Chapter 29 - SadavaThe Evolution of Seed Plants

29

Late in Devonian, some plants developed secondary growth– Thickened woody

stems of xylem– The first species with

secondary growth were progymnosperms seedless vascular plants, now extinct

http://www.futura-sciences.com/uploads/tx_oxcsfutura/comprendre/d/images/600/fig_1213.jpg

Earliest seed plants from Devonian– These seed ferns were also woody– Clades of seed ferns are known only

as fossils– Two of the clades are basal to

surviving seed plants

http://www.palaeos.com/Plants/Lists/Glossary/Images/SeedFern.jpg

Surviving seed plants fall into two groups:– Gymnosperms: conifers and cycads– Angiosperms: flowering plants

Gametophyte generation is reduced even further than it is in ferns– Haploid gametophyte develops partly or

entirely attached to sporophyte

Nonvascular – Mosses

Vascular, seedless – Ferns

Vascular, seed plants

Seed plants are heterosporous – Produce two types of spores

• One becomes female gametophyte, one becomes male gametophyte

• Seed plants form separate megasporangia (female) and microsporangia (male)

http://bio1151.nicerweb.com/doc/class/bio1151/Locked/media/ch30/30_03OvuleToSeedA.jpg

The Seed Plants - Megasporangium

• Megasporangium• Contains the

megagametophyte• develops into an

egg that is eventually fertilized next generation (sporophyte)

• Surrounded by integument made up of sporophyte structures

• Megasporangium and integument together form ovule

• Will become seed after fertilization

http://herbarium.usu.edu/teaching/4420/seed.GIF

Seed Plants - MicrosporangiumIn microsporangium,

microspores divide mitotically to produce the male gametophyte, or pollen grain– Walls of pollen grains

contain sporopollenin, the most resistant biological compound known protects pollen grain from chemicals and dehydration

Pollination– When pollen grain

lands near a female gametophyte

– Pollen tubes are produced that digests way through sporophyte tissue to megagametophyte

– Sperm are released from the tube, and fertilization results in a diploid zygote.

http://pix.botany.org/Setabot/abot-90-5_700.jpg

After Pollination

Resulting diploid zygote divides to produce an embryonic sporophyte – Growth is then

suspended– Embryo enters a

dormant stage, with the end product being a multicellular SEED

http://nosheteria.com/uploaded_images/PumpkinSeeds.jpg

Pumpkin seeds

SeedSeed contain tissues from three

generations• Seed coat (from integument)

and megasporangium develop from ovule tissues of diploid sporophyte parent

• 2n outside• Within megasporangium is

haploid female gametophyte tissue, contains nutrients for next generation

• 1n• Third generation, the embryo,

is the new diploid sporophyte is contained in the center of seed package.

• 2n inside – new generation

Seeds• Seeds are well-protected resting stages.

• May remain viable for many years, germinating when conditions are favorable.

• Seed coat protects from drying out as well as predators.

• Many seeds have adaptations for dispersal.

Besides seeds, secondary growth also contributes to success of seed plants– Wood: proliferated xylem,

gives support and allows plants to grow above their competitors for sunlight

http://cache.eb.com/eb/image?id=72251&rendTypeId=35 http://mayhem-chaos.net/photoblog/images/pine_tree_med.jpg

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

– Gymnosperm• “naked-seeded” • ovules and seeds are not protected by

ovary or fruit tissue

http://www.conifers.org/pi/pin/lambertiana5.jpg

Pinus longaeva, Bristlecone pine, cone and needles

Four major groups of living gymnosperms:

• Cycads: Cycadophyta

http://www.biologyreference.com/images/biol_02_img0213.jpg

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

Four major groups of living gymnosperms:

• Ginkgos: Ginkgophyta—one living species, Ginkgo biloba

Branch of male Ginkgo biloba with clusters of pollen-producing microsporophylls

Branch of female Ginkgo biloba with leaves and ovules

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

Four major groups of living gymnosperms:

• Gnetophytes: Gnetophyta

Ephedra viridis (Mormon tea or joint fir) with cones; Arizona

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

Four major groups of living gymnosperms:

• Conifers: Coniferophyta— the cone bearers

http://www.nasa.gov/centers/goddard/images/content/156030main_Conifers_JPG.jpg

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

• All living gymnosperms except gnetophytes have only tracheids for water conduction and support• Gymnosperms were

dominant during Mesozoic • Tallest gymnosperms are

California coastal redwoods

• over 100 m

• Angiosperms have vessel elements and fibers alongside of tracheids

Tracheids and fiber tracheids

http://www.uri.edu/cels/bio/plant_anatomy/images/47.gif

Gymnosperms were dominant during the Mesozoic, until about 65 Mya. – Today, conifers still

dominate many forests, especially at high latitudes and altitudes.

– The oldest living organism on Earth is a bristlecone pine that germinated about 4,800 years ago.

http://media.rd.com/rd/images/rdc/family-travel/poi/CA-poi-ancient-bristlecone-pine-forest-af.jpg

Male and female cones contain the reproductive structures of conifers.– Megastrobilus (female, seed-bearing Cone)

• Modified stem, bearing a tight cluster of woody scales (reduced branches)• Produces seeds

– Microstrobilus (male, pollen-bearing cone)• “Cone-like” structure• Scales are modified leaves; not woody• Microsporangia produces microspores pollen grains (microgametophyte)

Gymnosperms

Pine life cycle:– Wind carries pollen grains from

microstrobilus to megastrobilus

Gymnosperms

The Gymnosperms: Naked Seeds

• Pollen grains enter ovule through a small opening in the integument at tip of the ovule called the micropyle

• 2 sperm enter• 1 fertilizes• 1 degenerates

www.botany.hawaii.edu/faculty/webb/BOT201/Conifers

Pollen grains. Note the swollen bladders which helps them float in air currents.

Most conifer ovules are born on the upper surfaces of the cone scalesStone pine cone with pine nuts - note two nuts (seeds) that develop from the ovule under each cone scale

Gymnosperms

Gymnosperms• Some pine cone scales (e.g., longleaf and slash

pines) can only be opened by fire to release the seeds• helps insure new growth after fires.

• Some conifers have soft, fleshy, fruit-like tissue around seeds (e.g., juniper and yew “berries”)

• Animals may eat these and then disperse the seeds in their feces.

• Not true fruits

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsAngiosperms

• Angiosperms:• “enclosed seed”• Oldest fossils are

Jurassic, 150 my old• Explosive species

radiation angiosperms became dominant during Tertiary in only 60 million years

• Over 250,000 species exist today

http://gpc.edu/~pgore/myphotos/fossils/angiosperm-leaf.jpg

Extreme of evolutionary trends in vascular plants:– Sporophyte generation becomes larger,

gametophyte become smaller– Female gametophyte even more reduced

— usually only seven cells

• Synapomorphies (shared, derived traits) in angiosperms:

• Double fertilization• Triploid endosperm –

nutritive tissue in seeds• Ovules and seeds enclosed in

a carpel• Flowers• Fruits• Xylem with vessel elements

and fibers• Phloem with companion cells

http://www.gallerym.com/images/work/big/schatz_howard_Botanica-%20Orchid%20Phala%20001_L.jpg

Phalaenopsis Orchid

Angiosperms

Double fertilization:– Microgametophyte has two

male gametes• One nucleus combines

with egg• Second nucleus combines

with two haploid nuclei of female gametophyte to form triploid nucleus endosperm (3n)

–Endosperm nourishes developing sporophyte (embryo)

http://www.lclark.edu/~seavey/images%20/carpel.jpg

Angiosperms

Angiosperm (“enclosed seed”)– Ovules and seeds are

enclosed in a modified leaf called a carpel• Carpels provide

protection• May interact with pollen

to prevent self-pollination

– Angiosperms also produce flowers and fruits

Angiosperms

– Fiber cells: important in supporting plant body (associated with xylem)

http://student.nu.ac.th/u46410387/fiber.jpeg

Flowers– Stamens bear

microsporangia• Male

• consist of filament and anther

Angiosperms

Flowers– Carpels bear

megasporangia

• Female

• One or more carpels form the pistil — stigma, style, ovary and ovule

In this example, the pistil is a single carpel

Angiosperms

• Flowers often have other specialized leaves that often play a role in attracting pollinators• Petals – inner whorl

(collectively, the corolla)

• Sepals – outer whorl (collectively, the calyx)• Calyx protects

immature flower in bud before it opens

Angiosperms

Types of flowers:– Perfect flowers

• have both mega- and microsporangia

– Imperfect flowers• either mega or

microsporangia, but not both

http://leon.ifas.ufl.edu/images

Angiosperms – Perfect and Imperfect Flowers

– Monoecious: “one-housed”• male and female flowers occur on

the same plant or in perfect flowers

– Dioecious: “two-housed”• male and female flowers on

different plants• separate sexes• Have to have imperfect flowers

• Perfect flowers: • favors self-pollination, but

usually disadvantageous• many mechanisms have

evolved to circumvent this problem

• bush monkeyflower is constructed so that two different hummingbirds must participate in pollination

http://www.dkimages.com/discover/previews/931/50375608.JPG

Mimulus aurantiacus, bush monkeyflower

Most angiosperms are animal-pollinated by insects, birds, and bats– Many flowers entice

pollinators with nectar and pollen

– Plants and their pollinators have coevolved

– Some relationships are very specific—e.g., one species of moth pollinates one species of yucca plant.

http://bobklips.com/BOBS_WEBSITE/ILF-YUCCFILAlaying.jpg

Figure 29.14 The Life Cycle of an Angiosperm

Ovary and seeds develop into fruits– Fruit protects seed and aids in dispersal,

(e.g., can become attached to or eaten by animals)

http://www.hickerphoto.com/data/media/152/kiwi-fruit_3629.jpg

Angiosperms - Fruit

Types of fruits: – Simple fruits develop from one carpel

• Plums, cherries– Aggregate fruits develop from several carpels on same flower

• raspberries – Multiple fruits form from a cluster of flowers

• pineapples– Accessory fruits develop from parts other than carpels

• Apples, pears, strawberries

Angiosperms - Fruit

http://www.youtube.com/watch?v=bwCpQflmQG4

Angiosperm life cycle:– Zygote develops into an embryo

• consists of embryonic axis (will become stem and root), and 1 or 2 cotyledons — seed leaves

• Cotyledons absorb and digest endosperm, some become photosynthetic

www.botany.hawaii.edu/faculty/webb/Bot201/Angiosperm/MagnoliophytaLab99

cotyledons

embryonic axis

Angiosperms

Most angiosperms are in two major clades:– Monocots

• One cotyledon• Grasses, cattails,

orchids, palms– Eudicots (dicots)

• Two cotyledons• Majority of familiar

flowering plants, most herbs, trees, shrubs

– Other clades include star anise and relatives, water lilies, and magnoliids

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsAngiosperms

Monocots and Eudicots Are Not the Only Surviving Angiosperms

Water lilies

Star anise

Avocado

Magnoliid clade

Black pepper Dutchmean’s pipe

Amborella trichopoda

Date palm

Lilies

Wheat

Monocots

Prickly pear cactus

Dogwood

Catclaw Brier

Eudicots

How Do Plants Support Our World?

Plants contribute to ecosystem services: processes by which environment maintains resources that benefit humans– Plants are primary producers: photosynthesis

traps energy and carbon, making them available for their own needs and for herbivores and omnivores that consume them, and for the whole food chain

– Plants produce O2 and remove CO2 from the atmosphere

– Contribute to soil formation and soil fertility– Roots hold soil in place, preventing erosion– Moderate local climate by increasing humidity, providing

shade, and blocking wind.

Seed plants are our primary food source– Twelve are most

important: rice, coconut, wheat, corn, potato, sweet potato, cassava, sugarcane, sugar beet, soybean, common bean, banana

http://scienceafrica.com/cms/wp-content/uploads/2010/06/cassava-ethanol.jpg

http://3.bp.blogspot.com/_vhhw--dmXBE/SEKkOrSMjVI/AAAAAAAAAtw/E-3uz7yigtU/s400/

biopact_cassava_biofuels.jpg

Cassava root is an important

food in Africa

– Half the world’s population gets most of its food energy from rice (Oryza sativa)

• Rice been cultivated for more than 8,000 years

http://www.equita.qc.ca/images/produits/gn_008.jpg

Many medicines come from seed plants– Medicines are found

by screening large numbers of plants, or screening large numbers of chemical compounds

http://www.mountainvalleycenter.com/store/images/cherheal.JPG