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 coat Pollen is vehicle for sperm cells Coated with sporopollenin Reduction of the gametophyte Huge impact on human society Five Derived Traits of Seed Plants 1.Reduced gametophyte 2.Heterospory 3.Ovules 4.Pollen 5.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

Heterospory All 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 sporophyte Integuments Layer of sporophyte tissue that envelops and protects the megasporangium Gymnosperms have one; angiosperms have two Ovule 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 animals Pollination 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)

Ginkgophyta Ginkgo biloba Tolerates air pollution well Ornamental 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

Coniferophyta Largest of the 4 divisions Cone Reproductive structure Most are large trees Most are evergreens Needles 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 Pine 1.Most conifers have both ovulate and pollen cones 2.Haploid microscores produced by meiosis which develops into a pollen grain 3.Ovulate cone has two ovules, each containing a megasporangium 4.Pollination occurs; pollen tube germinates that digests the megasporangium 5.Megasporocyte undergoes meiosis producing 4 haploid cells, one survives as a megaspore 6.Megaspore develops into female gametophyte that contains 2-3 archegonia, each will form an egg 7.Egg matures and 2 sperm cells have formed in the pollen tube 8.Fertilization occurs (usually a year after pollination) –One zygote becomes the embryo; ovule becomes the seed

Life Cycle of a Pine

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 reproduction Compressed shoot with 4 whorls of modified leaves Sepals (nonessential) Petals (nonessential) Stamens (essential) Carpels (essential)

Flower Sepals Protect and enclose bud Petals Used to attract pollinators and enclose reproductive structures Carpels Female reproductive structures Pistil – fused carpels Stamens Male reproductive structures

Stamen Essential flower part Filament Supports the anther Anther Produces the pollen

Carpel Essential flower part Stigma Sticky to trap the pollen Style Leads to the ovary from the stigma Ovary Produces the ovules

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

Fruits Typically consist of a mature ovary (b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp (a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp (c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp (e) Walnut, a dry fruit that remains closed at maturity (d) Milkweed, a dry fruit that splits open at maturity

Fruits Simple fruit Single ovary of one flower

Fruits Aggregate fruit Several ovaries are part of the same flower (raspberry)

Fruits Multiple fruit Develops from several separate flowers (pineapple)

Angiosperm Life Cycle 1.Microsporocytes on anther divide by meiosis producing microspores 2.Microscpore develops into a pollen grain; generative cell divides  2 sperm; tube cell produces pollen tube 3.Megasporocytes divides by meiosis producing 4 megaspores; one survives forming the female gametophyte 4.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 seed 7.Seed germinates developing into a mature sporophyte

Angiosperm Life Cycle

Angiosperm Diversity Monocots Dicots Now called Eudicots Basal Angiosperms Magnolids

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 bee and dusts it with pollen, some of which will rub off onto the stigma of 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 CO 2 levels

Products from Seed Plants Humans depend on seed plants for Food Wood Many medicines