9.1 Plant structure and growth

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Plant Science

• 9.1 Plant structure and growth
• 9.2 transport in angiospermophytes
• 9.3 reproduction in angiospermophytes

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Remember

• Plant cell!

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Plant Evolution
Plants originated from green algae that lived in
ponds that occasionally dried out.
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Angiosperms

• Angiosperms have dominated the land for over 100
  million years.
• Known as flowering plants
• There are about 250,000 known species of
  flowering plants living today.
• Most of our food comes from flowering plants
• Roots, such as beets and carrots
• Fruits of trees and vines, such as apples, nuts,
  berries, and squashes
• Fruits and seeds of legumes, such as peas and
  beans
• Grains, such as rice, wheat, and corn

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Angiosperms

• Divided into two groups
• Names refer to the first leaves that appear on
  the plant embryo.
• Embryonic leaves are called seed leaves, or
  cotyledons
• Monocots (embryo has one seed leaf)
• Dicots (embryo has two seed leaves)

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Angiosperms

• Monocots
• Orchids, bamboos, palms, and lilies, as well as
  grains and other grasses
• Leaves have parallel veins
• Stems have vascular tissues arranged in a complex
  array of bundles.
• Flowers have petals and other parts in multiples
  of three.
• Roots form a fibrous system (a mat of threads)
  that spread out below the soil surface.
• Make excellent ground cover that reduces erosion.

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Angiosperms
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Angiosperms

• Dicots
• True dicots include most shrubs and trees (except
  for conifers), as well as many food crops.
• Leaves have a multibranched network of veins
• Stems have vascular bundles arranged in a ring.
• Flower usually has petals and other parts in
  multiples of four or five.
• Large, vertical root (called a taproot) goes deep
  into the soil
• You can see this if you try to pull up a
  dandelion

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Angiosperms
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Plant Body

• Composed of organs with various tissues
  reflective of their evolutionary history as
  land-dwelling organisms.
• Must draw resources from two environments
• Water and minerals from soil
• CO2 and light from air

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Plant Body

• Plant body is divided up to two main parts
• Subterranean part? root
• Aerial part? shoot

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Plant Body

• Root system
• Anchors in the soil, absorbs and transports
  minerals and water, and stores food.
• Monocots? Fibrous root system consists of a mat
  of generally thin roots spread out shallowly in
  the soil
• Dicots? have one main vertical taproot with many
  small secondary lateral roots growing outward.
• Both Monocots and Dicots have tiny projectsions
  called root hairs
• Enormously increase the root surface area for
  absorption of water and minerals.

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Plant Body

• Shoot system
• Made up of stems, leaves, and adaptations for
  reproduction (flowers)
• Stems are parts of the plant that are generally
  above ground and support the leaves and flowers.
  Composed of
• Nodes
• Points at which leaves are attached
• Internodes
• Portions of the stem between nodes
• Leaves are the main photosynthetic organs in most
  plants (green stems also perform photosynthesis)
• Consist of a flattened blade and a stalk, or
  petiole, which joins the leaf to a node of the
  stem.

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Plant Body

• Shoot system (continued)
• Two types of buds that are undeveloped shoots
• Terminal bud
• Found at the apex (tip) of the stem, has
  developing leaves and a compact series of nodes
  and internodes
• Axillary bud
• one of each of the angles formed by a leaf and
  the stem, are usually dormant.

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Plant Body

• Apical dominance
• Results from the terminal bud producing hormones
  that inhibit growth of the axillary buds.
• By concentrating resources on growing taller,
  apical dominance is an evolutionary adaptation
  that increases the plants exposure to light
• Important where vegetation is dense.
• Removing the terminal buds usually stimulates
  growth of the axillary buds.
• Branching is important for increasing exposure
  the environment

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Modified Roots, Stems, and Leaves

• Modified roots
• Some plants have unusually large taproots that
  store food in carbohydrates such as startch
• Carrots, turnips, sugar beets, and sweet potatoes

Sugar Beet
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Modified Roots, Stems, and Leaves

• Modified Stems
• Stolon
• runner has a horizontal stem that grows along
  the ground surface
• Plantlets form at nodes along their length,
  enabling a plant to grow asexually
• Example strawberry
• Rhizomes
• Look like large, brownish, rootlike structures
• Horizontal stems that grown just below or along
  the soil surface
• Store food, and having buds, can also spread and
  form new plants
• Potato plant has enlarged structures specialized
  for storage called tubers (the potatoes we eat)

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Modified Roots, Stems, and Leaves

• Modified stems (continued)
• Bulbs
• Modified stems that are also used for underground
  food storage (onions)

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Modified Roots, Stems, and Leaves

• Modified Leaves
• Tendrils
• Tips coil around a stem, help plants climb
• Examples grapevines, peas

Tendril- Pea Plant
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Plant Tissues in Stems and Leaves

• Each plant organ- root, stem, or leaf- is made up
  of three tissue systems
• Dermal
• Vascular
• Ground tissues

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Plant Tissues in Stems and Leaves

• Dermal Tissue
• Forms an outer protective covering.
• Acts as first line of defense against physical
  damage and infectious organisms.
• Consists of a single layer of tightly packed
  cells called the epidermis
• Epidermis of leaves and most stems is covered
  with a waxy layer called cuticle, which helps
  prevent water loss.
• Typical dicot leaf also has pores on its
  epidermis called stomata
• which allow CO2 exchange between the surrounding
  air and the photosynthetic cells inside the leaf.
• Surrounded by guard cells
• Regulate the size of the stoma

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Plant Tissues in Stems and Leaves
Plant Leaf
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Plant Tissues in Stems and Leaves

• Vascular Tissue
• Made up of
• Xylem
• type of vascular tissue that is made up of cells
  that transport water and dissolved ions from the
  roots to the leaves
• Phloem
• type of vascular tissue that is made up of cells
  that transport sugars from leaves or storage
  tissues to other parts of the plant

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Plant Tissues in Stems and Leaves

• Vascular Tissue (continued)
• In the stem..
• Vascular tissue forms vascular bundles
• Dicots? arranged in a circle

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Plant Tissues in Stems and Leaves

• Vascular Tissue (continued)
• In the leaf
• Vascular tissue form network of veins
• In the veins, the xylem and phloem are continuous
  with the vascular bundles of the stem.
• Allows them to be in close contact with
  photosynthetic tissues, ensuring water and
  mineral nutrients from the soil are supplied, and
  that sugars made in the leaves are transported
  throughout the plant

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Plant Tissues in Stems and Leaves

• Ground Tissue (continued)
• Accounts for the bulk of a young plant, by
  filling in spaces between the epidermis and
  vascular tissue.
• Functions include photosynthesis, storage, and
  support.
• Ground tissue inside vascular tissue is called
  pith
• Ground tissue external to vascular tissue is
  called cortex

Dicot Stem
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Plant Tissue in Stems and Leaves

• Ground Tissue (continued)
• Ground tissue of dicot stems
• consists of both a cortex region and pith region
• Ground tissue of the leaf
• Is called Mesophyll
• Sandwiched between the upper and lower epidermis
• Consists mainly of photosynthesis cells
• Loosely arranged to provide air spaces which CO2
  and O2 can circulate
• Main location of photosynthesis

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Plant Growth

• Growth in plants is made possible by tissues
  called meristems.
• A meristem consists of cells that divide
  frequently, generating additional cells.
• Some products of this division remain in the
  meristem and produce still more cells, while
  others differentiate and are incorporated into
  tissues and organs of the growing plant.

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Plant Growth

• Apical Meristems
• Meristems at the tips of roots and in the buds of
  shoots
• Cell division in the apical meristems produces
  the new cells that enable a plant cell to grow in
  length? primary growth
• Enables roots to push through the soil and allows
  shoots to increase exposure to light and CO2.
• Growth occurs behind the root tip in three zones
  of primary growth
• Zone of cell division, zone of elongation, and
  zone of maturation
• Zone of maturation brings about the three tissue
  systems (dermal, ground, and vascular)

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Plant Growth
Primary Growth of a Root
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Plant Growth

• Lateral meristems
• Associated with the increase in thickness of
  stems and roots? secondary growth
• Caused by the activity of two cylinders of
  dividing cells that extend along the length of
  roots and stems
• Vascular cambium
• Secondary growth adds layers of vascular tissue
  on both sides of the vascular cambium? wood
• Cork cambium
• Outer cambium that forms the secondary growth of
  the epidermis ?cork

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Control of Plant Growth

• Auxin is a term used for any chemical substance
  that promotes seedling elongation.
• Apical meristem at the tip of a shoot is a major
  site of auxin synthesis.
• As auxin moves downward, it stimulates growth of
  the stem by making cells elongate.
• Concentration of auxin determines its effect
• Too low to stimulate shoot cells will cause root
  cells to elongate
• High conc. stimulates shoots cell and inhibits
  root cell elongation.
• Stimulates stem elongation and root growth,
  differentiation, and branching.

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Control of Plant Growth

• Auxins also play a part in phototropism, an
  occurrence that involves plants bending or moving
  away from light.
• The shoot tip is responsible for directional
  movement by the plant in response to sunlight, as
  this is the area where auxins can be found.
• Sunlight eradicates auxin, meaning that the part
  of the shoot tip of the plant which is receiving
  direct sunlight will have the least amount of
  auxin.
• The extra auxin present on the shaded side
  promotes more cell division and elongation,
  causing the plant to bend towards the sunlight
  after this lop-sided growth.

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Control of Plant Growth
Cells on the darker side are larger and have
elongated faster causes the shoot to bend
towards the light.
If a plant receives sunlight uniformly from all
sides or is kept in the dark, the cells all
elongate at a similar rate.
Effect of Auxin on Phototropism
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Transport in Plants

• Several factors necessary for plant growth
• CO2 from air?absorbed by leaves
• O2 from air or soil?absorbed by leaves or roots
• H2O from soil? absorbed by the roots
• Minerals from the soild? absorbed by the roots
• Sugars are made in the leaves from the absorbed
  molecules and ions and used to build the plants
  body and provide energy

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Solute Uptake From The Roots

• Mineral ions from the soil can get into the root
  of plants by three different ways
• 1. Diffusion
• If the concentration of certain ions is lower
  inside of the root hair cells, they can simply
  diffuse into the root hair cells from the soil
• 2. Fungal hyphae
• Some plants live in a symbiotic relationship with
  fungi and use fungal hyphae to increase the
  surface of the root even more. The combination of
  plant root and fungal fibers are called
  mycorrhiza. The fungus benefit from a constant
  supply of sugar while the plant benefit from the
  increased surface area that the fungal hyphae
  provide, they also excrete growth factors and
  antibiotics
• 3. Mass flow of water into the root can also
  carry ions passively in dissolved form

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Solute Uptake From the Roots

• Roots hairs are extensions of epidermal cells
  that cover the root and form a huge surface area
• Allows the plant to absorb the water and minerals
  it needs for growth
• Watery solution has to be transported from the
  soil to epidermal cells to cortex of the root to
  the xylem (water-conducting vascular tissue)
• Plasma membrane of the xylem cells are
  selectively permeable, which helps regulate the
  mineral composition of a plants vascular system.

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Solute Uptake From the Roots

• Two possible routes to the xylem
• Intracellular route
• Extracellular route

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Solute Uptake From the Roots

• Intracellular route
• A.k.a. Symplatic route
• Cells within roots are connected via
  plasmodesmata (channels through the walls of
  adjacent cells) which allows for a continuum of
  living cytoplasm among the root cells
• Once inside epidermal cells, solution can move
  inward from cell to cell without crossing
  membranes

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Solute Uptake From the Roots

• Extracellular route
• Solution moves inward within the hydrophillic
  walls and extracellular spaces of the root cells
  but does not enter the cytoplasm of the epidermis
  or cortex cells.
• Solution passes through no plasma membranes, and
  there is no selection of solutes until they reach
  the endodermis.
• Endodermis has a waxy barrier called the
  Casparian strip which stops water and solutes
  from entering the xylem.
• Water and ions are forced to cross a plasma
  membrane into an endodermal cells, then are
  discharged into the xylem.

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Solute Uptake in the Roots

• In a real plant
• Water and solutes rarely follow just the two
  kinds of routes
• May take a combination of these routes, and may
  pass through numerous plasma membranes and cell
  walls en route to the xylem.
• All water and solutes must cross a plasma
  membrane at some point.

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Transpiration

• Why transpiration?
• As a plant grows upward toward sunlight, it needs
  to get water and minerals from the soil.
• Must be able to transport resources from the
  roots to thrive.

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Transpiration

• Xylem tissue is made of two types of conducting
  cells tracheids and vessel elements.
• When mature, but types of cells are dead,
  consisting only of cell walls, and both are in
  the form of very thin tubes that are arranged end
  to end.
• Because the cells have openings in their ends, a
  solution of water and inorganic nutrients, called
  xylem sap, can flow through these tubes.
• Xylem sap flows all the way up from the plants
  roots through the shoot system to the tips of the
  leaves.

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Transpiration
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Transpiration

• Forces that push xylem sap against gravity are
• Root pressure
• Root cells actively pump inorganic ions into the
  xylem, and the roots endodermis holds the ions
  there
• As ions accumulate in the xylem, water tends to
  enter by osmosis, pushing xylem sap upward ahead
  of it.
• Can push sap up a few meters
• For the most part, however, xylem sap is not
  pushed from below by root pressure by pulled
  upward by the leaves.
• Transpiration
• The pulling force caused by the loss of water
  from the leaves and other aerial parts of a
  plant.
• Water molecules leave the plant through the stoma
  of the leaf by diffusion.
• When the stoma is open, water concentration is
  higher in the plant cells than in the surrounding
  atmosphere.

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Transpiration

• Properties of water stimulate transpiration
• Cohesion
• Sticking together of molecules of the same kind.
• Because water is polar, they are attracted to
  each other by hydrogen bonds
• Water molecules form continuous strings in xylem
  tubes, extending all the way from the leaves down
  to the roots.
• Adhesion
• Sticking together of molecules of a different
  kind.
• Water molecules tend to adhere via hydrogen bonds
  to hydrophillic cellulose molecules in the walls
  of xylem cells.

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Transpiration

• Transpiration-Cohesion-Tension Mechanism
• Before a water molecule can leave the leaf, it
  must break off from the end of the string
• It is pulled off a steep diffusion gradient
  between the moist interior of the leaf and the
  drier surrounding air.
• Cohesion resists the pulling force of the
  diffusion gradient, but it is not strong enough
  to overcome it.
• The molecule breaks off, and the opposing forces
  of cohesion and transpiration put tension on the
  remainder of the string of water molecules.
• As long as transpiration continues, the string is
  kept tense and is pulled upward as one molecule
  exits the leaf and the one right behind it is
  tugged up into its place.
• Adhesion pulls the remaining water molecules
  upwards from the root against the downward pull
  of gravity.
• Process does not require energy from the plant,
  they are all extended by physical properties of
  water and the surrounding molecules.

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Transpiration

• Summary of Transpiration-Cohesion-Tension
  Mechanism
• Transpiration exerts a pull that is relayed
  downward along a string of water molecules held
  together by cohesion and helped upward by
  adhesion.
• Transpiration is an efficient means of moving
  large volumes of water upward from roots to
  shoots.
• http//www.phschool.com/science/biology_place/labb
  ench/lab9/transpull.html

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Guard Cells

• PROBLEM! Photosynthesis requires large leaf
  surfaces
• Results in constant transpiration and water loss
• If soil dries out, plants wilt and eventually die
• SOLUTION! Leaf stomata can open and close via the
  control of guard cells.
• Guard cells control the opening of a stoma by
  changing shape, widening or narrowing the gap
  between the two cells.

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Abscisic Acid

• Absicisic acid causes the closing of stomata.
• ABA crucial for plants to withstand drought.
• When the plant starts to wilt, ABA accumulates in
  the leaves and causes stomata to close.