Plant Diversity

II: The

Evolution of Seed Plants (Ch.30)

Transforming the World

Seed replaced the spore• Zygote develops into embryos with a food supply in a

protective seed coatPollen is vehicle for sperm cells• Coated with sporopolleninReduction of the gametophyteHuge impact on human societyFive Derived Traits of Seed Plants1. Reduced gametophyte2. Heterospory3. Ovules4. Pollen5. Seeds

Transforming the World

Advantages of Reduced Gametophytes

Tiny gametophytes can develop spores retained within the sporangia of the parental sporophyte• Protected

during environmental stress

• Obtain nutrients from sporophyte

HeterosporyAll are heterosporous• Megasporangia (female

gametophyte)• Microsporangia (male gametophyte)

Figure 30.3a

(a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.)

Integument

Spore wall

Megasporangium(2n)

Megaspore (n)

Ovule and Production of Eggs

Megasporangium is within the parent sporophyteInteguments• Layer of sporophyte tissue that envelops and

protects the megasporangium• Gymnosperms have one; angiosperms have twoOvule• Megaspore, megasporangium, and

integuments

Pollen and Production of Sperm

Pollen grain• Develops from a microspore• Male gametophyte enclosed

within the pollen wall• Outer wall contains sporopollenin• Can be carried long distances by

wind and animalsPollination• Transfer of pollen to the part of

the seed plant that houses the ovule

Gymnosperms

Gymnosperm Evolution

• “Naked seeds”• Lack ovaries• Water transported by

tracheid cells• Conifers

– Cone bearing gymnosperms– Spruce, pine, fir, redwood

Gymnosperms• Division

Coniferophyta (conifers)

• Division Cycadophyta (Cycads)

• Division Ginkgophyta (Ginkgo)

• Division Gnetophyta (Welwitschia)

GinkgophytaGinkgo bilobaTolerates air pollution wellOrnamental• Usually only plant males b/c seeds

smell rancid

• Cycads• Second largest

group• Large cones• Palmlike

leaves• Thrived during

age of the dinosaurs

Cycadophyta

• Tropical or deserts• Diverse but shared

molecular data• 3 genera

– Welwitschia (deserts of southwestern Africa)

– Ephreda (produces ephedrine used as a decongestant)

– Gnetum (tropical trees, shrubs, vines native to Africa and Asia)

Gnetophyta

ConiferophytaLargest of the 4 divisions Cone• Reproductive structureMost are large treesMost are evergreensNeedles• adaptation for dry climates• Reduces # of stomata and have a thick

cuticle

• The conifers, phylum Coniferophyta, is the largest gymnosperm phylum.

– Although there are only about 550 species of conifers, a few species dominate vast forested regions in the Northern Hemisphere where the growing season is short.

– The term conifer comes from the reproductive structure, the cone, which is a cluster of scalelike sporophylls.

• Much of our lumber and paper comes from the wood (actually xylem tissue) of conifers.

– One bristlecone pine, also from California, is more than 4,600 years old.

– Redwoods from northern California can grow to heights of over 100m.

• Coniferous trees are amongst the largest and oldest organisms of Earth.

– This tissue gives the tree structural support.

Conifers include pines, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods.

Life Cycle of a Pine1. Most conifers have both ovulate and pollen cones2. Haploid microscores produced by meiosis which

develops into a pollen grain3. Ovulate cone has two ovules, each containing a

megasporangium4. Pollination occurs; pollen tube germinates that digests

the megasporangium5. Megasporocyte undergoes meiosis producing 4

haploid cells, one survives as a megaspore6. Megaspore develops into female gametophyte that

contains 2-3 archegonia, each will form an egg7. Egg matures and 2 sperm cells have formed in the

pollen tube8. Fertilization occurs (usually a year after pollination)

– One zygote becomes the embryo; ovule becomes the seed

Life Cycle of a Pine

Angiosperms

Angiosperms• Division

Anthophyta• Flowering plants• Most diverse and

geographically widespread

• Split into monocots and dicots

• Xylem is more specialized for water transport– Vessel elements

Characteristics of Angiosperms

Flowers• Specialized structure for sexual reproductionCompressed shoot with 4 whorls of modified

leaves• Sepals (nonessential)• Petals (nonessential)• Stamens (essential)• Carpels (essential)

FlowerSepals• Protect and enclose budPetals• Used to attract pollinators and

enclose reproductive structuresCarpels• Female reproductive structures• Pistil – fused carpelsStamens• Male reproductive structures

StamenEssential

flower partFilament• Supports the

antherAnther• Produces the

pollen

Carpel

Essential flower partStigma• Sticky to trap the

pollenStyle• Leads to the ovary

from the stigmaOvary• Produces the

ovules

Figure 30.13a The structure of a flower

Figure 30.13a The structure of a flower

Characteristics of Angiosperms

Fruits• Mature ovary• Protects dormant

seeds• Aid in the dispersal of

seeds• Fleshy or dry• Propellers, burrs,

edible

FruitsFruits• Typically consist of a mature ovary

(b) Ruby grapefruit, a fleshy fruitwith a hard outer layer andsoft inner layer of pericarp

(a) Tomato, a fleshy fruit withsoft outer and inner layersof pericarp

(c) Nectarine, a fleshyfruit with a soft outerlayer and hard innerlayer (pit) of pericarp

(e) Walnut, a dry fruit that remains closed at maturity

(d) Milkweed, a dry fruit thatsplits open at maturity

FruitsSimple fruit•Single ovary of

one flower

FruitsAggregate fruit•Several ovaries are part of the

same flower (raspberry)

FruitsMultiple fruit•Develops from several separate

flowers (pineapple)

                                                                              

                    

Angiosperm Life Cycle1. Microsporocytes on anther divide by meiosis producing

microspores2. Microscpore develops into a pollen grain; generative cell

divides 2 sperm; tube cell produces pollen tube3. Megasporocytes divides by meiosis producing 4 megaspores;

one survives forming the female gametophyte4. After pollination, 2 sperm cells are discharged

• Pollen tubes absorbs water and germinates; penetrates through the micropyle

5. Double fertilization occurs• One sperm fertilizes the egg zygote with rudimentary root and

cotyledons• One sperm fertilized the central cell endosperm (3N; food source)

6. Zygote develops into the embryo and is packaged in a seed7. Seed germinates developing into a mature sporophyte

Angiosperm Life Cycle

Angiosperm Diversity

Monocots

Dicots• Now called Eudicots

Basal AngiospermsMagnolids

Monocots vs. Dicots

Evolutionary Links Between Angiosperms and Animals

• Pollination of flowers by animals and transport of seeds by animals– Are two important relationships in terrestrial

ecosystems

(a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the beeand dusts it with pollen, some ofwhich will rub off onto the stigmaof the next flower the bee visits.

(c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate.

(b) A flower pollinated by hummingbirds.The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive.

Global Impact• Agriculture is based almost entirely

on angiosperms– Provide nearly all of our food

• Provide a nutritional foundation for terrestrial ecosystems

• Contributed to the cooling of the Earth and maintaining temps. by lowering CO2 levels

Products from Seed PlantsHumans depend on

seed plants for• Food• Wood• Many medicines