Anthophyta – Flowering Plants

Systematics Phylum Anthophyta characterized by

flowers Double fertilization produces triploid

endosperm and zygote Xylem mostly contains vessel

elements Ovary developes into fruit

Systematics (cont.) Amborella the most primitive known

flowering plant– Uses tracheids instead of vessel

elements– Unisexual flowers, with spirally arranged

parts

Systematics (cont.) Magnoloids also seem to be primitive

– Flower parts– Nucleic acids

Still unsettled – water lilies and “paleoherbs” are contenders as well

Monocots and eudicots are derived

Monocots vs. dicots Monocots

– One cotyledon– Flower parts in threes– Pollen is monoaperturate– Parallel venation– Primary vascular bundles scattered– Usually no secondary growth

Monocots vs. dicots Eudicots

– Two cotyledons– Flower parts in fours or fives– Pollen triaperturate– Net venation– Primary vascular bundles in ring– Secondary growth common

The Flower Determinate shoot with sporophylls Carpel is definitive structure – it is the “vessel”

of angiosperms (“vessel seed”)– Contains ovules, which develop into seeds

Flowers clustered in various ways– Stalk of single flower or inflorescence is peduncle– Stalk of individual flower in inflorescence is pedicel– Part of stalk to which floral parts attached is

receptacle

Flowers (cont.) Most flowers have modified sterile

leaves called sepals and petals– Sepals collectively = calyx

Usually green, thick

– Petals collectively = corolla Usually colored, thin

– Calyx and corolla together = perianth

Flowers (cont.) Fertile parts of flowers called stamens and

carpels– Stamens collectively = androecium

Microsporophylls Consists of filament and two-lobed anther Anther contains four microsporangia (pollen sacs)

borne in 2 pairs (defining feature of angiosperms)

– Carpels collectively = gynoecium Megasporophylls Flowers may have more than one carpel Individual carpels or fused carpels called pistil Pistil differentiated into ovary, style and stigma In fused carpels, ovary partitioned into locules

Placentation Placenta is part of ovary where ovules

originate & remain attached until mature Arrangement of placenta is placentation

– Parietal – placenta on ovary wall or extensions– Axile – placenta on central column of partitioned

ovary– Free central – placenta on central column of

non-partioned ovary– Basal – single ovule at base of ovary– Apical – single ovule at apex of ovary

Flower variations Perfect flowers have both stamens and

carpels Imperfect flowers are missing either

stamens or carpels = unisexual– Carpellate (Pistillate) flowers have carpels– Staminate flowers have stamens– Monoecious plants have staminate and

carpellate flowers on one plant– Dioecious plants have staminate and carpellate

flowers on separate plantes

Flower variations (cont.) Four floral whorls (calyx, corolla, etc.)

– If all are present, the flower is complete– If one or more is missing, the flower in

incomplete Spiral vs. whorled Joining of floral parts

– Connation – parts of same whorl united– Adnation – parts of different whorls united– If parts not joined, prefixes apo- or poly- used– If parts are joined, prefixes syn- or sym- used

Flower variations (cont.) Sepals, petals and stamens vary in points of

attachment on floral axis– Attached to receptacle below ovary – superior– Attached above ovary – inferior– Intermediates as well (still considered inferior or

partially inferior)– Insertion points

Hypogynous – perianth and stamens on receptacle below ovary

Epigynous – perianth and stamens above ovary Perigynous – stamens and petals adnate to calyx and

forming short tube around ovary

Flower variations (cont.) Symmetry

– Radial symmetry (members of each whorl alike) – regular (actinomorphic) flowers

– Bilateral symmetry (members of a whorl may be different from other members) – irregular (zygomorphic) flowers

Angiosperm life cycle The “male” story

– Microsporogenesis Sporogenous cells develop in anther Sterile cells surround the pollen sacs

– Form tapetum, which nourishes growing microspores Sporogenous cells form microsporocytes Microsporocytes divide meiotically to produce

microspores Pollen grain features formed

– Exine – sporopollenin

– Intine

Angiosperm life cycle The “male” story continues Microgametogenesis

– Microspore (n) divides mitotically to form two cells

Large tube cell Smaller generative cell Most pollen shed in this two-celled stage

– Generative cell divides mitotically to form 2 sperm cells

Angiosperm life cycle The “female” story Ovule structure

– Funiculus– Nucellus enclosed by integument(s)– Micropyle

Megasporogenesis– Early in development of ovule, single megaspore

(2n) forms– Megaspore divides meiotically to produce 4

haploid megaspores arranged linearly– 3 of the 4 megaspores disintegrate, leaving the

megaspore farthest from the micropyle

Angiosperm life cycle The “female” story continues Megagametogenesis

– Three mitotic divisions produce 8 nuclei– One group of 4 is at chalazal end of developing

gametophyte, the other is at the micropylar end– One nucleus from each group migrates to

center; these two nuclei are polar nuclei– The 3 nuclei at micropylar end become egg

apparatus: an egg cell and two synergids– The 3 nuclei at chalazal end are the antipodals

Angiosperm life cycle Megagametogenesis continues

– Cell walls form around egg apparatus cells and antipodals, creating 7-celled, 8 nucleate embryo sac – the female gametophyte

– Variations on this theme; lily is an example

Two 4-nucleate stages, with triploid cells at chalazal end

Angiosperm life cycle Double fertilization

– Pollination Stigma is glandular, receives and nourishes pollen Style contains transmitting tissue, through with or by

which the pollen tube grows towards ovule Pollen enters ovule through micropyle, and then

penetrates embryo sac via one of the synergids

– Fertilization One of the sperm fuses with egg, making zygote The other fuses with the two polar nuclei, forming

triploid endosperm

– In Gnetophytes (gymnosperm), the second fertilization produces a second embryo

Seed and fruit formation Triploid endosperm nucleus may divide

mitotically to produce nutritive tissue (endosperm)

Alternatively, the nucellus may divide to produce nutritive perisperm

Cotyledon(s) serve either to store food obtained from the other nutritive tissues, or to serve as a conduit for those nutrients to embryo

Integuments form seed coat

Seed and fruit formation Ovary wall (the pericarp) thickens and

forms fruit– Often layered

Exocarp (outer layer), mesocarp (middle layer), and endocarp (inner layer); or sometimes just exocarp and endocarp

Other parts of flower may also be incorporated in fruit as it developes

Evolution of flower Early flower

– Probably didn’t have distinct sepals and petals In water lilies, petals evolved from sepals In most angiosperms, petals evolved from stamens

– Stamens had diverse function and structure In magnoliids, stamens scented, broad, brightly

colored Some become nectaries in other angiosperms

– Carpels unspecialized in early angiosperms

Evolution of flower Evolutionary trends in flower

– Flowers with few to many parts, indefinite in number flowers with few parts, definite in number

– Floral whorls declines from four to three, two or even one

– Ovary becomes inferior; perianth includes calyx and corolla

– Radial symmetry bilateral symmetry

Mechanisms of pollination

Outcrossing– Structural

Dioecious plants In monoecious plants, staminate and carpellate flowers

in different places, and may mature at different times Dichogamy – stamens and carpels mature at different

times, even if in same flower– Protandrous– Protogynous

Genetic self-incompatibility– Gametophytic self-incompatibility– Sporophytic self-incompatibility

Mechanisms of pollination

Outcrossing– Pollinators

Flowers and insects have co-evolved!– Beetle-pollinated flowers are typically broad (with landing

platforms), dull-colored, and strong odor– Bee and wasp-pollinated flowers typically blue or yellow

with nectar guides• Orchids and other flowers may parasitize bees• Bees may parasitize legumes

– Moth- and butterfly-pollinated flowers typically have long corolla tube

• Night pollinators vs. diurnal ones

Mechanisms of pollination

Outcrossing– Pollinators

Flowers and insects have co-evolved!– Fly-pollinated often dull-colored, smell like rotting

flesh; can also resemble other types Bird pollination

– Flowers often large, red or orange and odorless, lots of nectar inaccesible to other pollinators

Bat pollination– Flowers dull-colored, strong odor, lots of nectar,

protein-rich pollen

Mechanisms of pollination

Outcrossing– Wind pollination – flowers small, no

nectar, dull color, available to wind– Water pollination – pollen filiform or

released in chains Self-fertilization

– Common in temperate and arctic areas Flowers small, inconspicuous, may not even

open!

Mechanisms of pollination

Floral pigments– Flavenoids

Anthocyanins Flavonols

– Betacyanins

Fruits Nomenclature

– Fruit is mature ovary– Fruit which includes additional floral parts is an

accessory fruit– Parthenocarpic fruits have no seeds– Simple fruit developes from one carpel or

several united carpels– Aggregate fruits come from several carpels on

one gynoecium– Multiple fruits consist of gynoceia of more than

one flower

Fruits Simple fruits

– Berries (ex. Tomato) – one to several carpels each with many seeds, inner wall fleshy

– Drupes (ex. Peach) – one to several carpels each with one seed, inner layer stony and adheres to seed

– Pome (ex. Apple) – comes from compound inferior ovary; fleshy part developes from perianth. Endocarp is tough.

Fruits Simple fruits (cont.)

– Dry simple fruits Dehiscent

– Follicle – comes from single carpel that splits down one side (milkweed)

– Legume – like follicle but splits down both sides– Silique – comes from two fused carpels; two sides split

leaving seeds on central column– Capsule – from compound ovary

Indehiscent– Achene – single seed free in cavity

• Winged achene – samara– Caryopsis – grasses; seed attached to wall– Cypsela (asters) – achene-like but comes from inferior

ovary– Nut – resemble achenes but derived from compound ovary

and have stony wall– Schizocarp (maple) – fruit splits at maturity into two or

more one-seeded portions

Fruits Dispersal

– Wind Small seeds (orchids) Parachutes (dandelions) Whole plant (tumbleweed) Shoot seeds (Impatiens)

– Water

Fruits Dispersal

– Animals Eat the fruit, or some portion

– Ripening– Elaiosomes and ants

Adhere to fur, feathers, etc.

Biochemical coevolution Secondary metabolites

– Probably evolved to protect plant from herbivores

– Some herbivores become specialists on such plants

Mimicry systems

– Response to feeding Protease inhibitors Hormone mimics Attract predators or parasites