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Angiosperms250,000 spFlower power anddiversification

Gymnosperms~730 speciesSeed power

Cycads (oldest)

Conifers (largest)

Ginkgos

Gnetales (strangest?)

Seed Plants Gymnosperms and the Seed

1. Overview of evolutionary transition toseed plants

2. Adaptations of seed plants to land

3. Gymnosperms, one of the two cladesof seed plants

Fig. 15.7Remember: most seedless vascular plants arehomsporous, but some (e.g., the lycophyteSelaginella) are heterosperous

Homosporous

Sporophyte Single typeof spore

Bisexualgametophyte(with bothkinds ofgametangia)

Eggs &sperm

Heterosporous

SporophyteMicrospore

Megaspore

Malegametophyte

Sperm

Femalegametophyte(inside thespore wall)

Eggs

The heterosporous life cycle was important in the evolution of theseed. In fact, all seed plants are heterosporous.

Heterospory + reduced, dependent gametophyte = evolution of the seed

cf Figure 30.2 Three variations on gametophyte/sporophyterelationships. Note the progression in evolutionary time fromleft to right. Also note in the seed plant (far right) how thefemale gametophyte develops inside a spore (the megaspore)which is retained in the sporophyte.

Figure 30.2 Location and function of microsporangia and megasporangia in agymnosperm.

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Gymnosperms and the Seed

1. Overview of evolutionary transition toseed plants

2. Adaptations of seed plants to land

3. Gymnosperms, one of the two clades ofseed plants

Figure 30.3 The development of a seed from an unfertilized ovule.

The three most important new adaptations to land foundin the seed plants are all shown in this diagram:

1. Very small gametophytes that are nourished by and protectedinside the parental sporophyte

2. Pollen grains, which provide protection and dispersal for the malegametophyte

3. The seed, which protects and disperses the new sporophyteembryo

A seed represents 3 generations at once!

Embryo:Daughter Sporophyte

Food Supply:Mother Gametophyte

Seed Coat:Grand-Mother Sporophyte

Figure 30.6x2 A grain of pine pollen. Microspores develop into pollen grains,which are coated with protective sporopollenin. They can be dispersed (aftertheir release from the microsporangium) by wind or animals, and contain themale gametophyte.

Figure 38.6 If a pollen grain lands in the vicinity of an ovule, it will elongate atube and release one or more sperm into the female gametophyte. This iscalled pollination. How is this mechanism of fertilization an advantage onland, compared to the method employed by seedless land plants?

Winged seed of a White Pine (Pinus strobus), a gymnosperm.

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Advantages of the Seed

• Multicellular embryo gets a “head start” atthe germination stage

• Stored food for embryo also allows a headstart, and allows for extended dormancy

• Multicellular, larger and more complex thana spore = more resistant to harshconditions, seed coat can govern dormancy

• Larger and more complex = increasedcapacity to develop dispersal adaptations

Advantages of Pollen

• Fertilization does not require water forsperm to swim through

• Pollen disperses well, allowing matingover long distances

Disadvantage of the Seed

• More energetically costly to produce• To some extent, trade off quantity for

quality

Gymnosperms and the Seed

1. Overview of evolutionary transition toseed plants

2. Adaptations of seed plants to land

3. Gymnosperms, one of the two cladesof seed plants

Figure 29.7 Hypothetical phylogeny of the seed plants Hypothetical phylogeny of the seed plants

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Hypothetical phylogeny of the seed plants Figure 30.5 Archaeopteris, a Progymnosperm

Progymnosperms first appeared in the Carboniferous (360 mya)

Heterosporous, woody, fern-likeleaves, no seeds

Gymnosperms were favored by increasing aridity during the Permian (300 mya)

Gymnosperms were dominantduring the Mesozoic (~250-65 mya, “age of the dinosaurs”)

Hypothetical phylogeny of the seed plantsFigure 30.4 The only survivingspecies in PhylumGinkgophyta: Ginkgo biloba.Ginkgo’s nice traits include thatit is very tolerant of urbanpollution, and turns a beautifulgold color in fall. A not-so-nicetrait is that the seeds, whenthey decompose, smell likevomit, or worse. Ginkgos arealso dioecious, so, if you wantto play a bad horticultural jokeon someone, plant a femaleGinkgo in their yard.

Figure 30.4 Phylum Ginkgophyta: Ginkgo biloba Ginkgo: Male (left), female (right)

Ginkgos are dioecious

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Figure 30.4x2 Ginkgo sperm is flagellated. Coniferophyta have non-flagellatedsperm, as do all the flowering plants.

Key features of Ginkgophytes

• Exist today only as planted trees• Dioecious (male and female gametophytes

on separate plants)– (vs. monoecious)

• Flagellated sperm

Hypothetical phylogeny of the seed plants Phylum Cycadophyta: cycads. These gymnosperms superficially resemble thepalms, which are actually angiosperms. Only ~130 species today, the cycadsthrived during the Mesozoic. Many cycads are insect-pollinated.

Hypothetical phylogeny of the seed plantsGnetales: Division Gnetophyta

• A fuzzy window into seedplant evolution and phylogeny

• A rare glimpse into uniqueadaptations and extremedivergence

• A real puzzle

• What are the gnetales??

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Ephedra: ~45 speciesShrubs adapted to arid conditionsPhotosynthetic stemsReduced leavesSource of ephedrin

Gnetum: ~35 speciesMoist tropicsWide leavesVines, lianasUnique female gametophyte developmentmedicinal

Welwitschia: one speciesDry - extreme xerophytetwo leavesMostly below groundLong livedExtensive taproot

The gnetalesGnetales-general characteristics

• Three genera: Ephedra, Gnetum, Welwitschia• Ephedra and Welwitschia are adapted to dry

climates, while gnetum is moist tropical• Largely disjunct distributions• Mostly dioecious• Vessels in xylem (??)• Double fertilization (??)• Evolutionarily derived traits• Pollinated by wind and insects• Important medicinales

Phylogeny of the Gnetales

– Cosmopolitan: arid regions of North America, Mexico, SouthAmerica (Ecuador to Patagonia and lowland Argentina), SEurope, Asia, and N Africa (including the CanaryIslands)

– Photosynthetic stems– Stems resemble equisetum (horsetails)– Leaves opposite or whorled, scale-like– Double fertilization– Presence of vessels in the xylem (in addition to tracheids)

Genus Ephedra (mormon tea)

~ 45 species of shrubs

Ephedra:Source of the alkaloidephedrin

Used as a decongestant, adigestive aid, and when mixedwith caffeine, is a powerfulmetabolic stimulator (used inweight control and musclebuilding

Controversial

Plant-derived chemical: ephedrine• Ephedra plants contain

ephedrine & related alkaloids• Ma huang used in traditional

Chinese medicine• Mormon tea made use of

ephedrine’s stimulant effect• Modern dietary supplements for

weight loss (now illegal)• Asthma, cold medications

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Female (ovulate) cones

Male (staminate) cones

Ephedra

– 30-40 species– Moist tropics - Pantropical (America’s, Asia,

Africa)– Wide leaves– Netted venation in leaves– Climbing vines, one tree, lianas– Possible double fertilization– Unique female gametophyte development: the

embryo nourishing provision is a post-fertilization event

Genus Gnetum

“Cleanses your intestinal tract,boosts immunity & reduces allergies”

Local name: Uña de gatto

Harvested part: Bark

Preparation: Soak in boiled water

Medicinal application: arthritis, cancer, diabetes, aids

Peru

Gnetum as a medicinal

Ginkgo leaf:parallel venation

Gnetum leaf:reticulate venation

Flower-like reproductive structures (strobili)

Gnetophytes are the onlygymnosperms with vesselelements in their xylem

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Angola

Namibia

AFRICA

Distribution of Welwitschia mirabilis

Restricted to the skeleton coast region of the Namib Desert

– 1 species (mirabilis)

– Adapted to extremely dry conditions (extreme xerophyte)

– Extensive taproot

– Can live to over 1500 years!

– Most of the stem is underground

– One pair of leaves will last the entire life of the plant, but will splitwith exposure (continuous growth)

– Cone with female flowers is quite large

– Pollination likely by wind and by a beetle

Genus Welwitschia

Desert Bushmen made maps to locatewater and game. Welwitschia plantswere used both as a source of water andon maps!

A peculiarity of this species can beseen during fertilization. The egginside the large cone migrates to thepollen tube that has beentransported into the vicinity, insteadof the pollen migrating to the egg.

The controversy• The Gnetales (Gnetum, Ephedra, Welwitschia) have been

assumed to be the closest living relatives to flowering plants.• Evidence:• double fertilization• reduced female gametophyte• leaves with netted venation (Gnetum)• cones superficially resemble flowers• xylem with vessels• However, recent molecular phylogenetic analyses place

Gnetales with other conifers, leaving the closest relative of theflowering plants an extinct lineage of gymnosperms that wehave yet to discover in the fossil record.

Gnetales and other seed-plants - Morphological studies

Anthophyte Hypothesis(Crane, 1985;Crane et al. , 1995)

Fits well with:Double fertilization(Friedman, 1990, 1994,1996)

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Gnetales and other seed-plants - Molecular studies

1) Morphological studies placed gnetalessister to angiosperms

2) Molecular studies place gnetales sister toconifers

Phylogeny

Peculiarities: xylem structure and reproduction

Gnetales have vessels in the xylem in addition totracheids - an angiosperm-like feature

Gnetales undergo a form of double fertilization, unique innon-flowering seed plants, and more like angiosperms

These odd features (for a gymnosperm),have linked gnetales to angiosperms insome researchers’ eyes, and are simplyevidence of derived traits in the eyes ofothers.

Comparison to angiosperms• Angiosperm double fertilization results in a 2N zygote and a 3N

endosperm, in gnetales both are 2N

• In both it is a sister nucleus that is fertilized (ventral canalnucleus in is sister)

• Both sperm come from a single pollen tube

• In Gnetum, the embryo nourishing provision is a post-fertilizationevent (the only non-flowering seed plant!)

• In gnetales, nourishment of the embryo depends on maternalreserves in the female gametophyte, as in other gymnosperms

So??• Is double fertilization homologous?• Convergent evolution?• Did it arise independently in gnetales and angiosperms?• Useful result in angiosperms, but in gnetales?• This needs further investigation!! A thorough study of

basal angiosperm groups may provide insights….

Important features the gnetalesshare with angiosperms

• Vessel elements in addition to tracheids in xylem

• Double fertilization in Ephedra and possibly gnetum

• Insect attractants

None of the extant gnetophytes are relatives of angiosperms

These shared characteristics likely arose independently:convergent evolution rather than shared ancestry

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Key points

• Gnetales may provide clues to the origin ofendosperm and a link to angiosperms

• They are likely more related to (sister to) theconifers than angiosperms

• Double fertilization and vessels in the xylem may have arisenmore than once in plants (separately), and their presencedoes not automatically mean relatedness or ancestry

• Main characteristics:Disjunct distributionsDioeciousEvolutionarily derived traitsPollinated by wind and insectsImportant medicinals

Phylum Coniferophyta: the conifers. This group includes pines, firs, spruces,larches, yews, junipers, cedars, cypresses, and redwoods. Many are largetrees. Though there are only about 550 species of conifers, many of them arequite dominant where they occur. A forest of Douglas fir is shown below.

Phylum Coniferophyta: Sequoia is shown below. Most conifers are evergreens,i.e. they retain their leaves throughout the year. This is thought to be anadaptation to a short growing season at higher latitudes: In winter, they canphotosynthesize during the few sunny days. In the spring, they don’t need togrow new leaves, and therefore have functioning leaves as soon as the firstsunny days arrive.

Phylum Coniferophyta: Cypress. This photo is of the “Lone Cypress,” offshoreof Carmel, probably one of the most famous conifers in the world.

Monterey pine (Pinus radiata) near Ano Nuevo, just north of SantaCruz, CA. It is locally dominant in its small native range: 3 spotsalong the California coast, and two Mexican Islands. It is also themost widely planted tree in the world for forestry, being theprimary timber tree in New Zealand, Australia, Chile, Argentina,South Africa, and elsewhere. In some of those places, it hasbecome a problematic invasive plant as well.

Figure 30.6 The life cycle of a pine (Layer 3)

takes>1 year

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A closer look at pine cones (Pinus sp.): male (microsporangiate) cones

Pollen isdispersed bywind.

Conifer Ecology

• Often locally dominant species• Include some of the largest, oldest

organisms on Earth• Evergreens with needles - adapted to

conserve water, deal with short growingseason

• Wind-dispersed pollen• Multi-year lifecycle