P L A N T S

Mrs. Daniels

Advanced BiologyRevised Nov. 2006

Why Study Plants?• Plants form the foundation on which most terrestrial

ecosystems are built– Basis of food chain because they are producers

• Plants were first studied by humans who had to distinguish btwn edible and poisonous plants– These early humans later began to use plant products to make

useful tools and other items– What uses do plants have today?– Modern plant biology continues to center on how to use plant

and plant products to benefit humans

Alternation of Generation

• plants spend part of their life cycle as a haploid and part as a diploid organism

• The haploid portion is the gametophyte generation

• The diploid portion is called the sporophyte generation

Alternation of Generation

• The gametophyte generation is dominant in lower plants

• The sporophyte generation is dominant in more advanced plants

• All plants produce spores by meiosis

• Algae and fungi produce spores by either mitosis or meiosis

Figure 26-2Page 500

Embryo

Gametophyte

Sporophyte

Meiosis Fertilization

HAPLOID (n)GAMETOPHYTEGENERATION

DIPLOID (2 n)SPOROPHYTEGENERATION

Spore Sperm

Egg

Zygote

Plants move onto land

• Environmental challenges of living on land required adaptations over thousands of years

• These adaptations had to be:

• Anatomical

• Physiological

• Reproductive

Adaptations

• Alternation of generations

• Reproductive structures

• Seeds vs. spores

• Stomata for gas exchange

• Cuticle for prevention of water loss

• Vascular tissues and more structural support

• Etc.

Four major types of plants

• 1. Bryophytes: seedless, non-vascular

• 2. Tracheophytes: Seedless vascular plants– 3. Gymnosperms: non-flowering, seeded,

vascular plants– 4. Angiosperms: flowering, seeded,

vascular plants

Bryophytes• Mosses, liverworts, and hornworts• Colonial plants• Rhizoids anchor them• No true roots, stems, or leaves• Lack vascular tissue

• Small and easy to grow• Used for experiments in genetics, plant hormones,

tropisms, etc.

Sporophyte portion of moss

haircap moss(Polytrichum commune)

common hornwort(Anthoceros natans)

common liverwort(Marchantia polymorpha)

Tracheophytes• Vascular, seedless plants• Ferns (11,000 species)• Whisk ferns, horsetails, club

mosses• Dominant sporophyte generation• Used to study apical meristems• Are polyploidy, so used in gene

expression studies

Seeds Vs. Spores

• Both Bryophytes and Tracheophytes (ferns) are seedless

• The next two groups of plants we’ll discuss have produce seeds

• What are the advantages of seeds?

1. Further development than spores

2. Contain abundant food supply

3. Protected by a thick seed coat

Parts of a Seed

• We’ll examine this later in chapter 35

• For now, let’s see which plants produce them

Gymnosperms• Vascular, seeded plants• “naked seed” plants• Redwoods, Sequoias, Ginkgoes,

pines, spruce, firs, etc.• Divided into 4 phyla: Pinophyta,

Cycadophyta, Ginkgophyta, and Gnetophyta

• Tropical vines and ephedrine (shrub resembling horsetails) are oddly enough gymnosperms

Ginkgo Trees

• Phylum Pinophyta– Conifers that produce

• Wood

• Bark

• Needles

• Seeds in cones

– Most are monoecious: both male and female reproductive structures are on the same plant

– Phylum Cycadophyta• Look like palms or ferns• Dioecious (male plant and female plant), but

reproduce with pollen and seeds in conelike structures

• Once numerous, now few members left

Cycads

This female cycad in South Africa has a trunk that reaches a height of about 9 m. Note immense seed cones, to 0.8 m long

• Phylum Ginkgophyta– Sole member is

Ginkgo biloba– Deciduous– Dioceious– Female ginkgo produces seeds directly

on branches

Branch from a female ginkgo, showing

exposed seeds and distinctive leaves

– Gnetophytes• Consist of three genera

–Gnetum–Ephedra–Welwitschia

• Unique among gymnosperms, sharing traits with angiosperms

• Vessel elements in their xylem• Cone clusters resemble flower clusters• Life cycle details resemble those of angiosperms

Leaves of Gnetum gnemon resemble those of flowering plants

Male Ephedra has pollen cones clustered at the nodes

A specimen of Welwitschia mirabilis living in Namib Desert, Namibia – survives on fog

Angiosperms• Seeded, vascular plants that flower (and the seeds are

enclosed in a fruit)

• After fertilization, the ovules become seeds and the ovary becomes fruit

• Monocots and dicots

• Monocots: herbaceous ex. Lilies, grasses, corn, palms, orchids

• Dicots: herbaceous or woody

• Ex. Roses, oaks, potatoes

Monocots & Dicots

• Flower parts in 3’s• One pore in pollen grain• Parallel venation in leaves• Scattered vascular bundles• Fibrous root system• One cotyledon• Lack secondary growth

• Flower parts in 4’s & 5’s• 3 pores in pollen grain• Netted venation in leaves• Vasc bundles in ring• Taproot system• Two cotyledons• Often have secondary

growth

Plant Structure and Life Span

• Roots, stems, and leaves

• Herbaceous or woody

• Annuals, biennials, perennials

• Woody perennials can be either deciduous or evergreen

Ch31

The Root System

• Root systems (under ground)• Tap root=

• One large, vertical root produces many smaller secondary roots

• Provides firm anchorage

• Some taproots such as carrots, turnips, and sweet potatoes, are modified to store a large amount of reserve food

• Fibrous root=found primarily in monocots (palms, bamboo, grasses)

• A mat of threadlike roots spreads out below the soil surface

• Provides for extensive exposure to soil water and minerals

• Roots are concentrated in the upper few centimeters of soil, preventing soil erosion

Fxn -anchor plants, absorb & conduct water and nutrients,store food

• Adventitious roots= roots arising above ground from stems or leaves

• Form in addition to normal root system

• Some, such as prop roots of corn, help support the plant stem

The Shoot System

• “above ground”

• Stems, leaves, flowers, and fruits

• Air is the source for photosynthesis and sunlight cannot penetrate into the soil– Soil provides water and dissolved minerals to the plant

• Each system depends upon the other for survival of the whole plant– Roots depend on shoots for sugar and other organic

nutrients– Shoots depend on roots for minerals, water and support

• Materials are transported through the plant by vasc tissues– Xylem conveys water and dissolved minerals to the shoots– Phloem conveys food from shoots to roots and other

nonphotosynthetic parts. And from storage roots to actively growing shoots

Roots and Shoots

• Xylem consists of two cell types, both with secondary walls and both dead at functional maturity

• Tracheids= long, thin, tapered cells having lignin-hardened secondary walls with pits(thinner regions where only primary walls are present)

• Water flows from cell to cell through pits– Also function in support

• Vessel Elements are wider, shorter, thinner-walled, and less tapered than tracheids – Vessel elements are aligned end to end– The end walls are perforated, permitting the free flow of

water through long chains of vessel elements called xylem vessels

• Sieve-tube members are chains of phloem cells that transport sucrose, other organic compounds and some minerals

– The cells are alive at functional maturity

• Companion cells also help load sugar produced in the mesophyll into sieve-tubes of leaves of some plants

The Three Tissue Systems of a Plant

• Dermal Tissue System– Dermal tissue = epidermis = single layer of tightly packed cells covering

and protecting the young parts of the plant• Functions in protection and has special characteristics consistent with the

function of the organ it covers

• Vascular Tissue System– Vascular tissue = xylem and phloem that functions in transport and

support; is continuous throughout the plant

• Ground Tissue System– Ground tissue = predominantly parenchyma, with some collenchyma

and sclerenchyma, that fills the space btwn dermal and vascular tissue systems

– Functions in photosynthesis, storage and support

Plant cells are arranged into three tissue systems.

The many types of plant cells are organized into the three major tissue systems

Each type of plant cell has structural adaptations that make it possible to perform that cell’s function. Some are coupled with specific characteristics of the protoplast = the plant cell contents exclusive of the cell wall

• Parenchyma • Collenchyma• Sclerenchyma• Tracheids (already mentioned)• Sieve-Tube members (already mentioned)

Parenchyma Cells

• Primary cell walls are thin and flexible

• Most lack secondary walls

• The protoplast usually has a large central vacuole

• Function in synthesizing and storing organic products

• Most mature cells do not divide, but retain the ability to divide and differentiate into other cell types under special conditions - repair and replacement after injury

Parenchyma cells are the least specialized plant cells

Collenchyma Cells

• They are usually grouped in strands or cylinders to support young parts of plants without restraining growth

• They are living cells which elongate as the stems and leaves they support grow

Collenchyma cells usually lack secondary walls

Sclerenchyma Cells

• They have very rigid, thick secondary cell walls strengthened by lignin

• Many lack protoplast at functional maturity • they cannot elongate and may be dead, functioning

only as support• There are two forms:

– Fibers= long slender, tapered cells occurring in bundles

– Sclerids= shorter irregularly shaped cells

Sclerenchyma cells function in support

Plant Structure and GrowthMeristems generate cells for new organs throughout the lifetime of a plant

-Apical meristems extend roots and shoots

-Lateral meristems add girth to stems and roots

Meristems generate cells for new organs throughout the lifetime of a plant

• Indeterminate growth= growth as long as the plant lives

– Most animals cease growing after reaching a certain size(determinate growth)

– Certain plant organs, such as flower parts also show determinate growth

Plant growth begins with germination of the seed and continues for the lifespan of the plant

Indeterminate growth made possible by Meristems

• Meristematic (perpetually embryonic tissues) are unspecialized and divide to generate new cells near the growing point

• Apical meristems are located in root tips and shoot buds. They supply cells for plants to grow in length– Primary growth(elongation) is initiated by apical

meristems and forms primary tissues organized into the 3 tissue systems

• Secondary growth (increased girth) is the thickening of roots and shoots which occurs in woody plants due to development of lateral meristems= cylinders of dividing cells extending along the lengths of roots and shoots– Cell division in the lateral meristems produces

secondary dermal tissues which are thicker and tougher than the epidermis is replaces

– Adds new layers of vascular tissue

• Primary growth of roots is concentrated near the tip and results in roots extending through the soil

– The root tip is covered by a root cap, which which protects the meristem and secretes a polysaccharide coating that lubricates the soil ahead of the growing root

– The root tip contains three zones of cells in successive stages of primary growth. Although described separately, these zones blend into a continuum

• Zone of Cell Division

• Zone of Cell Elongation

• Zone of Maturation

The primary plant body with its three tissue systems is produced by primary growth

Primary Tissues of Stems• The vascular tissue of the stem is organized into strands of

vascular bundles that run the length of the stem– Converge at the transition zone (shoot to root) – Each bundle is surrounded by ground tissue, including pith

and cortex• In dicots, bundles are arranged in a ring with pith inside and

cortex outside– Xylem faces the pith, phloem faces the cortex– Pith and cortex are connected by pith-rays, thin layers of

ground tissue• In monocots, bundles are scattered throughout the ground tissue

of the stem– The stem ground tissue is mostly parenchyma, strengthened in

many plants by collenchyma located beneath the epidermis

Tissue Organization of Leaves

• Epidermis- tightly interlocked cells

– protect against physical damage, pathogens

– prevent excess water loss (waxy cuticle)

• Stomata- pores flanked by guard cells

– regulate gas exchange with the surrounding air and photosynthetic cells inside the leaf

– Allows for transpiration (water loss from plant by evaporation)

– More numerous on the bottom surface of the leaf. This location minimizes water loss

• Mesophyll=ground tissue of the leaf

– Consists mainly of parenchyma cells and equipped with chloroplasts for photosynthesis

– Dicots have two distinct regions of mesophyll:

• Palisade (mesophyll) parenchyma=one or more layers of culumnar cells of the upper half of a leaf

• Spongy mesophyll=irregularly shaped cells surrounded by air spaces through which oxygen and carbon dioxide circulate. Located in the lower half of the leaf

Leaves are cloaked by an epidermis of tightly interlocked cells

Lateral meristems add girth to stems and roots: a closer look at secondary growth

• The secondary plant body is comprised of the secondary tissues produced during growth in diameter

• Secondary growth results from two lateral meristems: the vascular cambium and the cork cambium– Vascular cambium produces secondary xylem and phloem– Cork cambium produces a tough, thick covering for roots

and stems that replaces the epidermis• Secondary growth occurs in all gymnosperms and most

dicot angiosperms– It is rare in monocots

Secondary growth of stems

Vascular cambium forms when meristematic parenchyma cells develop btwn the primary xylem and primary phloem of each vascular bundle and in the rays of ground tissue btwn the bundles

Wood• Accumulated layers of secondary xylem produces wood which

consists mostly of tracheids, vessel elements and fiber• The hardness and strength of wood results from the cells

which, while dead at maturity, have thick lignified walls• Forms annual growth rings due to yearly activity: cambium

dormancy, spring wood production and summer wood production

• The secondary phloem does not accumulate extensively. The secondary phloem, and all tissues external to it, develop into bark which eventually sloughs off the tree trunk

Cork• Cork cambium is a cylinder of meristematic tissue that forms

protective layers of the secondary plant body

• Forms first in the outer cortex of the stem

• Cork cells form to the outside of cork cambium

• As cork cells mature, they deposit suberin (a waxy material) in their walls and die

• The dead cork tissues protect the stem from damage and pathogens while reducing water loss

• The combination of cork cambium and layers of cork form the periderm (protective coat of the secondary plant body that replaces primary epidermis)

• The term bark refers to all the tissues external of the vascular cambium (phloem, cork cambium and cork)

Take time to stop and smell the roses…

Well, at least draw one.

• You had to draw a flower in your question packet…so we won’t actually do that again

• But…• What are the male reproductive parts?• Stamen (made up of anther and filament)• The female parts?• Pistil or carpel (made up of stigma, style, and

ovary)

Plant Sex

• Just kidding…sexual reproduction in plants involves the parts we just mentioned

• How does it work?

• There is a process called double fertilization

Double Fertilization

• Pollen is formed in the anther and is released

• It lands on the stigma at the tip of the carpel• A pollen tube grows from the pollen grain

down the carpel into the embryo sac• Two sperm are discharged and travel

through the tube to the egg• One sperm fertilizes the egg to become a

diploid zygote (develops into the embryo)

Double Fertilization

• The second sperm combines with the polar nuclei to form a triploid endosperm, which will become the food for the developing embryo

• The fertilized egg, endosperm, and ovule now make up a seed

• The ovary becomes the fruit, which can contain several seeds

Seeds

• Some of the major parts of a seed that you should know:

• Plumule: shoot tip

• Radicle: embryonic root

• Endosperm: provides nourishment in monocots

• Cotyledon:provides nourishment in dicots

• Seed coat:provides protection

Plant Hormones

• What drives all of this growing and reproducing?

• HORMONES!

• Yes, plants have them too.

• Quickly, before we look at the chemical causes of growth let’s look at what plants are growing in response to

Tropic Responses

• Tropism: directional growth movement to directional stimuli

• Phototropism - light

• Gravitropism - gravity

• Thigmotropism - touch

• Skototropism - shadow

Plant Hormones

• Hormone:– Organic chemical– Transported through the organism– Goes to a target site where action is required– Active in very small quantities

Plant Hormones1. Auxins:• Generally causes softening of the cell wall, which

allows cell expansion (of vacuole)• Active in fruit development and stem elongation• Role in abscission control• Synthesized in areas of cell division• Circulation in xylem and phloem through polar

transport (in only one direction - away from synthesis site)

Plant Hormones2. Gibberelins• help to control stem elongation, seed germination,

flowering, etc.• Moves very freely through xylem and phloem (no

polar transport)• Found in shoot, young leaves, root tips, and seeds• Effects membrane permeability and loosens cell walls

(may promote softening)

Plant Hormones3. Cytokinins:

• Cell division factors

• Synthesized in root tips, developing endosperm, embryo, and possible young leaves

• Found in phloem sap

• Initiates buds, promotes nutrient mobility, and prevents senescence ( loss of chlorophyll and break down of photosynthetic apparatus)

Plant Hormones4. Absisic Acid:

• Plant growth inhibitor by antagonizing other hormones, inhibiting DNA/RNA synthesis, and changing membrane permeability

• Transported through xylem and phloem

• Prepares plant for winter, induces dormancy of buds, and regulates water loss by controlling stomata opening and closing

Plant Hormones5. Ethylene:

• Any tissue can produce it under right conditions

• Environmental conditions (stress, pathogens, mechanical action) determine production

• Controls fruit ripening, abscision, flower senescence, and stem elongation

• Is counteracted by carbon dioxide

And that’s all I have to say about that…