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    Evolution of Anthophytes: Flowers and Fruit

    Two synapomorphies distinguish anthophytes other plant lineages.

      Flower, composed of whorls of

      • megasporophylls (carpels)
      • microsporophylls (anthers)
      • sterile, specialized leaves comprising the perianth

      Fruit

      • the mature ovary (sporophyte tissue)
      • often includes other flower parts
      • contains the seeds, which carry
        • female gametophyte (early)
        • new sporophyte (late)

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    The Flower

    A flower is a shoot that terminates in leaves specialized for reproduction.
    Even the sterile leaves may assist in procuring sex.

    Sterile structures:

    • petal (single) and corolla (collective petals)
    • sepal (single) and calyx (collective sepals)
    • perianth (the petals and sepals together)

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    Flower Microsporophyll: The Stamen

    Male structures:
    • stamen (microsporophyll)
      • anther (contains microsporangia)
      • filament (lower portion of microsporophyll)
    • pollen develops from a microspore
      • Pollen is NOT sperm, nor is it homologous to sperm.
      • Pollen is the mature male gametophyte.
      • Pollen lack antheridia.
      • Each pollen produces two sperm.

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    Flower Megasporophyll: The Carpel

    Female structures:
    • carpel (megasporophyll)
      • stigma (pollen landing pad)
      • style (filamentous tube)
      • ovary (ovulary) (contains megasporangia)

    A flower may have one or more carpels.
    The entire female structure comprises a pistil,
    which may be composed of one to many carpels.

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    Evolution of the Flower

    Flowers of a given plant family
    • have specific characteristics
    • tend not to vary within a family

    The earliest flowers most likely

    • did not have distinct sepals and petals
    • were not showy

    Natural selection exerted by pollination changed that.

    • parts of the perianth became showy
      and attractive to pollinators
    • either sepals or stamens evolved into petals
      • sepals - same number of vascular strands as the leaves (water lilies)
      • petals (and most stamens) - usually only one vascular strand (almost everyone else)

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Trends in Flower Evolution
As insect pollinators drove the earliest evolutionary radiations of angiosperm flowers, four major trends appeared.

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    1. Reduction of Metamerism

    Primitive condition
    • many copies of each flower part
    • indefinite number of each part

    Derived condition

    • reduced number of each flower part
    • specific number of each part
      • varies with species
      • multiples of 4-5 (dicots)
      • multiples of 3 (monocots)

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    2. Reduced number of floral whorls

    Primitive condition
    • many whorls of perianth components
    • flower parts separate

    Derived condition

    • reduced number of perianth components
    • flower parts often fused

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    3. Radial Symmetry --> Bilateral Symmetry

    Primitive condition
    • flower exhibits radial symmetry
    • flower is said to be actinomorphic
    • actin means "ray" in Greek

    Derived condition

    • flower exhibits bilateral symmetry
    • flower is said to be zygomorphic
    • zygo means "yoke" in Greek

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    4. Shift of Ovary Position

    From more primitive condition to more serived condition:
    • hypogynous flower = superior ovary
      • perianth and stamens attached below ovary
    • perigynous flower = intermediate ovary
      • perianth and stamens attached around ovary, forming a cup
    • epigynous flower = inferior ovary
      • perianth and stamens attached above ovary, surrounding it

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Complete flower: Hibiscus

    Flowers:
    Complete vs incomplete

  • A complete flower has
    sepals, petals, stamens, and pistils.

  • An incomplete flower lacks
    one or more of the parts listed above.


    The hibiscus on the left has all flower parts present.

    The bottlebrush on the right has flowers lacking petals.
    (The stamens have evolved into the pollinator-attractors.)

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Incomplete flower: Bottle Brush

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Imperfect flower: female begonia

    Flowers: Perfect vs Imperfect

  • A perfect flower has both stamens and pistils.

  • An imperfect flower has only stamens or only pistils.

    (By definition, an imperfect flower
    is also an incomplete flower.)

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Imperfect flower: male begonia

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    Flowers: Simple and Compound

    A flower may be simple or compound ( = composite).

    • simple flower - single bloom on one peduncle
    • compound flower - multiple blooms on one peduncle.

    A cluster of flowers arranged on a single stem
    is called an inflorescence.

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    The Final Product: Fruit

    A fruit is a mature ovary.
    It comprises three alternating generations.

    First generation (parent sporophyte tissue)

    • pericarp
      • exocarp
      • mesocarp
      • endocarp
    • seed coat

    Second generation (female gametophyte tissue)

    • 8-nucleate embryo sac is mostly gone when fruit is mature

    Third generation (offspring sporophyte tissue)

    • embryo (from ovum + sperm)
    • endosperm (from polar nuclei + sperm)

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    Simple, Aggregate, and Multiple Fruit

    A simple fruit is formed from a single flower with a single pistil.
    • tomato
    • blueberry
    • peanut
    • peach
    • banana

    An aggregate fruit is formed from a single flower with many pistils borne on the same receptacle. One peduncle per fruit.

    • strawberry
    • blackberry
    • raspberry

    A multiple fruit is formed via fusion of a cluster of individual flowers in a single inflorescence. Each "fruitlet" has its own receptacle and its own pedicel.

    • pineapple
    • kiwi fruit
    • mulberry
    • fig

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    Accessory Fruit

    An accessory fruit is one in which the fleshy pericarp is composed of floral tissue in addition to the walls of the ovaries.

    pistil of an apple blossom (an epigynous flower) develops into the papery apple "core" and a bit of adjacent tissue. The fleshy, edible part of the fruit is derived from the receptacle, which has grown up to surround the pistil.

    the "seeds" on the outside of a strawberry are the actual fruits. The fleshy, sugar-filled "fruit" designed to attract seed dispersers such as birds, monkeys or yourself is actually the receptacle that has evolved the function of attracting animal seed dispersers.

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    Categorizing Fruit

    At seed maturity, fruit can be

    • fleshy - mesocarp moist and pulpy at seed maturity
      • berry
      • drupe
      • pome

    • dry - mesocarp dry at seed maturity

      • dehiscent - splits open to release seeds
        • follicle
        • legume
        • capsule

      • indehiscent - does not split to release seeds

        • achene
        • nut
        • grain

      (There are several other types of dry fruit,
      but you can learn those in BIL 226.)

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    Fleshy Fruit: The Berry

  • true berry
    • fleshy endocarp, mesocarp and exocarp
    • one to many seeds

  • pepo
    • thick, rigid pericarp
    • numerous seeds separated from pericarp

  • hesperidium
    • aromatic, leathery exocarp
    • mesocarp and endocarp compartmentalized
      into sections (each representing a carpel)
    • each carpel contains one to several seeds
    • many parthenogenetic varieties are popular in the produce section

    Fleshy Fruit: The Drupe

    A drupe
    • has a single seed enclosed in a hard endocarp
    • endocarp may be called a "stone" or "pit."
    • exocarp soft and thin
    • mesocarp soft and fleshy

        These are drupes!
        • peach
        • apricot
        • plum
        • mango
        • olive

        These are drupes, not nuts!
        • coconut
        • walnut
        • almond
        • pecan
        These are clusters of drupelets, not berries!
        • raspberries
        • blackberries

    Fleshy Fruit: The Pome

    A pome
    • develops from an epigynous flower
    • has a papery/leathery endocarp formed by the ovary walls
    • Five carpels; endocarp is star-shaped in cross section
    • has a thin, soft exocarp
    • has a fleshy mesocarp
    • exocarp and mesocarp develop from the epigynous receptacle
    • Thus, a pome is an accessory fruit

    Most pome-bearing plants are in the Rosaceaea, the Rose Family.
    Notice the apple's faintly rose-like scent.

    Examples: apple, pear, loquat, quince

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Dry, Dehiscent Fruits

A dry fruit has a dry, hard pericarp at seed maturity.
A dehiscent fruit splits at maturity to release seeds.
(To dehisce means to split open.)


source: Wikimedia Commons

    Follicle

    A follicle consists of a single carpel that
    splits along one seam to release seeds.

    Examples: milkweed, Magnolia


(click on pic for source)

    Capsule

    The most common type of fruit, a capsule
    consists of multiple carpels that
    split along one seam per carpel to release seeds.

    Produced by many different angiosperm species.

    Examples: Many!
    Mahogany, cotton, eucalyptus, poppy, star anise
    (A Brazil "nut" is actually a seed from a giant
    "monkey pot" capsule.)


(By Alex Popovkin, Bahia, Brazil, Wikimedia Commons)


By Bill Ebbesen, Wikimedia Commons

    Legume

    Unique to the Pea Family (Fabaceae), a legume consists of
    one carpel that splits along two seams to release seeds.

    Though we use many as food, most legumes are poisonous.
    (Coumarin , a powerful anticoagulant found in clover
    and other legumes, is used to make warfarin .)

    Examples: green beans, peanuts, sweet peas, many tropical trees (e.g. Royal Poinciana)


By Texnik, Wikimedia Commons

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Dry, Indehiscent Fruits

A dry fruit has a dry, hard pericarp at seed maturity.
An indehiscent fruit does NOT split at maturity to release seeds.


(click on pic for source)

    Achene

    An achene has a thin, dry pericarp
    attached only at its base to a single seed inside.

    Many different species, from roses to quinoa, make achenes,
    including all members of the daisy family (Asteraceae).

    Examples: sunflower "seeds", strawberry "seeds",
    dandelion, quinoa, buckwheat


(click on pic for source)

    Nut

    A nut is similar to an achene,
    but with a thicker, harder pericarp.

    A nut develops in a little "cup" formed by fused bracts
    at the base of the flower.

    Examples: acorn, hazelnut, macadamia nut, chestnut


(source: Getty Images)


(click on pic for source)

    Grain

    A grain is a single-seeded fruit
    whose pericarp is completely fused to the seed.

    Because the pericarp and seed are so tightly united,
    whole grains are a good source of dietary fiber.

    The grain is unique to the Grass Family (Poaceae).

    Seventy five percent of the world's human population
    relies upon rice as a major staple food source.

    Corn farming is a multi-$billion/year industry in the U.S.

    Products range from the cornstarch coating
    on the pages of your textbook to the high-fructose corn syrup
    in your soda to the ethanol in your car's gas tank.

    Examples: rice, wheat, corn, barley, rye, bamboo, sugar cane


(click on pic for source)


(click on pic for source)