chapter 29 sadava the evolution of seed plants n.
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29. Chapter 29 - Sadava The Evolution of Seed Plants. 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.

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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


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

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

Vascular, seed plants

Vascular, seedless – Ferns

Nonvascular – Mosses

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)


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

seed plants microsporangium
Seed Plants - Microsporangium

In 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


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.

after pollination
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

Pumpkin seeds


Seed contain tissues from three generations

  • Seedcoat (from integument) and megasporangium develop from ovule tissues of diploid sporophyte parent
      • 2n outside
  • Within megasporangium is haploidfemale gametophytetissue, 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 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

embryophytes vascular plants euphyllophytes true leaves seed plants gymnosperms
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms



ovules and seeds are not protected by ovary or fruit tissue

Pinus longaeva, Bristlecone pine, cone and needles

Four major groups of living gymnosperms:


EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

Four major groups of living gymnosperms:

Ginkgos:Ginkgophyta—one living species, Ginkgo biloba

EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

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

Branch of female Ginkgo biloba with leaves and ovules

Four major groups of living gymnosperms:


EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms

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

Four major groups of living gymnosperms:

Conifers:Coniferophyta— the cone bearers

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

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.

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)


Pine life cycle:

Wind carries pollen grains from microstrobilus to megastrobilus



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

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 scales


Stone pine cone with pine nuts - note two nuts (seeds) that develop from the ovule under each cone scale


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

embryophytes vascular plants euphyllophytes true leaves seed plants angiosperms
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsAngiosperms


“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

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



Xylem with vessel elements and fibers

Phloem with companion cells


Phalaenopsis Orchid

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)


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


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


Stamens bear microsporangia


consist of filament and anther



Carpels bear megasporangia


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


In this example, the pistil is a single carpel

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


Types of flowers:

Perfect flowers

have both mega- and microsporangia

Imperfect flowers

either mega or microsporangia, but not both

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

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.

Ovary and seeds develop into fruits

Fruit protects seed and aids in dispersal, (e.g., can become attached to or eaten by animals)

Angiosperms - Fruit

Types of fruits:

Simple fruits develop from one carpel

Plums, cherries

Aggregate fruits develop from several carpels on same flower


Multiple fruits form from a cluster of flowers


Accessory fruits develop from parts other than carpels

Apples, pears, strawberries

Angiosperms - Fruit

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



embryonic axis

Most angiosperms are in two major clades:


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
Monocots and Eudicots Are Not the Only Surviving Angiosperms

Water lilies

Amborella trichopoda

Star anise

Black pepper

Dutchmean’s pipe


Magnoliid clade





Date palm



Prickly pear cactus

Catclaw Brier


how do plants support our world
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

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

Many medicines come from seed plants

Medicines are found by screening large numbers of plants, or screening large numbers of chemical compounds