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Angiosperms: PhylumAnthophyta, the flowering plants1. Overview of seed plant evolution

2. Traits of flowering plantsa) Flowersb) Fruits/Seedsc) Monocots vrs. dicots

3. The angiosperm life cycle

4. Pollen and seed (fruit) dispersal

Figure 29.7 Land plant evolution.

Three variations on gametophyte/sporophyte relationships.

In angiosperms, female gamteophyte is reduced even further than in gymnosperms.

Figure 30.3 Review: Development of a seed from an unfertilized ovule.

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

1. Very small gametophytes that are nourished by and protected insidethe parental sporophyte (reduced even further in angiosperms)

2. Pollen grains, which provide protection and dispersal for the malegametophyte (often animal-dispersed in angiosperms)

3. The seed, which protects and disperses the new sporophyte embryo(angiosperms: develop in flowers, dispersed in fruits)

Hypothetical phylogeny of the seed plants

• Angiosperms– Are commonly known as flowering plants– Are seed plants that produce the reproductive

structures called flowers and fruits– Are the most widespread and diverse of all

plants

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Angiosperm evolution

• First angiopserms appeared duringMesozoic, ~ 140 mya

• By the end of the mesozoic (~65 mya),angiosperms dominated many landscapes

• Coevolution between plants and animalsvery important– Herbivores– Pollinators– Seed (often via fruit) dispersers

Amborella• The most primitive

angiosperm has moderatesized, perfect flowers,

• Parts are spirally arrangedand have a moderate numberof parts.

• Floral evolution movestowards unisexual flowers,larger AND smaller flowersizes, other periantharrangements, and largerAND smaller numbers offlower parts.

Magnolias• In one view, the most

primitive angiospermresembled a Magnolia

• had large, showy,unspecialized, spirallyarranged flowers.

• No fusion of parts

• little differentiation ofparts.

Hypothesis for the evolutionary origin of the carpel from a reproductive leaf(sporophyll)

The ‘Mostly Male’ hypothesis:

• Angiosperms evolved from malegymnosperms

• Pollen and ovule producing structurescombined into a single flower: mutation

• Mutation: ovules developed on somemicrosporophylls = carpels

• Evidence: Flower development genes aresimilar to pollen producing gymnospermgenes

• Position of ovules can be easily changedwith mutations

Angiosperms: Phylum Anthophyta,the flowering plants

1. Overview of seed plant evolution

2. Traits of flowering plants

3. The angiosperm life cycle

4. Pollen and seed (fruit) dispersal

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Traits of angiosperms

• Vascular seed plants that produce flowersand fruits

• Extremely diverse: ~250,000 known species(compare to 720 gymnosperm spp.)

• Xylem tissue not only has tracheids, but alsofibers and vessel elements (exceptAmborella)

• Life cycle includes “double fertilization”

Flowers• A flower is a specialized shoot with modified

leaves– Sepals, which enclose the flower– Petals, which are brightly colored and attract

pollinators– Stamens, which produce pollen– Carpels, which produce ovules

Figure 30.7

Anther

Filament

Stigma

Style

Ovary

Carpel

Petal

ReceptacleOvule

Sepal

Stamen

Fruits• Fruits

– Typically consist of a mature ovary

Figure 30.8a–e

(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

• Can be carried by wind, water, or animals tonew locations, enhancing seed dispersal

Figure 30.9a–c

Wings enable maple fruits to be easily carried by the wind.

(a)

Seeds within berries and other edible fruits are often dispersed in animal feces.

(b)

The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals.

(c)

Figure 36.2 An overview of transport in whole plants (Layer 4)

Two kinds of vasculartissue facilitate flowthrough plants:

1. Xylem, whichconducts water andmineral nutrients

2. Phloem, whichconducts sugars

Cells in the xylem tissue of angiosperms

In gymnosperms,the only cell typefound in xylemtissue istracheids.

Angiosperms(exceptAmborella) haveboth tracheidsand vesselelements forconduction, aswell as fiber cellsfor support.

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Figure 38.6 Growth of the pollen tube and double fertilization Figure 38.6 Growth of the pollen tube and double fertilization

Figure 38.6 Growth of the pollen tube and double fertilization Angiosperm seeds consist of diploid andtriploid tissues

Embryo:Diploid (from fertilized egg)

Food Supply:TriploidEndosperm(from polar nucleiand second sperm)

Seed Coat:Diploid(from ovule wall)

Angiosperm seeds consist of diploid andtriploid tissues

Cotyledons(“seed leaves”)

Why double fertilization?

• According to one hypothesis, doublefertilization synchronizes development offood supply (endosperm) withdevelopment of the embryo: if nofertilization of the egg occurs, noresources are wasted on building the foodsupply

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Figure 30.7 The structure of a flower. Flower = modified reproductive shootwith 4 circles (“whorls”) of modified leaves

Protects flowerbefore it opens

When brightly colored, aid inattracting animal pollinators

“Male” parts,producemicrospores,whichdevelop intopollen grains

“Female” parts,producemegaspores &thus femalegametophytes,and ultimatelyovules andseeds

How does thisdiffer from thereproductivestructures ofthe other seedplants, thegymnosperms?

More flower terminology

All sepals together= “calyx”

All petals together =“corolla”

All stamenstogether =“androecium”

All carpelstogether =“pistil” or“gynoecium”

Ovules found incavity = “locule”

Ovary wall(pericarp) growsinto fruit

Flower terminology: symmetry Trillium: a complete flower (has all 4 basic floral organs)

Grass flower: incomplete (lacks petals)

Note: most grasseswind-pollinated

Why might petals beunimportant forgrasses?

Bisexual (perfect) flowers have both stamens and carpels, like this Lily

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Begonia, a monoecious species with unisexual (imperfect) flowers.(staminate flowers (left), carpellate flowers (right).

Begonia, a monoecious species with unisexual (imperfect) flowers.(staminate flowers (left), carpellate flowers (right).

In monoecious species, both sexes of flowers are found on a single plant.

Begonia, a monoecious species with unisexual (imperfect) flowers.(staminate flowers (left), carpellate flowers (right).

In monoecious species, both sexes of flowers are found on a single plant.

Why are the carpels and stamens similarly colored yellow?

Sagittaria: a dioecious species, with staminate flowers (left), and carpellateflowers (right) growing on separate plants.

Grape hyacinth has flowers clustered in inflorescences.

A solitary flower grows on a stalk called a peduncle

Individual flowers in an inflorescencegrow on pedicels

The whole inflorescence is attachedto a peduncle

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Pyrethrum, a composite flower (a special kind of inflorescence). Notice the twotypes of flowers, disc flowers (center) and ray flowers (at edge, each with apetal).

Hypogynous

Perigynous Epigynous

Figure 30.12 Representatives of major angiosperm clades

•Flowering plants usedto be divided into twogroups: monocots anddicots

•We now know thatthere are a number ofdifferent lineages thathave the dicotmorphology

•Monocots are amonophyletic group

Figure 30.12 Representatives of major angiosperm clades

•Flowering plants usedto be divided into twogroups: monocots anddicots

•We now know thatthere are a number ofdifferent lineages thathave the dicotmorphology

•Monocots are amonophyletic group

Figure 30.12 Monocots vs. Dicots Figure 30.12 Monocots vs. Dicots

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Monocots vs. non-monocots(“dicots” = eudicots and other

lineages)• Monocots (incl. lilies, orchids, yuccas, palms,

grasses) usually have parallel veins in leaves, asingle cotyledon, fibrous root systems, floralparts in multiples of three, and complexlyarranged vascular bundles in stem

• Dicots usually have net-like venation, twocotyledons, a taproot system, floral parts inmultiples of four or five, and vascular bundlesarranged in a ring in stem

Even more flower terminology• #-merous: having parts # of each kind. E.g. 3-

merous = flower parts in 3’s.

• a-: lacking. E.g. apetalous = no petals.

• Floral part arrangement:– Whorled: parts in a circle, on a single plane– Spiral: parts spiral around floral axis or receptacle

Review of flower terminology

• 4 floral organs: sepal, petal, stamen, carpel• radial vs. bilateral symmetry• complete vs. incomplete flowers• bisexual (perfect) vs. unisexual (imperfect)

flowers• monoecious vs. dioecious plants• inflorescences and composite flowers• ovary position

Figure 38.2 Simplified overview of angiosperm life cycle

Relationship between a pea flower and a fruit (pea pod). Fruit = mature ovary.Fruits protect seeds and aid in their dispersal. Endosperm-derived foods

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Why double fertilization?

• According to one hypothesis, doublefertilization synchronizes development offood supply (endosperm) withdevelopment of the embryo: if nofertilization of the egg occurs, noresources are wasted on building the foodsupply

Figure 38.7 The development of a dicot plant embryo

Figure 38.7 The development of a dicot plant embryo Figure 38.7 The development of a dicot plant embryo

Figure 38.7 The development of a dicot plant embryo Figure 30.10 Detailed life cycle of an angiosperm

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Seed adaptations for survival andgermination

• Many seeds exhibit dormancy, a temporary condition oflow metabolism and no growth or development. Someseeds can survive like this for decades or more. What arethe potential benefits of dormancy?

• Dormancy in some seeds is simply broken by favorableenvironmental conditions, but others only germinate afterspecific cues

• What would you expect the cues to be for seeds living indeserts, fire-prone habitats (such as California chaparral),or at high latitudes? How about for seeds borne in berrieseaten by mammals?

Figure 38. 8 Review: Three types of seed structure

Figure 38. 8 Review: Three types of seed structure Figure 38. 8 Review: Three types of seed structure

The four steps of seed germination: 1. imbibition of water,2. enzyme digestion of stored food, 3. embryo begins growth and radicle ispushed through the seed coat, and 4. shoot tip grows toward soil surface.

Germination of a barley seed is shown below.

Figure 38.10 Two ways that young shoots break through the soil surface.