Description
After
reading Chapter 4 Angiosperms from pages 47 – 57 in the Study Guide and
watching the Bidlack Sterns CH 23 Power Point Seed Plants Angiosperms,
complete the assignment Angiosperm Structure and Life Cycles draft 4 and
submit it in Canvas.I HAVE ATTACHED ALL THE INFORMATION YOU NEED BELOW, ALONG WITH THE ASSIGNMENT. IT IS RELATIVELY SIMPLE, PLEASE DO YOUR BEST FOR A GOOD GRADE!
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Angiosperm Structure and Life Cycles
Name _________________________
Please include the questions in your submission.
10 points each
1.
Compare monocots and dicots. Include 8 characteristics.
2.
List and describe the parts of a flower.
3.
Describe the development of a microspore mother cell into a pollen grain.
Define pollination as it takes place in the Angiosperms.
4.
Describe the development of the megaspore mother cell into the female
gametophyte.
5.
What is double fertilization? Describe each fertilization event.
6.
Define the term seed.
Define the term fruit.
What is the advantage of dispersal of fruits and seeds to the plant?
7.
What is dormancy?
What is the advantage of dormancy to the plant?
8.
Distinguish between the three main types of fruit: simple fruits, aggregate fruits,
and multiple fruits.
9.
Identify the requirements for the germination of a seed.
10.
Discuss the stages in the development of the embryo in the Capsella plant.
Chapter 23
Lecture Outline
Seed Plants: Angiosperms
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
©McGraw-Hill Education.
Outline
Introduction
Phylum Magnoliophyta – The Flowering Plants
Development of Gametophytes
Pollination
Fertilization and Development of the Seed
Apomixis and Parthenocarpy
Trends of Specialization and Classification in
Flowering Plants
Pollination Ecology
Herbaria and Plant Preservation
©McGraw-Hill Education.
Introduction
Angiosperms = flowering plants
Seeds enclosed in carpel that resembles a leaf
that has folded over and fused at the margins.
Pistil composed of a single carpel, or two or more
united carpels.
Seed develops from ovule
within carpel.
Ovary becomes a fruit.
Bleeding hearts
©McGraw-Hill Education.
Introduction (2)
Phylum Magnoliophyta – The Flowering Plants
Has been divided into two large classes:
Magnoliopsida – Dicots
DNA and cladistic evidence suggest that two
groups of dicots should be recognized.
Liliopsida – Monocots
Flower is modified stem bearing modified leaves.
Most primitive flower:
Long receptacle
Many spirally arranged flower parts that are
separate and not differentiated into sepals
and petals
Flattened and numerous stamens and
carpels
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Phylum Magnoliophyta – The Flowering Plants
Heterosporous
Sporophytes dominant.
Female gametophytes wholly
enclosed within sporophyte tissue
and reduced to only a few cells.
At maturity, male gametophytes
consist of a germinated pollen
grain with three nuclei.
©McGraw-Hill Education.
Phylum Magnoliophyta
Development of gametophytes – Female:
Diploid megasporocyte differentiates in ovule.
Undergoes meiosis and produces four
haploid megaspores.
Three degenerate.
Remaining cell enlarges and nucleus divides to
produce 8 nuclei (without walls).
Outer two layers of ovule differentiate into
integuments that later become seed coat.
Micropyle at one end of ovule
©McGraw-Hill Education.
Phylum Magnoliophyta (2)
Development of gametophytes – Female:
• 8 nuclei form two groups, 4 near each end of
cell.
• One nucleus from each group migrates to cell
middle and form central cell.
•
Cell walls form around remaining six
nuclei.
–
–
•
Egg and two synergids
closest to micropyle
Three antipodals at
opposite end – No
apparent function
Female gametophyte
(megagametophyte, embryo sac) =
large sac containing 8 nuclei and 7
cells
©McGraw-Hill Education.
Phylum Magnoliophyta (3)
Development of gametophytes – Male:
•
• Formation of male gametophytes takes place in
anthers.
• Four patches, corresponding to pollen sacs, of
microsporocyte cells differentiate in anther.
Each
microsporocyte
undergoes meiosis
to produce quartet
of haploid
microspores.
Anther with microspores
©McGraw-Hill Education.
Phylum Magnoliophyta (4)
Development of gametophytes – Male:
Microspores undergo three important changes:
Divide once by mitosis to form a small generative cell
inside the larger tube cell
Nucleus of tube cell = vegetative nucleus
Members of each quartet of microspores separate.
–
Wall becomes two-layered.
o
Outer layer = exine
« Finely sculptured
« Contains chemicals that
may react with chemicals
in stigma
•
Generative nucleus will later divide to
produce two sperm.
©McGraw-Hill Education.
Pollen grain
Phylum Magnoliophyta (5)
Pollination:
Pollination – Transfer of pollen grains
from anther to stigma
Self-pollination – Pollen grains
germinate on stigma of same
flower.
Fertilization – Union of sperm and egg
Pollination by insects, wind, water,
animals or gravity.
©McGraw-Hill Education.
Phylum Magnoliophyta (6)
Fertilization and development of the seed:
After pollination, further development of male
gametophyte may not take place unless pollen grain is:
From a different plant of the same species.
From a variety different from that of the receiving
flower.
Pollen tube grows between cells of stigma and style until it
reaches ovule micropyle.
Vegetative nucleus stays at tips of pollen tube, while
generative cell lags behind and divides into two sperm.
Pollen tube enters female gametophyte, destroying
synergid in the process, and discharges sperms.
©McGraw-Hill Education.
Phylum Magnoliophyta (7)
Fertilization and
development of the seed:
• Mature male
gametophyte =
germinated pollen grain
with its vegetative
nucleus and two sperms
within tube cell
©McGraw-Hill Education.
Phylum Magnoliophyta (8)
Fertilization and development of the seed:
Double fertilization:
One sperm unites with egg, forming zygote, then embryo.
Other sperm unites with central cell nuclei, producing
triploid endosperm nucleus that develops into endosperm
tissue.
Endosperm tissue = nutritive tissue for embryo
Endosperm becomes extensive part of seed in some
monocots, such as corn and other grasses.
Wheat, rice and corn – Major source of nutrition for
humans due to nutritional quality of endosperm
Endosperm absorbed into cotyledons in most dicots.
Ovule becomes seed, ovary matures into fruit, integuments
harden into seed coat.
©McGraw-Hill Education.
Phylum Magnoliophyta (9)
Lifecycle of typical flowering plant:
©McGraw-Hill Education.
Phylum Magnoliophyta (10)
Fertilization and development of the seed:
Other types of (female) gametophyte development:
Female gametophyte can have from 4 to 16 nuclei or
cells at maturity.
Endosperm may be 5x, 9x or 15x.
Example in lilies:
All four haploid megaspore nuclei produced from
megasporocyte remain functional.
Three of nuclei unite forming 3x nucleus and forth
nucleus remain haploid.
Results in female gametophyte with four 3x
nuclei and four 1x nuclei
One central cell nucleus is 3x and other is 1x.
Fertilization results in 5x endosperm nucleus.
©McGraw-Hill Education.
Phylum Magnoliophyta (11)
Fertilization and development of the seed:
Lily (female) gametophyte development:
©McGraw-Hill Education.
Phylum Magnoliophyta (12)
Apomixis and parthenocarpy:
Apomixis – Without fusion of gametes but with the
normal structures otherwise being involved
Embryo from diploid nutritive cell or other
diploid cell of ovule, instead of from zygote.
Results in a vegetatively propagated plant
Parthenocarpy – Fruits develop from ovaries with
unfertilized eggs.
Results in seedless fruits
Navel oranges and bananas
©McGraw-Hill Education.
Phylum Magnoliophyta (13)
Trends of specialization and classification in
flowering plants:
First historical classifications for
convenience.
Modern botanists group plants according to
natural relationships based on evolution.
Fossil record suggests flowering plants first
appeared about 160 million years ago
during late Jurassic.
Flowering plants then developed during
Cretaceous and Cenozoic.
Dominant plants today
©McGraw-Hill Education.
Phylum Magnoliophyta (14)
Trends of specialization and classification in flowering plants:
First pistil from leaflike structure with ovules along margins
= carpel.
Edges of blade rolled inward and fused together.
• Separate carpels of
primitive flowers
fused together to
form compound
pistil consisting of
several carpels.
©McGraw-Hill Education.
Phylum Magnoliophyta (15)
Trends of specialization and classification
in flowering plants:
•
Inferior ovary (epigynous flower) –
Receptacle or other flower parts
fused to ovary and grown up
around it.
– Calyx and corolla appear to be
attached to top of ovary.
•
•
Superior ovary (hypogynous flower) –
Ovary produced on top of
receptacle.
– Other flower parts attached
around ovary base.
Perigynous flowers – Flower parts
attached to corolla tube of fused
petals, creating floral tube that is
not attached to ovary.
©McGraw-Hill Education.
Phylum Magnoliophyta (16)
Trends of specialization and classification in
flowering plants:
•
Complete flower – Has calyx, corolla,
stamens and pistil
•
Incomplete flower – Corolla or other
flower parts missing.
•
Perfect flower – Both stamens and
pistil present.
•
Imperfect flower – Either stamens or
pistil missing.
–
–
Male
flower
Monoecious species – Male
and female imperfect flowers
on same plant.
Dioecious species – Plant
bears only male flowers and
other plants bear only female
flowers.
Female flower with inferior ovary
©McGraw-Hill Education.
Phylum Magnoliophyta (17)
Trends of specialization and classification in flowering
plants:
Primitive flowering plants:
Simple leaves.
Flower with numerous, spirally arranged parts, not
fused to each other
Flowers radially symmetrical = regular.
–
–
–
©McGraw-Hill Education.
Flowers with both
stamens and pistils
o Complete and
perfect flowers
Superior ovary
(hypogynous flower)
Still many plants
today whose flowers
have primitive
features.
Magnolia
Phylum Magnoliophyta (18)
Trends of specialization and classification in flowering plants:
Specialized flowering plants:
Flower parts fewer and definite in number.
Spiral arrangements compressed to whorls
Bilaterally symmetrical flowers = irregular
Reduction and fusion of parts
Incomplete or imperfect flowers
Inferior ovary
Orchid
©McGraw-Hill Education.
Pollination Ecology
Pollinators have coevolved with plants.
Twenty thousand bee species among current-day pollinators.
Bee-pollinated flowers:
Generally brightly colored, mostly blue or yellow
Often have lines or other distinctive markings, which may
function as honey guides to lead bees to nectar.
Bees see UV light (humans do not).
Some flower markings visible only in UV light.
In ordinary light
©McGraw-Hill Education.
In UV light
Pollination Ecology (2)
Beetle-pollinated flowers:
Strong, yeasty, spicy or fruity odor
White or dull in color – Beetles do have keen visual senses.
Some do not secrete nectar, but furnish pollen or food on petals in
special storage cells.
Fly-pollinated flowers:
Smell like rotten meat
Dull red or brown
©McGraw-Hill Education.
Pollination Ecology (3)
Butterfly- and moth-pollinated flowers:
Often have sweet fragrances
White or yellow for night-flying moths
Sometimes red, often blue, yellow or orange for butterflies
Nectaries at bases of corolla tubes or spurs for long tongues.
Bird-pollinated flowers (hummingbirds and sunbirds):
•
Often bright red or yellow
• Little if any odor – Birds do
not have a keen sense of
smell.
• Large and part of sturdy
inflorescence
• Copious amounts of nectar
– Birds highly active.
• Long floral tubes
©McGraw-Hill Education.
Pollination Ecology (4)
Bat-pollinated flowers:
•
Primarily in tropics
•
Open at night when bats are
foraging
•
Dull in color
•
Large enough for bat to insert
head or consist of ball-like
inflorescence containing large
numbers of small flowers
©McGraw-Hill Education.
Pollination Ecology (5)
Orchid flowers:
• Have pollinators among all types mentioned
• Some of adaptations between orchid flowers and pollinators are
extraordinary.
• Pollen grains produced in little sacs called pollinia (singular:
pollinium) with sticky pads at base.
• Members of Ophrys have modified petal that resembles female
bumble bee or wasp.
–
Male bees or wasps try to
copulate with flower.
o Pollinia deposited on
their head.
Ophrys
©McGraw-Hill Education.
Herbaria and Plant Preservation
Herbaria (singular: herbarium) – Libraries of
dried, pressed plants, algae, and fungi, arranged
and labeled.
Methods:
• Fungi and
bryophytes dried
and stored in small
packets.
• Plant press used for
vascular plants.
©McGraw-Hill Education.
Herbaria and Plant Preservation (2)
Methods:
• Vascular plant specimens
mounted on 100% rag
herbarium paper.
• Specimens stored so
retrieval of specimens is
easily accomplished.
©McGraw-Hill Education.
Review
Introduction
Phylum Magnoliophyta – The Flowering Plants
Development of Gametophytes
Pollination
Fertilization and Development of the Seed
Apomixis and Parthenocarpy
Trends of Specialization and Classification in
Flowering Plants
Pollination Ecology
Herbaria and Plant Preservation
©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.
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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.
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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
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The life cycle is characterized by an Alternation of Generations.
The gametop
