Download Plants notes

Introduction to Plants
What is a plant?
Plants are multicellular eukaryotes that have cell walls
made of cellulose.
Plants develop from multicellular embryos and carry out
photosynthesis using the green pigments chlorophyll a
and b.
Plants include trees, shrubs, and grasses, as well as other
organisms, such as mosses and ferns.
Most plants are autotrophs, although a few are parasites
or saprobes that live on decaying materials.
The Plant Life Cycle
Plant life cycles have two alternating phases, a diploid
(2N) phase and a haploid (N) phase, known as alternation
of generations.
Alternation of Generations
During the two phases of the life cycle, mitosis and
meiosis alternate to produce the two types of
reproductive cells—gametes and spores.
The diploid (2N) phase is called the sporophyte, or sporeproducing plant.
The haploid (N) phase is called the gametophyte, or
gamete-producing plant.
Plant spores are haploid (N) reproductive cells formed in
the sporophyte by meiosis.
The spores can grow into new organisms called
gametophytes.
A gamete is a reproductive cell produced by mitosis, and
it can fuse with another gamete to produce the
sporophyte.
What do plants need to survive?
In order to survive, plants need:
• sunlight
• water and minerals
• gas exchange
• transport of water and nutrients
throughout the plant body
Sunlight
Plants use energy from sunlight to carry out
photosynthesis.
Photosynthetic organs such as leaves are broad and flat
to maximize light absorption.
Water and Minerals
All cells require a constant supply of water.
Water is used up quickly when the sun is shining. As a
result, plants have structures that limit water loss.
As they absorb water, plants also absorb minerals.
Minerals are nutrients in the soil needed for plant
growth.
Gas Exchange
Plants require oxygen to support cellular respiration as
well as carbon dioxide to carry out photosynthesis.
They must exchange these gases with the atmosphere
without losing excessive amounts of water through
evaporation.
Movement of Water and Nutrients
Plants take up water and minerals through their roots,
but they make food in their leaves.
Most plants have specialized tissues that carry water and
nutrients from the soil and distribute products of
photosynthesis throughout the plant body.
Simpler plants carry out these functions by diffusion.
Early Plants
When plants first appeared, life on Earth changed.
As plants colonized the land, they changed the
environment so other organisms could develop.
New ecosystems arose, and organic matter began to form
soil.
The first plants evolved from an organism similar to
the multicellular green algae living today.
Multicellular green algae have the size, color, and
appearance of plants.
They have reproductive cycles similar to those of plants.
Green algae also have cell walls and photosynthetic
pigments that are identical to those of plants.
The First Plants
DNA sequences confirm that plants are closely related to
certain groups of green algae, suggesting that the
ancestors of the first plants were indeed algae.
The oldest known plant fossils, about 450 million years
old, are similar to today’s mosses.
They had a simple structure and grew close to the
ground.
Fossils suggest that the first plants needed water to
complete their life cycles.
The demands of life on land favored the evolution of
plants that were:
• more resistant to the drying rays of the
sun.
• more capable of conserving water.
• more capable of reproducing without
water.
From these plants, several major groups of plants
evolved.
• One group developed into the mosses and
their relatives.
• Another group gave rise to all other plants.
All plants have evolved different adaptations for a
variety of terrestrial environments.
Overview of the Plant Kingdom
Plants are divided into four groups based on these
features:
• water-conducting tissues
• seeds
• flowers
Plants are also classified by other features, including
reproductive structures and body plan.
Evolutionary Relationships Among Plants
Today, scientists can classify plants more precisely by
comparing the DNA sequences of various species.
Bryophytes
Mosses and their relatives are called bryophytes, or
nonvascular plants.
They do not have vascular tissues, or specialized tissues
that conduct water and nutrients.
What adaptations of bryophytes enable them to live
on land?
Bryophytes have life cycles that depend on water for
reproduction.
Bryophytes draw up water by osmosis only a few
centimeters above the ground.
During one stage of their life cycle, bryophytes produce
sperm that swim through water to reach eggs of other
individuals.
Therefore, bryophytes must live where there is rainfall
or dew for part of the year.
Groups of Bryophytes
Bryophytes are low-growing plants found in moist, shaded
areas.
What are the three groups of bryophytes?
The three groups of bryophytes are:
• mosses
• liverworts
• hornworts
The Structure of a Moss
Each moss plant has a shoot that looks like a stem with
leaves. These are not true stems or leaves, because they
do not contain vascular tissue.
When mosses reproduce, they produce thin stalks, each
containing a capsule.
This is the sporophyte stage.
The “leaves” of mosses are one cell thick, so they lose
water quickly if the surrounding air is dry.
Mosses have rhizoids, which are long cells that anchor
them in the ground and absorb water and minerals from
the soil.
Water moves through rhizoids and into the rest of the
plant.
Liverworts
Liverworts’ gametophytes form broad, thin structures
that draw up moisture from the soil surface.
Mature gametophytes produce structures that look like
tiny green umbrellas.
These carry the structures that produce eggs and sperm.
Some liverworts can reproduce asexually by means of
gemmae.
Gemmae are small multicellular reproductive structures.
In some species, gemmae form in gemma cups.
When washed out of the cup, the gemmae can divide by
mitosis to produce a new individual.
Hornworts
Hornworts are found only in soil that is damp nearly yearround.
Their gametophytes look like those of liverworts.
The hornwort sporophyte looks like a tiny green horn.
How do bryophytes reproduce?
Bryophytes reproduce and develop by alternation of
generations.
The gametophyte is the dominant stage of the life cycle
and is the stage that carries out most of the plant's
photosynthesis.
Life Cycle of a Moss
The life cycle of a moss illustrates how bryophytes
reproduce and develop.
Life Cycle of a Bryophyte
When a spore lands in a moist place, it germinates and
grows into a mass of tangled green filaments called a
protonema.
As the protonema grows, rhizoids grow into the ground
and shoots grow into the air.
These shoots grow into green moss plants, which are the
gametophyte stage of its life cycle.
Gametes form in structures at the tips of the
gametophytes.
Sperm are produced in antheridia, the male reproductive
structure.
Eggs are produced in archegonia, the female
reproductive structure.
Some species produce both sperm and eggs on the same
plant.
Fertilization produces a diploid zygote.
The zygote grows directly from the gametophyte and
depends on it for water and nutrients.
The mature sporophyte is a long stalk ending in a capsule.
Inside the capsule, haploid spores are produced by
meiosis.
When the capsule ripens, it opens and spores are
scattered.
Human Use of Mosses
Sphagnum mosses thrive in the acidic water of bogs.
Dried sphagnum acts as a natural sponge. It can
accumulate to form peat deposits.
Peat can be cut from the ground and used as fuel.
Peat can be used to improve the soil’s ability to retain
water and to increase soil acidity.
Seedless Vascular Plants
420 million years ago, mosslike plants on land were joined
by taller plants.
Evidence shows that these plants had vascular tissue,
which is specialized to conduct water and nutrients
throughout the plant.
Evolution of Vascular Tissue
The first vascular plants contained tracheids which are
cells specialized to conduct water.
Tracheids make up xylem, a transport subsystem that
carries water from the roots to every part of a plant.
Tracheids are hollow with thick cell walls that resist
pressure.
They connect end to end to allow water to move
efficiently.
Vascular plants have a second transport subsystem
composed of vascular tissue called phloem.
Phloem transports solutions of nutrients and
carbohydrates produced by photosynthesis.
How is vascular tissue important to ferns and their
relatives?
Both xylem and phloem can move fluids through the plant
body, even against the force of gravity.
Together xylem and phloem move water, nutrients, and
other materials throughout the plant.
In many plants, xylem and lignin (a substance that makes
cell walls rigid) enable them to grow upright and tall.
What are the characteristics of the three phyla of
seedless vascular plants?
Seedless vascular plants include:
• club mosses
• horsetails
• ferns
Ferns and Their Relatives
The most numerous phylum is the ferns.
Ferns and their relatives have true roots, leaves, and
stems.
Roots are underground organs that absorb water and
minerals.
Leaves are photosynthetic organs that contain one or
more bundles of vascular tissue. Tissue is gathered into
veins made of xylem and phloem.
Stems are supporting structures that connect roots and
leaves, carrying water and nutrients between them.
Club Mosses
Ancient club mosses grew into trees and produced
forests.
Fossilized remains of these exist today as huge beds of
coal.
Today, club mosses are small plants that live in moist
woodlands.
Horsetails
The only living genus of Arthrophyta is Equisetum.
Equisetum has true leaves, stems, and roots.
Equisetum is called horsetail, or scouring rush.
Ferns
Ferns probably evolved 350 million years ago, when club
moss forests covered Earth.
Ferns thrive in wet areas with little light.
Life Cycle of Ferns
Ferns have vascular tissues, strong roots, underground
stems called rhizomes, and leaves called fronds.
What are the stages in the life cycle of ferns?
Life Cycle of Ferns
Ferns and other vascular plants have a life cycle in which
the diploid sporophyte is the dominant stage.
Fern sporophytes develop haploid spores on the
underside of their fronds in structures called sporangia.
Sporangia are grouped into clusters called sori.
The Underside of a Fern Frond
Fern Life Cycle
When the spores germinate, they develop into haploid
gametophytes.
The gametophyte first grows a set of rootlike rhizoids.
It then flattens into a mature gametophyte.
The gametophyte grows independently of the sporophyte.
The antheridia and archegonia are found on the
underside of the gametophyte.
Fertilization requires water, which allows the sperm to
swim to the eggs.
The diploid zygote formed by fertilization, develops into
a new sporophyte.
As the sporophyte grows, the gametophyte withers away.
Fern sporophytes often live for many years.
In some species, fronds die in the fall, but rhizomes live
through the winter and produce new leaves in the spring.
Seed Plants
Seed plants are the most dominant group of
photosynthetic organisms on land.
Seed plants are divided into two groups:
• Gymnosperms bear seeds directly on the
surfaces of cones.
• Angiosperms, or flowering plants, bear
seeds within a layer of tissue that
protects the seed.
Gymnosperms include conifers, cycads, ginkgoes, and
gnetophytes.
Angiosperms include grasses, flowering trees and shrubs,
and all species of flowers.
Reproduction Free From Water
Seed plants have a life cycle that alternates between a
gametophyte stage and a sporophyte stage.
They do not need water for fertilization of gametes.
Seed plants can live just about anywhere.
What adaptations allow seed plants to reproduce
without standing water?
Adaptations that allow seed plants to reproduce without
water include:
• flowers or cones
• the transfer of sperm by pollination
• the protection of embryos in seeds
Cones and Flowers
Gametophytes grow within sporophytes called cones,
which are the seed-bearing structures of gymnosperms,
and flowers, which are the seed-bearing structures of
angiosperms.
Gametophyte generations live inside these structures.
Pollen
The male gametophyte is contained in a tiny structure
called a pollen grain.
Sperm do not need water to fertilize eggs; instead the
pollen grain is carried to the female reproductive
structure by wind, insects, or small animals.
This transfer of pollen is called pollination.
Seeds
A seed is an embryo of a plant that is encased in a
protective covering and surrounded by a food supply.
An embryo is an organism in its early stage of
development.
The seed coat surrounds and protects the embryo and
keeps contents of the seed from drying out.
Internal Structures of a Seed
Seeds may have special tissues or structures that aid in
their dispersal to other habitats.
• Some seed coats stick to the fur or
feathers of animals.
• Other seeds are within tissues eaten and
dispersed by animals.
After fertilization, the zygote grows into a plant—the
embryo.
The embryo can stop growing while it is within the seed,
and it can remain this way for a long time.
When it grows, it uses nutrients from the stored food
supply.
Seeds can survive extreme cold or heat, or even drought.
Evolution of Seed Plants
The fossil record indicates that ancestors of seed plants
evolved adaptations enabling them to survive where most
mosses and ferns could not.
The most important of these adaptations was the seed
itself.
A seed can survive dry conditions and extreme
temperatures.
What are the four groups of gymnosperms?
Gymnosperms—Cone Bearers
The four groups of gymnosperms are:
• gnetophytes
• cycads
• ginkgoes
• conifers
Gnetophytes
About 70 present-day species of the phylum Gnetophyta
are known, placed in just three genera.
Reproductive scales of these plants are clustered into
cones.
Cycads
Cycads are palmlike plants that reproduce with large
cones.
They first appeared during the Triassic, 225 million
years ago.
Today, only nine genera of cycads exist.
They grow naturally in tropical and subtropical places.
Ginkgoes
Today the phylum Ginkgophyta contains only one species,
Ginkgo biloba. The living Ginkgo species looks like its
fossil ancestors.
Ginkgo trees are planted in U.S. cities because of their
resistance to air pollution.
Conifers
Conifers are the most common gymnosperms, with more
than 500 known species.
Conifers include pines, spruces, firs, cedars, sequoias,
redwoods, junipers, and yews.
Ecology of Conifers
Conifer leaves have specific adaptations to dry
conditions.
Most developed long, thin leaves, which reduce
evaporation.
Their leaves have a thick, waxy layer.
Most conifers are “evergreens” and retain leaves all year.
Angiosperms—Flowering Plants
Flowers and Fruits
The majority of living plant species are flowering plants,
or angiosperms.
What are the characteristics of angiosperms?
Flowers and Fruits
Angiosperms develop unique reproductive organs known as
flowers.
Flowers are an evolutionary advantage because they
attract animals, which then transport pollen from flower
to flower.
Flowers contain ovaries, which surround and protect
the seeds.
After pollination, the ovary develops into a fruit.
A fruit is a wall of tissue that surrounds a seed. A fruit
protects the seed and aids in its dispersal.
Diversity of Angiosperms
Angiosperms are categorized in many ways:
• monocots and dicots
• woody and herbaceous plants
• annuals, biennials, and perennials
An angiosperm can belong to more than one category.
Diversity of Angiosperms
Monocots and Dicots
There are two classes within the angiosperms—monocots
and dicots.
What are monocots and dicots?
Monocots and dicots are named for the number of seed
leaves, or cotyledons, in the plant embryo. Monocots have
one seed leaf, and dicots have two.
A cotyledon is the first leaf or the first pair of leaves
produced by the embryo of a seed plant.
Monocots include corn, wheat, lilies, orchids, and palms.
Dicots include roses, clover, tomatoes, oaks, and daisies.
Woody and Herbaceous Plants
Flowering plants are also categorized by the woodiness of
the stem.
Woody plants are made primarily of cells with thick cell
walls that support the plant body.
Woody plants include trees, shrubs, and vines.
Plant stems that are smooth and nonwoody are
characteristic of herbaceous plants.
Herbaceous plants do not produce wood as they grow.
Herbaceous plants include dandelions, zinnias, and
petunias.
Plant life span is determined by genetic and
environmental factors.
What are the three categories of plant life spans?
Annuals, Biennials, and Perennials
There are three categories of plant life spans: annual,
biennial, and perennial.
Annuals are plants that complete a life cycle in one
growing season.
Biennials complete their life cycle in two years. In the
first year, they germinate and grow roots, short stems,
and sometimes leaves. In the second year, they grow new
stems and leaves, produce flowers and seeds, and die.
Perennials live for more than two years.