What is a plant? Section 22.1

What do plants need to survive? • Plants form part of the kingdom Plantae

• Cell wall containing cellulose• Photosynthesize using chlorophyll a and b• Most are autotrophs, a few are parasites or saprobes

• Plants need Sunlight• Required to carry out photosynthesis

• Plants needs to exchange gas• Oxygen for cellular respiration• Carbon dioxide for photosynthesis

• Issue is to exchange gasses and not loose too much water at the same time

• Plants need water and minerals • Plants essentially sweat• Plants have evolved to limit water loss and speed up

uptake from the ground• Minerals in the soil are needed for plant growth

How have plants adapted to life on land? • It is thought that all land plants

originated in the water•Most photosynthetic organisms were

unicellular eukaryotes, similar to today’s green algae• Confused evolutionary biologists at first

– how can algae and an oak tree be in the same kingdom? • Cell walls and photosynthetic segments

are the same! Have the same reproductive cycles• Genomes are also closely related

The first land plants• Fossil spores of land plants have been dated to 475 million years ago

• No actual plant fossils

• Oldest fossilized land plants are approximately 425 million years ago• No leaves and roots, only a few cm tall

• Difficulty obtaining water• Growed close to the ground in damp locations

• Plants evolved to be more resistant to the drying rays of the sun, capable of conserving water and reproducing without water

• Emergence changed land, changed ecosystems• Many groups developed – mosses, ferns, cone bearing plants and

flowering plants• Adapted differently to a range of terrestrial environments

The plant kingdom•Can be divided into five major groups based on four important

features• Embryo formation• Specialized water conducting tissues• Seeds• Flowers

What defines most plant life cycles?

•The life cycle of land plants has two alternating phases – a diploid phase and a haploid phase

•Alternation of generations

• Sporophyte – multicellular diploid phase

•Gametophyte – multicellular haploid phase

Trends in plant evolution• Gametophyte size has decreased

and sporophyte size has increased

•Most green algae have a diploid sporophyte phase, some don’t• Only multicellular bodies are

gametophytes

• Ferns and their relatives have a small gametophyte and a larger sporophyte

• Seed plants have an even smaller gametophyte which is contained within sporophyte tissues.

Key point questions

•What do plants needs to survive?

•How did plants adapt to life on land?

•What features define most plant life cycles?

•How have plants in general changed over time?

Seedless plantsSection 22.2

What is algae? • Seaweed is the most common

form of algae

•Algae are not a single group of organisms

•Applied to any photosynthetic eukaryote that isn’t a land plant

• Some are protists, some are plants• Those grouped with plants are

called green algae

The first plants• Green algae were the first plants on earth

• Large algal mats from the Cambrian – 550 million years ago

•Mostly aquatic

• Found in fresh and salt water, and some moist areas on land

• Photosynthetic pigment and cell wall composition shared with larger more complex plants

• Absorb moisture and nutrients directly from their surroundings

• Do not contain the specialized tissues found in other plants

Multicellularity

•Many green algae from colonies

• In some cases, (eg. Volvox) algae are connected through strands of cytoplasm and can communicate• Allows movement of the colony

•Up to 50,000 cells can form a hollow sphere

• Straddles the fence between colonial and multicellular life

Mosses and Bryophytes•Mosses are like nature’s carpet

• Thin waxy coating allows resistance to dry conditions

• Rhizoids anchor them to the soil and absorb water and minerals from surrounding soil

•Mosses are a bryophyte

• Bryophytes have specialized reproductive organs enclosed by other non reproductive cells

• Higher degree of cell specialization than algae

•Other examples are hornwarts and liverworts (Phylums)

Features of bryophytes

• Found in damp places where lots of water

•Because they do not contain vascular tissues to carry water

•Also means that they can’t grow above 1 meter in height

•They don’t produce lignin, meaning they can’t support large structures

What does vascular tissue do? Why is it important to plants?

•420 million years ago plants suddenly got taller!

•They were the first plants to have a transport system• Contain true vascular tissue• Efficient at carrying water and

nutrients

How did the transport system evolve?• Vascular plants are also called tracheophytes,

because they contain a specialized water conducting cell• A tracheid – hollow tubelike cell with thick cell walls

strengthened by lignin• Massive evolutionary step in plant kingdom

• Tracheids are found in the xylem - a tissue that carries water upwards from roots to all parts of the plant• Connected end to end• Openings (pits) allow water to move more efficiently

than simply by diffusion

• Phloem – transports both nutrients and carbohydrates produced by photosynthesis • Fluids are moved throughout the plant against gravity

Seedless plants•All seed bearing plants have a

vascular system• So do some seedless plants

•Common examples – Club mosses, horsetails and ferns

• Ferns are the most common- 11,000 species

•Can thrive in areas with little light and are abundant in seasonally wet habitats

Key summary questions

•What are the characteristics of green algae?

•What factors limit the size of bryophytes?

•How is vascular tissue important? What tissues to most plants contain? What function do they perform?

Seed plantsSection 22.3

What is a seed?

•A plant embryo and food supply encased in a protective coating

• Living plant is diploid

•Early development stage of sporophyte stage of plant life cycle

The first seed plants

• First fossils date back to 360 million years ago

• Several evolutionary stages chart the development of the seed

•DNA evidence suggests all seed plants share a common ancestor

•Key evolutionary steps:• No standing water required for fertilization • Through reproductive process involving cones or

flowers • Pollination• Embryos in seed

What is the role of pollen?•Entire male gametophyte is contained within a pollen grain

• Sperm do not swim through water

•Carried by wind or insects to fertilize eggs

•Process called pollination

How does a seed form?

• After fertilization, zygote in seed grows into a tiny plant

• Embryo will stop growing whilst it is small and contained within a seed• Can remain this way for weeks, months or years

• Seed coat provides protection• Can survive long periods of bitter cold, extreme

heat or drought• Embryo will begin to grow again when conditions

are once again right• Uses nutrients from stored food supply until it can

carry out photosynthesis on its own

How does fertilization take place in gymnosperms?

• Name means – ‘naked seed’• Seeds exposed on scales of cones

• Cones are produced by mature sporophyte plant

• Two types of cones produced – pollen cones and seed cones

• One pollen grain is entire male gametophyte – one haploid nuclei will divide to produce two sperm cells

• Familiar seed cones/female cones produce female gametophytes• Larger than pollen cones• Two ovules found near base• In ovules meiosis produces female gametophytes that contain hundred

and thousands of egg cells – when mature each gametophyte contains a few large eggs cells ready for fertilization

Key points questions

•What adaptations allow seed plants to reproduce without water?

•What is a Gymnosperm

•How does fertilization of a Gymnosperm take place?

Flowering plantsSection 22.4

Flowering plants (angiosperm) overview

•Most abundant plant species on Earth

• First appeared 135 million years ago

• Flower – reproductive organ• Contain ovaries – surround and protect

seeds

•Gives rise to name angiosperm – ‘enclosed seed’.

•Reproduce via sexual reproduction• After fertilization, fruit development

provides protection for the seed

Why do they have flowers?

•Attract animals such as bees, moths and hummingbirds

•Attracted by scent, color or shape

•Take pollen away when they leave

•Pollinate as they move – more efficient than wind

Why do they have fruits?

•All fruits contain one or more mature ovaries

•Wall helps to disperse seeds inside – carrying away from parent plant

•Can be spread by animals….

How do you classify angiosperms? • Originally classified on the basis of number of seed

leaves (Cotyledons)

• One seed leaf – monocots

• Two seed leaves – dicots

• Actually, life is more complicated • Ancient chinese fruiting plant fossil (Archaefructus –

oldest known plant with reproductive organs found resembling modern flowers)

• Can not be classified as either monocot or dicot

• Amborella – from pacific island of New Caledonia also can’t be classified as either

• Now monocots are placed in a single group, but dicots are divided into a variety of distinct and different categories

• Dicot not used for classification, but still used today

What If you aren’t a scientist?

•Alternative widely used classifications exist

•Angiosperms can be grouped according to:• Number or seed leaves• Strength and composition of stem• Number of growing seasons

Monocots vs Dicots

Woody and Herbaceous plants• Flowering plants can be

divided on the basis of stem characteristics

•Woody plants – plants made with cells that have a thick cell wall which supports plant body• Include trees, shrubs and bines

•Herbaceous plants – do not produce wood as they grow• Smooth and nonwoody

Annuals, Biennials and Perennials

• Some flowers plants will grow, flower and die in a single year

•Other grow from year to year

• Lifespan determined by genetic and environmental factors

•Harsh environmental conditions can shorten life cycles

Summary questions

•What are the key features of angiosperm reproduction? What aspects are unique to angiosperms?

•What is the importance of a fruit?

•How can angiosperms be categorized?

•How are scientific and everyday classification methods different?

Specialized tissues in plants Section 23.1 – Plant structures and function

What are the three principal organs of seed plants? • Roots

• Anchor plants, hold in soil, prevent erosion, help form mutualistic relationships with bacteria and fungi that help root absorb water and nutrients, help transport these things to other parts of the plant, store food and provide support.

• Stems• Support system, host transport system,

defense system against predators, produce leaves and reproductive organs,

• Leaves• Main photosynthetic organ, large surface area

for capturing sunlight, have adaptations against water loss,

What are the main tissue systems in plants?

•Dermal tissue • Covers a plant like skin

•Vascular tissue• Act as a plants bloodstream

•Ground tissue • Produces and stores food

Dermal Tissue• The protective outer covering of the

plant

• In young plants - one layer of cells called the epidermis • Covered with a thick waxy layer called the

cuticle• Protects against water loss

• Some have trichomes - aid in leaf protection

• In older plants, epidermis may be several layers thick and covered with bark

• In roots, root hairs are an example of dermal tissue

Vascular tissue• Support plants body and transport water and

nutrients

• Xylem – water

• Called tracheids

• Found in all seed plants• Long and narrow

• Xylem is made up of dead cells – lignin in cell walls resists water and provides strength• Opening in walls connect neighboring cells

• Pits (thinner regions) allow water to flow to ground tissue by diffusion

Xylem continued…•Angiosperms do not just contain

tracheids

•Vessel element – wider than tracheid and stacked on top of each other

•After cells mature and die, cell walls are left with slit like openings allowing movement of water

• In some cases you can be left with a continuous tube

The Phloem• Unlike Xylem, Phloem cells are alive at

maturity•Main cells are called sieve tube elements• Arranged end to end• End walls have many small holes – allowing

nutrients to move from cell to cell• As they mature, sieve tube elements lose

their nuclei and many organelles• Remaining organelles sustained by

companion cells • Surround sieve tube elements• Support phloem and aid in the movement of

material in and out

Ground Tissue

• Tissues that are neither vascular or dermal• Main functions are to:

• Produce and Store sugars• Contribute to physical support of the plant

• From the edible part of most plants• Most consist of Parenchyma

• Thin cell wall, large vacuole and thin layer of cytoplasm• In leaves, these cells contain chloroplasts for photosynthesis

• Collenchyma form another type of ground tissue• Strong flexible cell walls that help support plant organs

• Example – celery stem

• Sclerenchyma – thick rigid wall – makeup seed coats• Used to make rope from hemp• Found in nut casings

Ground tissue overview

What is a meristem? • A region of unspecialized

cells in which mitosis produces new cells that are ready for differentiation• Explains why plants unlike

animals never stop growing!

• Apical meristem – tip of stem or root (As tip is called apex)

• All start out similar, but over time will differentiate into dermal, vascular or ground tissue

Meristems and flowers

•Cones and flowers (reproductive organs) are also formed from meristems

•Development begins with a change in the pattern of gene expression changes in a stem’s apical meristem

•Transform apical meristem into floral meristem• Produces flower tissues

Key points questions

•What are the three main organs of a seed plant?

•What are the three main types of tissues?

•How do the functions of the different tissues differ?

•What is a meristem? What is its function?

•How do meristems relate to plant cutting? How does taking a cutting work?

RootsSection 23.2

What are the two different types of root systems?

• Taproots – Primary roots grows long, thin and thick• Gives rise to smaller branch roots• In some cases, such as oak and hickory trees – grow

so large they can reach water several meters down• Carrots, dandelions and beets – all examples of short

taproots that store sugars and starches

• Fibrous root systems - many equally sized branch roots• All grow separately from the base of the stem• No single root grows larger than the rest• Help prevent soil erosion

Anatomy of a root• Roots contain, dermal (epidermis), vascular and ground tissues

• Key role in water and mineral transport

• Epidermis – protection and absorption• Covered with root hairs that produce large surface area – maximizing

efficiency

• Ground tissue – Cortex• Material flows through the cortex and acts as storage area for

products of photosynthesis• Endodermis encloses vascular cylinder

• Vascular tissue – enclosed in vascular cylinder• Apical meristem – produce new cells near root tip, causing roots

to grow• Protected by root cap

• Cap also secretes slippery substance easing progress through the soil• Cap cells constantly destroyed and replaced

What are the main functions of a root? • Roots support a plant, anchor it to the

ground, store food, and absorb water and dissolved nutrients from the soil

• Uptake of nutrients• Soil can vary a lot from place to place –

texture and composition• Determine which kinds of plants can grow in

the soil

• Plants require a lot of different nutrients• Main ones are Nitrogen, Phosphorus,

Potassium, Magnesium and Calcium • Alongside trace elements

• Possible to have too much of a good thing!

What nutrients does a plant need?

Root functions continued…• Active transport of dissolved nutrients

• Cell membranes of root hairs and other epidermis cells contain active transport proteins

• Energy required to move particles against a concentration gradient across a membrane

• Osmosis • Due to accumulation of nutrients, water will follow nutrients into the roots• Although water is not pumped, it will accumulate

• Movement into vascular cylinder • Water and minerals pass the inner boundary of the cortex and move

towards the vascular cylinder • Endodermis encloses vascular cylinder

• Caspian strip – waxy layer that surrounds vascular tissue• Forces water and minerals to pass through cell membrane of endodermis • Endodermis cells can filter and control the water and dissolved nutrients

that enter vascular cylinder • Ensure that nutrients do not leak back out • Results in one way passage

Root pressure explained

•The one way system in plants is very important

•Allows plants to generate enough pressure to move water out of the soil and into the body of the plant

•As water is contained by Caspian strip – the only way it can go is up!

•But, this is just the beginning….

Key points questions

•What are the main functions of a root?

•How are tissues distributed in a plant root?

•How is osmosis involved in the the absorption of water and minerals?

•What is the Caspian strip? What role does it play in water transport? Why is this so important?

StemsSection 23.3

What are the 3 main functions of stems? •Produce leaves, branches and flowers

•Hold leaves up to the sun

• Stems transport substances through the plant

Stem anatomy

• Stems contain all three tissue types – dermal, vascular and ground tissue

• Layer of epidermal cells that have thick cell walls and a waxy protective coating

•Node – where leaf is attached to the stem

•Bud – contains apical meristem that can produce new stems and leaves

• Stem can develop woody tissues that help support leaves and flowers

Vascular bundle patterns

•Monocots have clusters of xylem and phloem tissues called vascular tissues

• In dicots vascular bundles are arranged in a cylinder or a ring

Plant stem growth • Primary vs Secondary growth • Primary growth - of new cells produced by the

apical meristems of roots and stems• Increase in length in a plant is due to primary growth

from year to year

• Primary growth of stems is the result of elongation of cells produced in the apical meristem • Occurs in all seed plants

• Secondary growth – Stems must increase in thickness as well as length if the plant gets big and have more mass to support • Common in dicots and non flowering seed plants such

as pines• Rare in monocots• Explains why most monocots have limited girth

Secondary growth continued…•Most dicots have meristems within their stems

and roots• Enable true secondary growth• Increased width is able to support increased

height of tree• In conifers and Dicots, secondary growth occurs

in meristems called the vascular cambium and the cork cambium• Vascular cambium – produces vascular tissue and

increases stem thickness over time• Cork cambium – produces the outer covering of

stems• Similar types of cambium are also found in roots]

Growth from the Vascular Cambium

• In a young dicot stem, bundles of xylem and phloem are arranged in a ring

• Secondary growth begins as a thin, cylindrical layer of cells between clusters of vascular tissue

• New meristem forms between the xylem and the phloem of each vascular bundle

• Division of vascular cambium gives rise to new layers of the xylem and phloem

• Each year the cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker.

How does wood form?•Most of what we call wood is layers of

secondary xylem produced by the vascular cambium• These cells build up year after year, layer

upon layer•Older xylem – heartwood – found in the

center no longer conducts water• Darkens with age as it collects colored

deposits• Sapwood surround heartwood – is

lighter in color, and is active in fluid transport

What is a tree ring? How does it form?• Tree growth is seasonal in temperate zones• When growth begins in the spring, the vascular cambium begins to

grow rapidly, producing large, light colored xylem cells with thin cell walls

• Results in light colored earthy wood

• As growing season continues, cells grow less and have thicker cell walls, forming a layer of dark late wood

• Alternation of dark and light wood produces tree rings

• Usually one ring is a year of growth

• Tree ring cross sections can be used to estimate a trees age

• Size of rings can provide additional information about weather conditions

How does Bark form?

• In a mature stem all of the tissue outside of the vascular cambium makes up bark

• Includes Phloem, Cork Cambium and cork• As a tree expands in width, the phloem layer must

grow as well• Expansion may cause the oldest tissues to split and

fragment as the expanding stem stretches them• The cork cambium surrounds the cortex and

produces a thick , protective layer of waterproof cork that prevents water loss from the stem

• As stem increases in size, outer layers of dead bark often crack and flake off the tree

Key points questions

•What are three important functions of a stem?

•How does the arrangement of vascular bundles differ between monocots and dicots?

•How do the functions of a stem relate to the functions of roots ad leaves?

•Define primary and secondary growth

Leaves Section 23.4

How is the leaf adapted to make photosynthesis more efficient?

• Most leaves have a thin flattened part called the blade – maximize area over which light can be absorbed

• Blade attached to stem by a thin branch called a petiole

• Leaves are covered top and bottom by dermal tissue• Thick outer wall that resists tearing• Covered by a waxy cuticle - waterproof barrier that protects tissues and limits the loss of water through evaporation

• Vascular tissue flows through leaves in veins – connecting the leaves to the rest of the plants

• Leaves contain a special ground tissue called mesophyll, where photosynthesis occurs

Leaf structure •Photosynthesis predominantly occurs in pallisade mesophyll layer

•Transpiration – loss of water through leaves• Walls of mesophyll cells are kept moist to facilitate gas exchange• Also keeps leaves cool on a hot day• Water replaced through xylem

What role do stomata play in maintaining homeostasis? • When Photosynthesizing, leaves take in Carbon

Dioxide and give off Oxygen• When respiring they take in oxygen and give off

carbon dioxide• Stomata control gas exchange between spongey

mesophyll and exterior• Why can’t the stomata be left open all the time?• Water loss would be too great• Guard cells maintain the balancing act between gas

exchange and water loss• Guard cells open and close in response to changes in

water pressure• General rule – stomata are open during day time and

closed during night time

Transpiration and wilting

•Osmotic pressure keeps a plant’s leaves and stems rigid and stiff

• High transpiration rates can lead to wilting

•Wilting leads to loss of water and pressure in a plants cells

•When a leaf wilts it’s stomata close

• Therefore, wilting helps plants to conserve water

Leaf adaptations

• Leaves can adapt to live in a variety of different environments

Key point questions

•How is the structure of a leaf adapted to make photosynthesis more efficient?

•What is the role of the palisade mesophyll layer?

•Why might desert plants have two or more palisade mesophyll layers?

•How do stomata help maintain photosynthesis?

•Will stomata be open or closed on a hot day?

Reproduction in flowering plants

Section 24.1

Why do plants have flowers?

•They are the result of the stunning evolutionary success of flowering plants

• Flowers are the reproductive organ composed of four parts• Sepal• Petal• Stamen• Carpel

What role do each of these 4 parts do? • Sepal and Petals

• Sepal encloses bud before it opens, protecting flower during development

• Petal: Attract insects and other pollinators to the flower

• Stamens• Male part of the flower• Consists of filament (stalky bit) and Anther (where pollen

grains produced)• Flowers often have numerous stamens

• Carpels• Female part of the flower• Produce and shelter both female gametophyte and seeds• Ovary at base contains one or more Ovules • Stigma – area which captures pollen• Style – protrudes from ovary

How do flowers vary?

•Although most species of angiosperms produce both male and female gametophyte, some species will produce them on different plants

• In some species, many flowers grow close together – forming a composite structure

What is pollination? • Transfer of pollen to female portion of flower

• Can be Insect or wind pollinated• Wind pollination can be hit or miss• Some plants have adapted for insect or animal pollination – bright colors, food source etc…

• When a pollen grain lands on a stigma, it begins to grow a pollen tube• Two cells in pollen grain• One produces two sperm cells, one produces pollen tube

• Double fertilization occurs • One sperm fuses with egg nucleus to produce diploid zygote• The other sperm fuses with two polar nuclei in the embryo sac to form a triploid (3N) cell -

Endosperm (nourishes seedling as it grows)• So two distinct stages of fertilization

• Energy efficient process - allows many seeds to be produced

A visual overview

What is vegetative reproduction?

•A form of asexual reproduction in angiosperms• Produces offspring genetically identical

to itself

•New individuals formed by mitosis – no gametes, flowers or fertilization

•Allows for quick reproduction of a successful plant

•But does not introduce genetic variation

Plant propagation• Horticulturists take advantage of vegetative

reproduction • Use cutting and grafting

• Cutting• Cuts a length of stem that contains buds

containing meristem tissues• Stem partially buried – roots will form

• Grafting• A piece of stem is cut from parent and attached

to another plant• Plants must be closely related• Example – lemon and orange, or different

varieties of apple

Review questions

•What are the four parts of a flower?

•Which are male, which are female?

•How does fertilization of an angiosperm occur?

•What are the advantages of double fertilization?

•What is vegetative reproduction?

Fruits and SeedsSection 24.2

How do fruits form?

• Seeds – great evolutionary step

• Fruit are however an even better adaptation!

• Fruit is simply a mature angiosperm ovary, that usually contains seeds

• Develops as angiosperm seed matures

• Exception – commercially grown seedless fruits – grapes!

• Peas, corn, rice, cucumber and tomatoes are all technically fruits

How are seeds dispersed? • By animals

• Fruits designed to attract animals• Sweet, juicy goodness - seeds get eaten alongside

fruit• Seeds are covered with tough coatings - pass

through digestive system unharmed• Seeds sprout from feces

• By wind and water• Seeds contained in lightweight fruits that allow

them to be carried by air or in buoyant fruits that allow them to float

• Coconut – dispersed by water• Dandelion – by wind

For how long does a seed stay a seed?

•This is highly variable – and depends on a number of environmental factors• Temperature• Moisture

•Dormancy – when seed is alive but not growing

•Germination – resumption of growth of the plant embryo

What does a seed need to germinate?

•Water!

•Water caused food storing tissues to swell• Seed coat is cracked open• Young root emerges first• Followed by the shoot

What is a Cotyledon? Why is it important? • A cotyledon is a flowering plants first

leaves• Job: Store nutrients, and transfer

them to the growing embryo as a seed germinates•Monocot: Single cotyledon

• Usually remains underground as nutrients passed to young plant

• Young shoot protected by sheath

• Dicot: No sheath protects tip of young plant• Upper end of shoot bends to form a

hook – forcing its way through the soil

What are the advantages of seed dormancy? •Allows for long distance dispersal

•Allows seeds to germinate under ideal growth conditions• Seeds can survive winter colds but young plants cannot

• In some cases, extreme environmental conditions are needed to end seed dormancy

• Some pine trees produce seed cones that only release seeds in high heats generated by forest fires• Mechanism for repopulating a forest quickly after a fire

Key point questions

•What is a fruit? How do they form?

•What does something need to be classified as a fruit? Give 3 examples of fruits commonly mistaken as a vegetable

•How can seeds be dispersed?

•What environmental factors affect seed germination?

•What is the importance of dormancy for seeds?

Plant hormonesSection 24.3

What is a hormone?

•A chemical signal produced by a living cell that affects the growth, activity or development of cells and tissues• Controls development of cells,

tissues or organs

•Either act where they are made or will travel to other parts of the plant• Unlike animal hormones which

typically act far away from where they are produced

How do plant hormones work?

• Target cells – cells affect by a particular hormone

• Receptor – needed for a cell to respond to a hormone• Proteins that hormone molecules bind to

• Response dependent upon the types of receptors present in target cell

• Possible effects are:• Alter metabolism

• Speed Growth

• Inhibit cell division

• A given hormone may affect roots differently from stems or flowers

• Cells without receptors are generally unaffected by hormones

What is an Auxin?

• First discovered by Charles Darwin and his son, Francis•Wanted to understand why a plant

seedling bends towards light as it grows• Hypothesized that the tip produces a

substance that regulates cell growth• 40 years later auxins discovered –

stimulate cell elongation and growth of new roots• Produced in shoot apical meristem, and

are transported to the rest of the plant

How does an Auxin work?

•Auxins stimulate cell elongation

•When light hits a shoot, auxins collect in the shaded part of the stem

•Change in concentration stimulates cells on the dark side to lengthen

• So, shoot bends towards the light

But it doesn’t end there…• Auxins also regulate cell division in

meristems• As stems grow they are able to produce lateral

buds• Buds near apex grow more slowly than at the

base• Growth at lateral buds is inhibited by auxins

• The closer a bud is to an apical meristem, the more it is inhibited• As auxins are produced at meristem• Called apical dominance

• This is why gardeners prune plants – to remove apical meristems

What is a Cytokinin?

•A plant hormone produced in growing roots and developing fruits and seeds• Role is to stimulate late cell division

• Interact with auxins to balance root and shoot growth• Also stimulate regeneration• And prevent aging

•Produced in the roots, and have the opposite effect of auxins

Other plant hormones

• Gibberellins• Stimulate growth and can cause dramatic increases in size of both stems and fruits

• Produced in meristems of shoots, roots and seed embryo

• Abscisic acid• Inhibits cell division and promotes seed dormancy. Can be washed away by strong

spring rain, resulting in germination

• Opposite effect to gibberellins – balancing acts in plant

• Ethylene• Stimulate fruits to ripen

• Also results in plants shedding unnecessary organs – petals, leaves, fruit

• Causes plant to seal off unneeded plant from rest of plant

What stimuli do plants respond to? (tropisms)

• Light• Phototrophism – tendency of plant to

grow towards light source• Controlled by auxins

•Gravity• Gravitotrophism - still not well

understood, but auxins migrate to the lower sides of horizontal roots and stems

•Touch• Thigmotropism – Vines and climbing

plants exhibit thigmotropism – encounter an object and wrap around it

Are all plant responses tropisms?

•No - Some plant responses are too quick to be tropisms

• Some plants respond immediately to human touch, and are able to fold in leaves quickly – Mimosa Pudica

•Venus fly trap responds to fly landing immediately, and snaps leaves shut

How do plants respond to seasonal changes? •Photoperiodism – major factor

in the timing of seasonal activities such as flowering and growth

•Phytochrome – responsible for plan responses to photoperiod.• Absorbs red light and activates a

number of pathways in the cell

• Plants respond to changes in these pathways

Relationship between phytochrome and winter dormancy

• Phytochrome regulates changes in activity that prepare many plants for dormancy as winter approaches• As cold weather approaches, deciduous plans turn off

photosynthetic pathways, transport materials from leaves to roots, and seal off leaves from the rest of the plant

• Many plants loose their leaves during the colder months• At end of summer, phytcohrome in leaves absorbs less light• Auxin production drops, ethylene production increases• Starts series of events leading to shutdown of leaf• Chlorophyll breaks down, yellow and orange carotenoids become

visible

• Hormones also produce important changes to apical meristems - instead of leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds• Allows terminal bud to survive cold winter day• Xylem and Phloem pump themselves full of ions and organic

compounds - acting like antifreeze

Key points questions

•What are the main plant hormones?

•How do hormones help maintain homeostasis?

•How does pruning a plant make it more bushy?

•What external stimuli do plants respond to?

•How do plants respond to seasonal changes?

•What is the difference between a sort day and a long day planyt?

Plants and HumansSection 24.4

How has agriculture changed the Earth?

• The onset of crop cultivation 10,000 – 12,000 years ago changed human history• People were given a reason to stay in one

place

• Nowadays, large amount of diversity in crops grown, BUT humankind largely depends on rice, wheat , soybeans and corn for the bulk of it’s food supply• Endosperms (seeds) of grasses

• These crops are also used to feed livestock

How has agriculture changed? •As travel has become easier, new crop

plants have been introduced around the world• For example – potatoes, peanuts and corn

where unknown in Europe 400 years ago

• Selective breeding and changes to farming techniques has increased efficiency of agriculture• Example – cauliflower, broccoli and

Brussels sprouts all originate from a single species of wild mustard

• Green revolution – 1950 – 1970s • Increased world food supplies

The Green Revolution• Came about through use of high yield seeds and

fertilizer • Natural fertilizer (dung) had been used for

thousands of years• Artificial fertilizers ensure that plants get what

they needs• Nitrogen, phosphorous and potassium • Must be used with care – too much will kill a crop• Can also effect groundwater

• Pesticide use has increased crop production• Must be used with caution – are poisonous, potential

to harm wildlife, and leave dangerous concentrations in food.

What else do Humans use plants for, besides food?•Cotton

•Wood

•Paper

•Medicine

•Without plants, life as we know it would be very different…

Key points questions

•Which four crops form the basis of the world’s food supply?

•How was selective breeding been used to increase yield form crops?

•What benefits beside food do humans derive from plants?