Plants

1
Plants

• Diversity, Structure, and Function

2
Introduction

• Plants are the most dominant life forms on this
  earth. Without them no life would exist, they are
  the basis of all life.
• 280,000 species of plants inhabit Earth today.
• Most plants live in terrestrial environments,
  including deserts, grasslands, and forests.
• Some species, such as sea grasses, have returned
  to aquatic habitats.
• Land plants (including the sea grasses) evolved
  from a green algae
• four main groups of land plants bryophytes,
  pteridophytes, gymnosperms, and angiosperms.

3
Cont

• most common bryophytes are mosses.
• The pteridophytes include ferns.
• The gymnosperms include pines and other conifers.
• The angiosperms are the flowering plants

4
Vascular and Non-Vascular

• Mosses and other bryophytes have evolved several
  adaptations. For example, the offspring develop
  from multicellular embryos that remain attached
  to the mother plant which protects and
  nourished the embryos.
• The other major groups of land plants evolved
  vascular tissue and are known as the vascular
  plants.
• In vascular tissues, cells join into tubes that
  transport water and nutrients throughout the
  plant body.
• Most bryophytes lack water-conducting tubes and
  are sometimes referred to as nonvascular plants
• Ferns and other pteridiophytes are sometimes
  called seedless plants because there is no seed
  stage in their life cycles.

5
Seed Development

• evolution of the seed in an ancestor common to
  gymnosperms and angiosperms facilitated
  reproduction on land.
• A seed consists of a plant embryo packaged along
  with a food supply within a protective coat.
• The first seed plants evolved about 360 million
  years ago, near the end of the Devonian.
• The early seed plants gave rise to the diversity
  of present-day gymnosperms, including conifers
• majority of modern-day plant species are
  flowering plants, or angiosperms.
• Flowers evolved in the early Cretaceous period,
  about 130 million years ago.
• A flower is a complex reproductive structure that
  bears seeds within protective chambers called
  ovaries

6
Evolution

• four great episodes in the evolution of land
  plants
• the origin of bryophytes from algal ancestors
• the origin and diversification of vascular plants
• the origin of seeds
• the evolution of flowers

7
The Green Algae Connection

• Plants are multicellular, eukaryotic,
  photosynthetic autrotrophs.
• But red and brown seaweeds also fit this
  description.
• Land plants have cells walls made of cellulose
  and chlorophyll a and b in chloroplasts.
• However, several algal groups have cellulose cell
  walls and others have both chlorophylls.
• Land plants connected to green algae in 2 ways 1.
  Both possess rosette cellulose-synthesizing
  complexes that synthesize the cellulose
  microfibrils of the cell wall. 2. presence of
  peroxisomes, help minimize the loss of organic
  products due to photorespiration.

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Separation From Green Algae

• 1. apical meristems
• 2. multicellular embryos dependent on the parent
  plant
• 3. alternation of generations
• 4. sporangia that produce walled spores
• 5. gametangia that produce gametes
• resources that a photosynthetic organism requires
  are found in 2 diff. places. Light and carbon
  dioxide are mainly aboveground. Water and mineral
  resources are found mainly in the soil.
• The elongation and branching of the shoots and
  roots maximize their exposure to environmental
  resources.
• This growth is sustained by apical meristems,
  localized regions of cell division at the tips of
  shoots and roots.

9
Cont

• Multicellular plant embryos develop from zygotes
  that are retained within tissues of the female
  parent.
• parent provides nutrients, such as sugars and
  amino acids, to the embryo.

• embryo has specialized placental transfer cells
  that enhance the transfer of nutrients from
  parent to embryo.

10
Cont.

• All land plants show alternation of generations
  in which two multicellular body forms alternate
• One of the multicellular bodies is called the
  gametophyte with haploid cells.
• Gametophytes produce gametes, egg and sperm.
• Fusion of egg andsperm duringfertilizationform
  a diploidzygote.

11
Cont

• Mitotic division of the diploid zygote produces
  the other multicellular body, the sporophyte.
• Meiosis in a mature sporophyte produces haploid
  reproductive cells called spores.
• A spore is a reproductive cell that can develop
  into a new organism without fusing with another
  cell.
• Mitotic division of a plant spore produces a new
  multicellular gametophyte.
• humans do not have alternation of generations
  because the only haploid stage in the life cycle
  is the gamete, which is single-celled.

12
Cont

• Multicellular organs, called sporangia, are found
  on the sporophyte and produce these spores.
• Within a sporangia, diploid spore mother cells
  undergo meiosis and generate haploid spores.
• The outer tissues of the sporangium protect the
  developing spores until they are ready to be
  released into the air

13
Cont

• gametophytes of bryophytes, pteridophytes, and
  gymnosperms produce their gametes within
  multicellular organs, called gametangia.
• A female gametangium, called an archegonium,
  produces a single egg cell in a vase-shaped organ

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Other Adaptations

• adaptations for acquiring, transporting, and
  conserving water,
• adaptations for reducing the harmful effect of UV
  radiation,
• adaptations for repelling terrestrial herbivores
  and resisting pathogens.
• epidermis of leaves and other aerial parts is
  coated with a cuticle of polyesters and waxes.
• The cuticle protects the plant from microbial
  attack.
• The wax acts as waterproofing to prevent
  excessive water loss.

15
Cont

• Pores, called stomata, in the epidermis of leaves
  and other photosynthetic organs allow the
  exchange of carbon dioxide and oxygen between the
  outside air and the leaf interior.
• Stomata are also the major sites for water to
  exit from leaves via evaporation.
• Changes in the shape of the cells, guard cells,
  bordering the stomata can close the pores to
  minimize water loss in hot, dry conditions.

16
Cont

• Except for bryophytes, land plants have true
  roots, stems, and leaves, which are defined by
  the presence of vascular tissues.
• Vascular tissue transports materials among these
  organs.
• Tube-shaped cells, called xylem, carry water and
  minerals up from roots.
• When functioning, these cells are dead, with only
  their walls providing a system of microscopic
  water pipes.
• Phloem is a living tissue in which
  nutrient-conducting cells arranged into tubes
  distribute sugars, amino acids, and other organic
  products

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Xylem and Phloem
18
Cont

• Land plants produce many unique molecules called
  secondary compounds
• Examples of secondary compounds in plants include
  alkaloids, terpenes, tannins, and phenolics such
  as Flavonoids.
• Various secondary compounds have bitter tastes,
  strong odors, or toxic effects that help defend
  land plants against herbivorous animals or
  microbial attack.
• Flavonoids absorb harmful UV radiation.
• Lignin, a phenolic polymer, hardens the cell
  walls of woody tissues in vascular plants,
  providing support for even the tallest of trees

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Those Smart Humans

• Humans have found many applications, including
  medicinal applications, for secondary compounds
  extracted from plants.
• For example, the alkaloid quinine helps prevent
  malaria. Other Chemicals used in perfumes ect.
• Up to 40 of medications are plant derived
• Ex. Asprin from the bark of a Willow tree

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Origin and Evolution of Plants

• Several lines of evidence support the
  phylogenetic connection between land plants and
  green algae, especially the charophyceans,
  including homologous chloroplasts, homologous
  cell walls, homologous peroxisomes,
  phragmoplasts, homologous sperm, and molecular
  systematics.
• Homologous chloroplasts - The chloroplasts of
  land plants are most similar to the plastids of
  green algae and of eulgenoids which acquired
  green algae as secondary endosymbionts.
• Similarities include the presence of chlorophyll
  b and beta-carotene and thylakoids stacked as
  grana.
• Comparisons of chloroplast DNA with that of algal
  plastids place the charophyceans as most closely
  related to land plants.

21
Cont

• Homologous cellulose walls - In both land plants
  and charophycean algae, cellulose comprises
  20-26 of the cell wall.
• Also, both share cellulose-manufacturing
  rosettes.
• Homologous peroxisomes - Both land plants and
  charophycean algae package enzymes that minimize
  the costs of photorespiration in peroxisomes.
• Phagmoplasts - These plate-like structures occur
  during cell division only in land plants and
  charopyceans.
• Many plants have flagellated sperm, which match
  charophycean sperm closely in ultrastructure.

22
Cont

• Molecular systematics - similarities derived from
  comparing chloroplast genes, analyses of several
  nuclear genes also provide evidence of a
  charophycean ancestry of plants.
• In fact, the most complex charophyceans appear to
  be the algae most closely related to land plants.
• All available evidence upholds the hypothesis
  that modern charophyceans and land plants evolved
  from a common ancestor.
• oldest known traces of land plants are found in
  mid-Cambrian rocks from about 550 million years
  ago.

23
Look
plant kingdom is monophyletic, the differences in
life cycles among land plants can be interpreted
as special reproductive adaptations as the
various plant phyla diversified from the first
plants.
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Bryophytes

• represented by 3 phyla Hepatophyta liverworts,
  Anthocerophyta hornworts, Bryophyta mosses
• diverse bryophytes are not a monophyletic group.
• Several lines of evidence indicate that these
  three phyla diverged independently early in plant
  evolution

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Cont..

• gametophyte is the dominant generation in the
  life cycles of bryophytes
• Bryophytes are anchored by tubular cells or
  filaments of cells, called rhizoids. Rhizoids are
  not composed of tissues.
• They lack specialized conducting cells.
• do not play a primary role in water and mineral
  absorption.
• Most bryophytes lack conducting tissues to
  distribute water and organic compounds within the
  gametophyte.
• Those with specialized conducting tissues lack
  the lignin coating found in the xylem of vascular
  plants.
• Lacking support tissues, most bryophytes are only
  a few centimeters tall.

26
Cont

• gametophytes of mosses and some liverworts are
  more leafy because they have stemlike
  structures that bear leaflike appendages.
• They are not true stems or leaves because they
  lack lignin-coated vascular cells.
• The leaves of most mosses lack a cuticle and
  are only once cell thick, features that enhance
  water and mineral absorption from the moist
  environment.
• Some mosses have more complex leaves with
  ridges to enhance absorption of sunlight.
• ridges are coated with cuticle.
• Some mosses have conducting tissues in their
  stems and can grow as tall as 2m. See Pic Next
  Slide

27
Cont
bryophyte sporophyte does have photosynthetic
plastids, and cannot live apart from the maternal
gametophyte. A bryophyte sporophyte remains
attached to its parental gametophyte throughout
the sporophytes lifetime. It depends on the
gametophyte for sugars, amino acids, minerals and
water. Bryophytes have the smallest and simplest
sporophytes of all modern plant groups
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Life Cycle
29
Cont.

• Moss sporophytes consist of a foot, an elongated
  stalk (the seta), and a sporangium (the capsule).
• The foot gathers nutrients and water from the
  parent gametophyte via transfer cells.
• The moss capsule (sporangium) is the site of
  meiosis and spore production.
• One capsule can generate over 50 million spores
• Wind dispersal of lightweight spores has
  distributed bryophytes around the world. They are
  common and diverse in moist forests and wetlands.
• Some even inhabit extreme environments like
  mountaintops, tundra, and deserts.
• Mosses can loose most of their body water and
  then rehydrate and reactivate their cells when
  moisture again becomes available.

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Cont

• Sphagnum, a wetland moss, is especially abundant
  and widespread
• Sphagnum has been used in the past as diapers and
  a natural antiseptic material for wounds
• Bryophytes were probably Earths only plants for
  the first 100 million years that terrestrial
  communities existed.
• Then vegetation began to take on a taller profile
  with the evolution of vascular plants.

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Vascular Plants

• Modern vascular plants (pteridophytes,
  gymnosperms, and angiosperms) have food transport
  tissues (phloem) and water conducting tissues
  (xylem) with lignified cells
• The first vascular plants, pteridophytes, were
  seedless.
• Cooksonia, an extinct plant over 400 million
  years old, is the earliest known vascular plant.
• Its fossils are found in Europe and North
  America.
• The branched sporophytes were up to 50cm tall
  with small lignified cells, much like the xylem
  cells of modern pteridophytes.

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Pteridophytes

• Are seedless vascular plants
• consists of two modern phylaLycophyta
  lycophytes, Pterophyta - ferns, whisk ferns, and
  horsetails
• probably evolved from different ancestors among
  the early vascular plants
• have true roots with lignified vascular tissue.
• uncertain if the roots of seed plants arose
  independently or are homologous to pteridophyte
  roots.

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Cont..

• the sporophyte generation is the larger and more
  complex plant.
• the leafy fern plants that you are familiar with
  are sporophytes.
• The gametophytes are tiny plants that grow on or
  just below the soil surface.
• reduction in the size of the gametophytes is even
  more extreme in seed plants.
• Ferns also demonstrate a key variation among
  vascular plants the distinction between
  homosporous and heterosporous plants.
• A homosporous sporophyte produces a single type
  of spore.

34
Cont

• This spore develops into a bisexual gametophyte
  with both archegonia (female sex organs) and
  antheridia (male sex organs).
• A heterosporous sporophyte produces two kinds of
  spores.
• Megaspores develop into females gametophytes.
• Microspores develop into male gametophytes.
• Regardless of origin, the flagellated sperm cells
  of ferns, other seedless vascular plants, and
  even some seed plants must swim in a film of
  water to reach eggs.
• Because of this, seedless vascular plants are
  most common in relatively damp habitats.

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Life Cycle
36
Lycophyta

• Phylum Lycophyta - Modern lycophytes are relicts
  of past.
• By the Carboniferous period, lycophytes existed
  as either small, herbaceous plants or as giant
  woody trees with diameters of over 2m and heights
  over 40m.
• The giant lycophytes thrived in warm, moist
  swamps, but became extinct when the climate
  became cooler and drier.
• The smaller lycophytes survived and are
  represented by about 1,000 species today.
• Modern lycophytes include tropical species that
  grow on trees as epiphytes, using the trees as
  substrates, not as hosts.
• Others grow on the forest floor in temperate
  regions.

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Pterophyta

• phylum Pterophyta consists of ferns and their
  relatives.
• Psilophytes, the whisk ferns, used to be
  considered a living fossil, but comparisons of
  DNA sequences and ultrastructural details,
  indicate that the lack of true roots and leaves
  evolved secondarily.
• Sphenophytes are commonly called horsetails
  because of their often brushy appearance.
• During the Carboniferous, sphenophytes grew to
  15m, but today they survive as about 15 species
  in a single wide-spread genus, Equisetum.
• Horsetails are often found in marshy habitats
  and along streams and sandy roadways

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Wisk FernHorsetail
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Ferns

• first appeared in the Devonian and have radiated
  extensively until there are over 12,000 species
  today.
• Ferns are most diverse in the tropics but are
  also found in temperate forests and even arid
  habitats.
• Fern leaves or fronds may be divided into many
  leaflets.

Ferns produce spores and many have built in
catapult devices to spring them away from the
parent and get carried by the wind.
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Dead Plant Energy

• Lycophyta and Pterophyta formed forests during
  the Carboniferous period about 290-360 million
  years ago.
• These plants left not only living
  represent-atives and fossils, but also fossil
  fuel in the form of coal.

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Plants With Seeds

• The evolution of plants is highlighted by two
  important landmarks
• (1) the evolution of seeds, which lead to the
  gymnosperms and angiosperms, the plants that
  dominate most modern landscapes
• (2) the emergence of the importance of seed
  plants to animals, specifically to humans.
• Agriculture, the cultivation and harvest of
  plants (primarily seed plants), began
  approximately 10,000 years ago in Asia, Europe,
  and the Americas.
• This was the single most important cultural
  change in the history of humanity, for it made
  possible the transition from hunter-gather
  societies to permanent settlements.

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Cont.

• Seed plants are vascular plants that produce
  seeds.
• 3 important reproductive adaptations
• continued reduction of the gametophyte
• the advent of the seed
• the evolution of pollen.
• gametophytes of seed plants are even more reduced
  than those of seedless vascular plants such as
  ferns.
• delicate female gametophyte and young embryos are
  protected from many environmental stresses
  because they are retained within the moist
  sporangia of the parental sporophyte.

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Small Gametophytes
44
Seed Development

• Spores were the main way that plants spread over
  Earth for the first 200 millions years of life on
  land
• seed represents a different solution to resisting
  harsh environments and dispersing offspring.
• seed consists of a sporophyte embryo packaged
  along with a food supply within a protective coat
• All seed plants are heterosporous, producing 2
  different types of sporangia that produce two
  types of spores.
• Megasporangia produce megaspores, which give rise
  to female (egg-containing) gametophytes.
• Microsporangia produce microspores, which give
  rise to male (sperm-containing) gametophytes.

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Seed Cont..

• seeds protective coat is derived from the
  integuments of the ovule.
• Within this seed coat, a seed may remain dormant
  for days, months, or even years until favorable
  conditions trigger germination.
• When the seed is eventually released from the
  parent plant, it may be close to the parent, or
  be carried off by wind or animals.

46
Pollen No More Water

• microspores, released from the microsporangium,
  develop into pollen grains.
• These are covered with a tough coat
• They are carried away by wind or animals until
  pollination occurs when they land in the vicinity
  of an ovule.
• The pollen grain will elongate a tube into the
  ovule and deliver one or two sperm into the
  female gametophyte
• In bryophytes and pteridophytes, flagellated
  sperm must swim through a film of water to reach
  eggs cells
• The evolution of pollen in seed plants led to
  even greater success and diversity of plants on
  land.

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2 Types of Seed Plants

• gymnosperms and angiosperms
• most familiar gymnosperms are the conifers, the
  cone-bearing plants such as pines.
• The ovules and seeds of gymnosperms (naked
  seeds) develop on the surfaces of specialized
  leaves called sporophylls.
• In contrast, ovules and seeds of angiosperms
  develop in enclosed chambers (ovaries).
• Gymnosperms appears in the fossil record much
  earlier than angiosperms
• descended from progymnosperms, a group of
  Devonian plants.
• earliest progymnosperms lacked seeds, by the end
  of the Devonian, some species had evolved seeds.

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Gymnosperms Cont

• Adaptive radiation during the Carboniferous and
  early Permian produced the various phyla of
  gymnosperms.
• flora and fauna of Earth changed during the
  formation of the supercontinent Pangaea in the
  Permian.
• This likely led to major environmental changes,
  including drier and warmer continental interiors.
• Many groups of organisms disappeared and others
  emerged as their successors.Ex. amphibians
  decreased in diversity while reptiles increased.
• lycophytes, horsetails, and ferns that dominated
  in Carboniferous swamps were largely replaced by
  gymnosperms, which were more suited to the drier
  climate.

49
Cont

• 4 phyla of extant gymnosperms are ginko, cycads,
  gnetophytes, and conifers

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Phylum Ginkgophyta

• consists of only a single extant species, Ginkgo
  biloba.
• popular ornamental species has fanlike leaves
  that turn gold before they fall off in the
  autumn.
• Landscapers usually only plant male trees because
  the seed coats on female plants decay, they
  produce a repulsive odor (to humans, at least).

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Phylum Cycadophyta

• Have palm like leaves, but are not palms

52
Phylum Gnetophyta

• Phylum Gnetophyta consists of three very
  different genera.
• Weltwitschia plants, from deserts in southwestern
  Africa, have straplike leaves.
• Gentum species are tropical trees or vines.
• Ephedra (Mormon tea) is a shrub of the American
  deserts.

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Phylum Coniferophyta

• The term conifer comes from the reproductive
  structure, the cone.
• about 550 species of conifers
• include pines, firs, spruces, larches, yews,
  junipers, cedars, cypresses, and redwoods, up to
  and over 100 meters tall.
• 1994 the Wollemi pine was found in the rain
  forest of Sydney, Australia. It was thought
  extinct for over 100 million years
• The bristlecone pines in the Rockies are about
  5000 years old.
• Most conifers are evergreen, retaining their
  leaves and photosynthesizing throughout the year

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Conifer Pics
55
Pine Tree
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Angiosperms

• better known as flowering plants, are vascular
  seed plants that produce flowers and fruits.
• They are by far the most diverse and
  geographically widespread of all plants.
• There are abut 250,000 known species of
  angiosperms.
• placed in a single phylum, the phylum Anthophyta.
• As late as the 1990s, most plant taxonomists
  divided the angiosperms into two main classes,
  the monocots and the dicots. Cotyledon-seed leaf
• Most monocots have leaves with parallel veins,
  while most dicots have netlike venation

57
Monocot and Dicot
58
Cont
59
Why So Much Better Then Gymnosperms

• Refinements in vascular tissue, especially xylem,
  probably played a role in the enormous success of
  angiosperms in diverse terrestrial habitats.
• reproductive adaptations associated with flowers
  and fruits contributed the most.
• Flowers- reproductive structures that produce
  pollen and seeds. 1st appeared 130 mya they made
  reproduction much more efficient now it is not
  just the random chance of wind now plants could
  attract many different animals and get them to
  pollinate for them. Pretty smart!!!!
• And Done Many Different Ways!!!!!!!! Some
  relationships are specific Ex. Darwin

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Parts Of A Flower
61
Fruit

• fruit is a mature ovary.
• As seeds develop from ovules after fertilization,
  the wall of the ovary thickens to form the fruit.
• Fruits protect dormant seeds and aids in their
  dispersal
• Pass through an animals digestive tract unharmed
  and conveniently land in a pile of fertilizer
  some time later
• Various modifications in fruits help disperse
  seeds.
• In some plants, such as dandelions and maples,
  the fruit functions like a kite or propeller,
  enhancing wind dispersal.
• Many angiosperms use animals to carry seeds.
• Fruits may be modified as burrs that cling to
  animal fur.
• Edible fruits are eaten by animals

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Fruits
63
Fruit Classification
64
We Still Rely on Fruit

• selectively breeding plants, humans have
  capitalized on the production of edible fruits.
• Apples, oranges, and other fruits in grocery
  stores are exaggerated versions of much smaller
  natural varieties of fleshy fruits.
• The staple foods for humans are the dry,
  wind-dispersed fruits of grasses.
• These are harvested while still on the parent
  plant.
• The cereal grains of wheat, rice, corn, and other
  grasses are actually fruits with a dry pericarp
  that adheres tightly to the seed coat of the
  single seed inside

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Angiosperms Coevolved With Animals

• Ever since they colonized the land, animals have
  influenced the evolution of terrestrial plants
  and vice versa.
• The fact that animals must eat affects the
  natural selection of both animals and plants.
• Natural selection must have favored plants that
  kept their spores and gametophytes far above the
  ground, rather than dropping them within the
  reach of hungry ground animals.
• In turn, this may have been a selective factor in
  the evolution of flying insects

67
Cont

• some herbivores may have become beneficial to
  plants by carrying the pollen and seeds of plants
  that they used as food.
• Natural selection reinforced these interactions,
  for they improved the reproductive success of
  both partners.
• Pollinator-plant relationships are partly
  responsible for the diversity of flowers

68
Dependence

• Flowering plants provide nearly all our food.
• All of our fruit and vegetable crops are
  angiosperms.
• Corn, rice, wheat, and other grain are grass
  fruits
• grow angiosperms for fiber, medications,
  perfumes, and decoration.
• Agriculture allowed us to stay in one place and
  move from a hunter-gather society
• Building Materials---at a cost---As the forests
  disappear, thousand of plants species and the
  animals that depend on these plants also go
  extinct.

69
Destruction
tropical rain forests and other plant communities
may be a medicine chest of healing plants that
could be extinct before we even know they exist.
70
Medicines

• We have explored the potential uses for only a
  tiny fraction of the 250,000 known plant species

71
Structures and Functions

• plant body consists of organs that are composed
  of different tissues, and these tissues are teams
  of different cell types
• 3 basic organs roots, stems, and leaves
• Plants must simultaneously inhabit and draw
  resources from two very different environments
• So a subterranean root system and an aerial shoot
  system of stems and leaves are the answer
• Both systems depend on the other-- Lacking
  chloroplasts and living in the dark, roots would
  starve without the sugar and other organic
  nutrients imported from the photosynthetic
  tissues of the shoot system.
• Conversely, the shoot system depends on water and
  minerals absorbed from the soil by the roots

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Monocot---Dicot
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3 Tissue System

• dermal, vascular, and ground tissue systems
• dermal tissue, or epidermis, is generally a
  single layer of tightly packed cells that covers
  and protects
• has other specialized characteristics consistent
  with the function
• the roots hairs are extensions of epidermal
  cells
• The epidermis of leaves and most stems secretes a
  waxy coating, the cuticle, that helps the plant
  retain water.

75
Cont.

• Vascular tissue, continuous throughout the plant,
  is involved in the transport of materials
• water conducting elements of xylem, the tracheids
  and vessel elements, are elongated cells that are
  dead at functional maturity
• Tracheids are long, thin cells with tapered ends.
• Water moves from cell to cell mainly through
  pits.
• walls are hardened with lignin, tracheids
  function in support as well as transport.
• Vessel elements are aligned end to end, forming
  long micropipes, xylem vessels
• phloem, sucrose, other organic compounds, and
  some mineral ions move through tubes formed by
  chains of cells, sieve-tube members

76
Cont..

• Ground tissue is tissue that is neither dermal
  tissue nor vascular tissue
• divided into pith, internal to vascular tissue,
  and cortex, external to the vascular tissue
• functions of ground tissue include
  photosynthesis, storage, and support

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Phloem
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Plant Tissue---3Cell Types

• parenchyma, collenchyma, and sclerenchyma
• distinguishing characteristics may be present in
  the protoplast, the cell contents exclusive of
  the cell wall.
• parenchyma cells have primary walls that are
  relatively thin and flexible, and most lack
  secondary walls
• they generally are the least specialized, but
  there are exceptions
• sieve-tube members of the phloem are parenchyma
  cells.
• perform most of the metabolic functions of the
  plant, synthesizing and storing various organic
  products
• fleshy tissue of most fruit is composed of
  parenchyma cells

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Cont

• parenchyma cells do not generally undergo cell
  division.
• Most retain the ability to divide and
  differentiate into other cell types under special
  conditions - during the repair and replacement of
  organs after injury to the plant.
• In the laboratory, it is possible to regenerate
  an entire plant from a single parenchyma cell
• Collenchyma cells have thicker primary walls than
  parenchyma cells Grouped into strands or
  cylinders, collenchyma cells help support young
  parts of the plant shoot
• cells are living and flexible and elongate with
  the stems and leaves they support

81
Cont

• Sclerenchyma cells also function as supporting
  elements of the plant, with thick secondary walls
  usually strengthened by lignin
• cannot elongate and occur in plant regions that
  have stopped lengthening
• Many sclerenchyma cells are dead at functional
  maturity
• Vessel elements and tracheids in the xylem are
  sclerenchyma cells
• fibers and sclereids, are specialized entirely in
  support.
• Fibers are long, slender and tapered, and usually
  occur in groups.
• Those from hemp fibers are used for making rope
  and those from flax for weaving into linen.
• Sclereids, shorter than fibers and irregular in
  shape, impart the hardness to nutshells and seed
  coats

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Growth and Development

• has perpetually embryonic tissues called
  meristems in its regions of growth
• Apical meristems, located at the tips of roots
  and in the buds of shoots, supply cells for the
  plant to grow in length.
• elongation, primary growth--- secondary growth,
  progressive thickening of roots and shoots
• Secondary growth is the product of lateral
  meristems extending along the length of roots and
  shoots.
• root tip is covered by a thimble-like root cap,
  which protects the meristem as the root pushes
  through the abrasive soil

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Cont..

• primary meristems the protoderm, procambium, and
  ground meristem will produce the three primary
  tissue systems of the root dermal, vascular, and
  ground tissues
• epidermis develops from the dermal tissues.
• ground tissue produces the endodermis and
  cortex.
• The vascular tissue produces the stele, the
  pericycle, pith, xylem, and phloem.
• stele, which in roots is a central cylinder of
  vascular tissue where both xylem and phloem
  develop
• ground tissue between the protoderm and
  procambium gives rise to the ground tissue system
  They store food and are active in the uptake of
  minerals
• innermost layer of the cortex, the endodermis

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• root may sprout lateral roots from the outermost
  layer of stele, the pericycle

86
Cont

• secondary plant body consists of the tissues
  produced during this secondary growth in
  diameter.
• The vascular cambium acts as a meristem for the
  production of secondary xylem and secondary
  phloem.
• The cork cambium acts as a meristem for a tough
  thick covering for stems and roots that replaces
  the epidermis like bark on a tree

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References

• Jack Brown M.S. Biology
• Microsoft Encarta Encyclopedia 2004
• Starr and Taggart The Unity and Diversity of
  Life 10th edition 2004 Thomson Brookes/Cole
• Campbell and Reece Biology 6th edition 2002
  Benjamin Cummings.
• Raven and Johnson Holt Biology 2004 Holt,
  Rinehart and Winston.