS
1) Plant cells are eukaryotic. Their DNA is contained within a nucleus surrounded by a
nuclear membrane.
2) Plants are photosynthetic. They contain chlorophyll a and chlorophyll b, xanthophylls
(yellow pigments) and carotenes (orange pigments). The photosynthetic pigments are
concentrated in organelles known as chloroplasts.
3) Food is stored in the form of starch within plastids.
4) Plants have cell walls that are composed of cellulose. In addition, vascular plants have
lignin in the cell walls that functions in support and conduction, enabling plants to grow
tall.
5) Plant cells have large central vacuoles.
6) Cell division is by means of a cell plate that forms across the mitotic spindle.
2
COMPARISON OF PLANT CELLS AND ANIMAL CELLS
Characteristic
Cell Walls
Presence of Chloroplasts
Presence of Centrioles
Plant Cells
Have cell wall composed
of cellulose
Plants are photosynthetic.
The photosynthetic
pigments are concentrated
in chloroplasts.
Plant cells lack centrioles.
Storage of Food
Food is stored in the form
of starch within plastids.
Cell Division (Cytokinesis)
Plant cell divide by cellplate formation.
Vacuoles
Plant cells have a large
central vacuole
3
Animal Cells
Lack cell walls
Animal cells are not
photosynthetic. Animal
cells lack chloroplasts.
Animal cells have
centrioles.
Food is stored in the form
of glycogen. Plastids are
lacking.
Animal cells divide by
constriction of cytoplasm.
( a cleavage furrow)
Animal cells have many
smaller vesicles
THE IMPORTANCE OF PLANTS
PLANTS PRODUCE O 2 AND TAKE IN CO 2
Photosynthesis is the production of food in green plants by utilizing light energy absorbed by
chlorophyll to combine carbon dioxide and water producing carbohydrate and releasing oxygen,
a by-product. Photosynthesis is an autotrophic form of nutrition. Autotrophic literally means
“self-feeding”. It means that plants can make their own food. In contrast, animals are
heterotrophic. This means “other-feeding”. Animals must eat plants or other animals in order
to survive.
EQUATION
The overall equation for photosynthesis:
light
6CO2 + 6H2O ————————-> C6H12O6 + 6O2
IMPORTANCE
All aerobic organisms must take in oxygen. Oxygen is needed for cellular respiration, a set of
reactions in which glucose is broken down in the presence of oxygen into carbon dioxide and
water. Almost all of the oxygen in the atmosphere, the oxygen that is needed for this reaction
that produces energy for all aerobic organisms, has been put there by green plants carrying out
the reactions of photosynthesis.
PLANTS ARE A SOURCE OF FOOD
Plants are at the base of the food chain. Animals are heterotrophic. This means that they must
eat plants or other animals to survive. Even if animals are carnivorous and eat other animals,
those animals must feed upon plants. Plants on the other hand, are autotrophic, they can produce
their own food. Fortunately for us and for other animals, plants produce more food than they
require for their own use. As a result, they can sustain all the animals with the food that they
need.
4
Plants supply our staple foods including grains such as wheat, rice, corn, oats, and barley etc.
Produce such as lettuce, carrots, beets, tomatoes, etc. is supplied by farming plants. Plants
produce fruit, such as apples, pears, peaches, plums, cherries, etc. The production of food from
animal sources, such as meat, milk, cheese, poultry, eggs, and fish is also dependent upon plants.
Beverages, such as coffee come from a plant. Alcoholic beverages are produced from plants.
Spices are produced from plants.
PLANTS SUPPLY LUMBER AND PULP FOR PAPER
The lumber that is needed for the construction of homes and commercial buildings comes from
trees. The pulp for the production of paper, including the paper in your study guide, comes from
trees.
Cotton is used to make clothing.
PLANTS ARE USED TO MAKE MEDICINES AND DRUGS
The drug taxol, which was the first drug that was found to be effective against ovarian cancer,
was first derived from the Pacific Yew tree.
PLANTS ARE SOURCES OF ENERGY
Oil
Oil comes from algae that lived in the oceans and carried out photosynthesis millions of years
ago. After they died, the bodies of these organisms fell to the bottom and were converted to
sediments. As they were buried and subjected to great heat and pressure, they were gradually
converted to oil.
It is interesting to consider that we may once again turn to algae to produce oil, only this time we
will use living algae and oil will be produced as a renewable resource.
Coal
Coal is derived from ferns and related plants that lived during the Carboniferous period
approximately 300 to 260 million years ago. After the bodies of these plants died, they did not
decay but were buried by sediments. As they were subjected to great heat and pressure within
the earth’s crust, they were gradually converted to our deposits of coal.
Alcohol
Corn and other plants are used to produce alcohol, which is added to gasoline to make “gasohol”.
Methane
Methane is produced by the decomposition of material from plants and animals. It can also be
used as a source of fuel.
5
CLASSIFICATION OF PLANTS
PLANT TAXONOMY
Plant taxonomy is the science concerned with describing, naming, and classifying plants.
DEVELOPMENT OF THE BINOMIAL SYSTEM OF NOMENCLATURE
Each living organism is given a scientific name that consists of two parts. This name is the
species name. The first part of the word is the genus. The second part of the name is a modifier,
or epithet. For example, the scientific name of man is Homo sapiens. The scientific name of the
red oak is Quercus rubra. It is important to know that the species name for the organism is the
entire two-part name. The second word in the name, for example sapiens, has no meaning by
itself. It is an adjective or modifier. The term taxon (plural taxa) is used to refer to a group such
as genus or species. The binomial system of nomenclature was developed by Carolus
Linnaeus (1707-1778).
Species – groups of actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups.
MAJOR FEATURES OF THE CLASSIFICATION SYSTEM
1. The name of an organism is always written in Latin.
2. The species name is written in italics.
3. No two organisms can have the same scientific name.
4. The classification system is universal, it is used all over the world.
5.
The classification system used in Biology is a hierarchical system, meaning that groups
are placed within groups. Species are grouped within genera, genera are grouped within
families, families are grouped within orders, orders are grouped within classes, classes
are grouped within phyla (divisions), and phyla are grouped within kingdoms.
6
The major groups used in classification are:
Kingdom
Phylum
Class
Order
Family
Genus
Species
An example showing the classification of the red oak is given below:
Kingdom Plantae
Phylum Magnoliophyta
Class Magnoliopsida
Order Fagales
Family Fagaceae
Genus Quercus
Species Quercus rubra L.
CLADISTICS
Cladistics is a new approach to classification that is based upon an organism’s phylogeny, that
is, its ancestry. In cladistics, organisms are grouped together on the basis of whether they have
one or more shared derived characteristics that come from the group’s ancestor. The
relationships are portrayed on diagrams known as cladograms.
MAJOR PLANT GROUPS
The major groups of plants are as follows:
Nonvascular Land Plants
This group consists of mosses, liverworts, and hornworts
Vascular Nonseed Plants
This group consists of ferns, and fern allies
Seed Plants
This group is divided into
Gymnosperms – plants with exposed seeds, and
Angiosperms – the flowering plants
7
EVOLUTIONARY TRENDS DISPLAYED BY PLANTS
Plant evolution is primarily, the history of plant adaptations in structure and reproduction that
have allowed them to make a transition from living in water to living on land. The following
major features of plant evolution can be recognized below:
CHANGES IN MORPHOLOGY THAT REDUCE THE LOSS OF WATER
Structural features appeared to reduce the loss of water. Nonvascular land plants have developed
a cuticle, a waxy layer that prevents the excessive loss of water and stomata, microscopic pores
that permit gaseous exchange on land. Vascular tissue, which first appeared in ferns, developed
in plants to transport water and minerals from the roots up to higher plant parts and carbohydrate
from the photosynthetic tissue to tissues throughout the plant.
CHANGES IN REPRODUCTION THAT FREED PLANTS FROM DEPENDENCE
UPON WATER
As plants evolved to live on land, they developed adaptations which freed them from dependence
upon water for sexual reproduction. Nonvascular land plants and ferns depend upon water for
reproduction. This is because their sperm are flagellated and must swim through water to reach
the egg. Innovations appearing in Gymnosperms, including pollen, the pollen tube, the ovule
and the seed have freed them from dependence upon water for reproduction. Rather than
flagellated sperm swimming through the water, pollen grains dispersed by the wind are carried
from the male cone to the female cone. Primitive gymnosperms retain several features that link
them to more ancestral plants that depended upon water for reproduction. Primitive
gymnosperms retain flagellated sperm. They have a pollen tube, but it does not carry the sperm
to the egg, it serves a nutritive function instead. In higher gymnosperms and flowering plants
the sperm are nonflagellated and are carried to the egg by the pollen tube. The ovule protects the
female gametophyte as it develops. The ovule develops into a seed. The seed contains the
embryo, stored food, and is surrounded by a protective seed coat, allowing the embryo to
develop until it can be dispersed onto the land. The seed may contain specialized structures for
dispersal, and may remain dormant for long periods, surviving freezing weather, or drought.
CHANGES IN THE LIFE CYCLE
The life cycle of plants is characterized by an alternation of generations. During the plant life
cycle, a stage of the life cycle known as the gametophyte alternates with the sporophyte. The
gametophyte stage is the haploid, gamete-producing stage of the life cycle. The sporophyte stage
is the diploid, spore-producing stage of the life cycle.
8
SHIFT FROM DOMINANCE OF THE GAMETOPHYTE TO DOMINANCE OF THE
SPOROPHYTE
During plant evolution there is a transition from the dominance of the gametophyte to the
dominance of the sporophyte. The sporophyte becomes larger and larger in relation to the
gametophyte, which becomes smaller and smaller.
CHANGE IN THE RELATIONSHIP BETWEEN THE GAMETOPHYTE AND THE
SPOROPHYTE
As plants evolved, the relationship between the gametophyte and the sporophyte has changed. In
the nonvascular land plants the sporophyte is dependent upon the gametophyte. In the ferns, the
sporophyte is dependent upon the gametophyte, but only for a short time early in development.
In the seed plants the gametophyte becomes dependent upon the sporophyte.
9
THE NONVASCULAR LAND PLANTS
CHARACTERISTICS
1) Lack True Xylem and Phloem. In general, nonvascular plants lack xylem and phloem, the
specialized vascular tissues that are found in higher plants. In higher plants, xylem conducts
water and minerals, and phloem conducts carbohydrates that are produced in photosynthesis
2) Morphology. The body or thallus of nonvascular land plants lacks true roots, stems, and
leaves. The bodies of nonvascular plants are stratified, that is they are composed of several
layers of parenchyma cells. In contrast, algae are composed of filaments that are chains of
cells linked end to end. Nonvascular plants have a cuticle over much of their bodies; many
have stomata. Because of the lack of vascular tissues, water cannot be transported to higher
plant structures. As a result, these are generally low-growing plants.
3) Alternation of Generations. Nonvascular plants, like vascular plants, have a life cycle with
an alternation of gametophyte and sporophyte generations. In nonvascular plants, the
gametophyte is the dominant stage of the life cycle. The sporophyte is dependent upon the
gametophyte.
4) Multicellular Reproductive Structures. Nonvascular plants have multicellular reproductive
structures (sporangia and gametangia); one or several layers of sterile cells always surround
reproductive cells. This characteristic distinguishes the nonvascular plants from the algae.
5) Production of airborne spores. Spores are produced in sporangia by meiosis and released
into the air.
6) Multicellular Embryos
7) Nonvascular Plants Require Water for Sexual Reproduction. The nonvascular plants
require free water to reproduce sexually. They have flagellated, motile sperm cells which
must swim through a film of water in order to reach the egg.
8) Habitat. Because mosses lack vascular tissues and true roots, and depend upon water for
reproduction, they generally live in moist locations.
10
CLASSIFICATION
The nonvascular land plants are classified into three divisions:
Division Bryophyta
Division Bryophyta is made up of the mosses.
Division Hepatophyta
Division Hepatophyta is made up of the liverworts.
Division Anthocerotophyta
Division Anthocerotophyta is made up of the hornworts.
Division Bryophyta: Mosses
MORPHOLOGY OF THE GAMETOPHYTE GENERATION
The plant body of the moss is known as a thallus. The thallus or plant body lacks true roots,
stems, and leaves. These structures are not considered true roots, stems, or leaves because they
occur in the gametophyte generation, and as such are haploid cells; they also lack true xylem and
phloem. The thallus is composed of
1) A vertical axis known as a gametophore surrounded by whorls of leaflike structures.
The moss gametophyte has radial symmetry; the leaves are arranged around a central
axis. Moss gametophores lack true xylem and phloem. However, some mosses have
cells called hydroids that can conduct water and others called leptoids that can conduct
sugars.
2) Leaflike structures that are photosynthetic. A cuticle occurs only on the upper surface.
Water is absorbed directly through the uncutinized lower surface from rain, dew, and fog.
3) Rhizoids anchor the stem in the ground and do not appear to be involved in absorbing
either water or minerals. Rhizoids are multicellular in the moss.
11
LIFE CYCLE
The moss life cycle illustrates the concept of Alternation of Generations. In the life cycle there
is an alternation between a haploid gametophyte and a diploid sporophyte. The gametophyte
is the haploid gamete-producing stage of the life cycle. The sporophyte is the diploid spore
producing stage of the life cycle. In the mosses, as in all the nonvascular plants, the gametophyte
is the dominant stage of the life cycle. The gametangia are the gamete-producing structures.
The gametangia include: antheridia, which are male reproductive structures that produce sperm
and archegonia, which are female reproductive structures that produce eggs. The archegonium
is made up of a rounded base called the venter, which contains a large egg, a narrow neck, and a
canal that runs down the neck.
Some species of mosses are monoecious, the male and female reproductive structures
(antheridia and archegonia) occur on the same individual plant. Others are dioecious, the male
and female reproductive structures may occur on separate individuals. In the moss, the sperm
form by mitosis. The gametophytes have the haploid number of chromosomes. Division by
mitosis results in daughter cells that have the same number of chromosomes. Therefore the
sperm are also haploid. When sperm cells are mature, the antheridium breaks open and liberates
sperm cells. Water is necessary for sexual reproduction. The sperm are flagellated and must
swim through water to reach the egg. The eggs also form by mitosis and are haploid. The
sperm are guided to the egg by following a chemical that is secreted from the archegonia. The
sperm reach the archegonia and travel down the neck to the egg, where one sperm cell fertilizes
the egg. The fusion of the egg and the sperm is fertilization. Fertilization results in a diploid
cell known as a zygote. The zygote is the first cell of the sporophyte generation.
THE SPOROPHYTE GENERATION
The zygote undergoes development within the archegonium to form the sporophyte. In the moss,
the sporophyte develops from and remains attached to the gametophyte. The sporophyte consists
of:
1) The foot that attaches the sporophyte to the gametophyte. It also absorbs water,
minerals, and sugar from the gametophyte.
2) The seta or stalk. This is a long narrow structure that supports the capsule.
3) The capsule. This structure produces spores.
The capsule consists of
1) A layer of sterile cells comprising a cylindrical outer wall.
2) The operculum, a lid that covers the capsule.
12
3) Peristome teeth, form a ring below the operculum, move in and out in response to
changes in humidity to disperse the spores.
4) A central axis of sterile tissue inside the capsule known as the columella.
5) A ring of sporogenous cells that surrounds the columella. These cells undergo
meiosis to produce haploid spores that fill the cavity within the capsule.
Note: The apex of the capsule is covered by a structure called the calyptra. Technically,
this is not part of the capsule proper. Unlike the capsule, which is composed of diploid
tissue, the calyptra is haploid. The calyptra is composed of gametophytic tissue derived
from the neck of the archegonium. As the capsule develops, the seta elongates and pulls
away the calyptra which forms a cap over the capsule.
Mechanism of Spore dispersal: The caplike lid of the capsule, the operculum breaks off from
the capsule. Inside the capsule is a ring consisting of two rows of teeth. These are called
peristome teeth. The teeth move in and out in response to changes in humidity. They bend
outward, releasing the spores when the air is dry and bend inward preventing their release when
the air is humid. Spores are released when they are light and dry and easily carried by air
currents.
The spore germinates to form a filamentous structure known as a protonema (pl.: protonemata)
that grows into a new gametophyte thallus. The similarity of the protonema to filamentous green
algae supports the suggestion that mosses evolved from green algae.
SUMMARY OF THE LIFE CYCLE
•
•
•
•
The life cycle is characterized by an Alternation of Generations.
The gametophyte is the dominant stage in the life cycle of the moss.
The sperm are flagellated. Water is required for sexual reproduction.
In the moss life cycle, the sporophyte is dependent upon the gametophyte.
ECONOMIC IMPORTANCE
One moss that is important economically is Sphagnum, the peat moss. Peat is used as fuel. Peat
is also used extensively in gardening and lawn care because of its moisture-holding capacity.
13
DIVISION HEPATOPHYTA (THE LIVERWORTS)
These plants are called liverworts because in Medieval times, physicians believed that they could
treat diseases by using plants (worts) that resembled the appearance of the organ that they were
trying to treat. Liverworts are divided into lobes, so is the human liver. Seeing this resemblance,
the physicians used liverworts to treat liver disease. The plants became known as liverworts and
the name has stayed with them until the present time.
Liverworts are found in moist environments, such as on moist rocks and soil along
woodland streams. Like mosses, liverworts are small plants that have an alternation of
heteromorphic generations. The sporophyte is even less conspicuous than in mosses and is also
completely dependent upon the gametophyte.
STRUCTURE OF THE MARCHANTIA GAMETOPHYTE
The dominant stage of the life cycle is the haploid gametophyte. The body of the liverwort
gametophyte is known as a thallus. It is a flat, ribbon-like structure composed of layers of
parenchyma cells. The thallus is bilaterally symmetrical and branches repeatedly into two
approximately equal divisions as it grows. This type of branching is known as dichotomous
branching. The thallus is composed of:
1) The upper epidermis, a single layer of cells that lines the upper surface of the thallus. It
is provided with pores that function in gaseous exchange. As seen from above, the
surface is divided into diamond-shaped areas with a pore in the center of each.
2) Below the upper epidermis is a chamber that contains groups of photosynthetic,
branching cells. The branching arrangement of these cells increases the surface area of
the cells in contact with the gases in the air spaces. This facilitates the diffusion of
carbon dioxide into the cells and leads to an increase in the rate of photosynthesis. The
region of air spaces is divided into diamond-shaped compartments by walls of layered
cells. These walls of cells support the upper epidermis. Each compartment opens to the
outside through a single pore described above.
3) Below the region of air spaces is a region of compactly arranged parenchyma cells.
These parenchyma cells are thin-walled cells that generally lack chloroplasts and have
little chlorophyll. They probably serve as storage cells for the accumulation of starch.
4) The lower epidermis, a single layer of cells that lines the lower surface of the thallus of
Marchantia. Arising from the lower epidermis are two types of filamentous structures:
rhizoids and scales. The rhizoids are unicellular; the scales are multicellular. The
rhizoids anchor the thallus to the soil. The scales function to retain moisture.
14
REPRODUCTION
ASEXUAL REPRODUCTION
1) Asexual multiplication may occur by fragmentation. As the younger regions at the tips
of Marchantia continue to grow and spread outward, the older portions in the rear die and
decay. This causes the young branches to separate from the original thallus, forming new
individual plants.
2) Marchantia also reproduces asexually by means of gemmae (singular: gemma). The
gemmae, or buds, are formed in small cupules on the upper surface of the thallus.
Raindrops splash into the cups scattering the gemmae as much as a meter (3 feet away).
Each gemma is capable of growing into a new thallus. This is a method of rapid
multiplication.
SEXUAL REPRODUCTION
Male and female organs are produced on separate gametophytes (Marchantia is dioecious). The
reproductive structures are known as gametophores.
The male gametophore is known as an antheridiophore. It consists of:
1) The antheridial receptacle
2) A stalk
The sperm-producing structures, the antheridia are embedded within the upper surface of the
antheridial receptacle. This location facilitates the release and dispersal of the sperm. In the
liverwort, the sperm form by mitosis and are haploid .
Female gametophores are known as archegoniophores. The archegoniophore consists of:
1) The archegonial receptacle
2) A stalk
The egg-producing structures, the archegonia, hang down from the lower surface of the
archegonial receptacle. This location affords the archegonia protection.
The archegonium is a flask-shaped structure. It consists of an enlarged, rounded base or venter,
a long neck containing a canal, and a short stalk attaching the organ to the thallus. The venter
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of the archegonium contains the egg and a ventral canal cell, which is just above the egg like a
plug, holding it in place. The eggs form by mitosis and are haploid.
FERTILIZATION (SYNGAMY)
Water is necessary for fertilization. The sperm are flagellated and must swim through water to
reach the egg. The mature sperm are released from the antheridium when liquid water is
available for the sperm. Chemicals given off by the archegonium attract the sperm, which swim
down the canal in its neck to the egg. One of the sperms unites with the egg, forming a diploid
zygote.
How can the sperm reach the egg if the archegonia on the receptacle are elevated high above the
surface by a stalk? The explanation is that the archegonia mature before much elongation has
occurred in the stalks of the gametophores. As a result, the sperm can fertilize the egg while the
archegonium is close to the surface
THE SPOROPHYTE
The zygote is the first cell of the sporophyte. The zygote divides to form a multicellular embryo
that develops into the sporophyte. The sporophyte is retained in the venter of the archegonium
during its embryonic stages. The old archegonium is at this stage called the calyptra.
Structurally the mature sporophyte consists of the following structures:
1) A foot
The foot attaches the sporophyte to the female receptacle. It also transports nutrients from the
gametophyte to the developing sporophyte.
2) A seta or stalk
The stalk transports nutrients from the gametophyte to the sporophyte. It connects the foot and
the capsule.
3) A capsule
The function of the capsule is spore production. The capsule is an ovoid structure at the distal
end of the sporophyte. It has a wall consisting of a single layer of cells. Within the capsule
spore mother cells divide by meiosis to produce spores. Spore dispersal is aided by the elaters.
These are narrow elongated cells, dead at maturity, with spirally thickened cell walls that
resemble springs. As the spore capsule dries, it splits open, exposing the elaters to the air. As
the elaters dry, they twist, helping to disperse the spores from the ca
