Evolution of the flower chapter 20
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Evolution of the Flower Chapter 20 Two classes - Monocotyledons and Dicotyledons Distinctive reproductive feature - carpels

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  • Two classes - Monocotyledons and Dicotyledons

  • Distinctive reproductive feature - carpels

  • Angiosperms enclose their seeds in structures known as carpels, instead of lying naked on the scales of a strobilus as in gymnosperms. Hence the name "angiosperm" which means "seed in vessel".


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Review of Flower Structure

  • Flowers are reproductive structures that are formed from four sets of modified leaves on a shortened stem. In other words, the flower is a modified strobilus.

  • Sepals - protect floral parts in the bud

  • Petals - attract pollinators

  • Stamens - anthers and filaments

  • Carpels - stigma, style, and ovary (collection of carpels referred to as a pistil)

  • The carpel is a unique structure found only in angiosperms.


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  • The ovary wall forms a fruit to help disperse the seeds

  • There is an amazing diversity of floral structures. Linnaeus used these differences to classify plants.


  • Evolution of the carpel l.jpg
    Evolution of the Carpel chambers, each chamber holding one or more sporangia on tiny stalks.

    • Goethe, German writer, philosopher, and (in his spare time) noted botanist, proposed in 1790 that carpels evolved from leaves.

    • Chambers in the pistil were probably formed from a sporophyll - a fertile leaf bearing ovules.

    • Sporophyll had ovules (modified sporangia) on its outer edges.

    • Edges of the leaf folded over and fused together to form a protective chamber - the carpel.


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    Derived features of angiosperms l.jpg
    Derived Features of Angiosperms along the midrib of the modified leaves.

    • Leaves with finely divided venation

    • Complex xylem - incl. vessels and parenchyma

    • Complex phloem - sieve tube elements w/companion cells

    • Herbaceous habit - rapid life cycle (some angios)

    • Ovary to protect ovules ("seeds in vessels")

    • Double fertilization and formation of triploid endosperm


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    Origin of the angiosperms l.jpg
    Origin of the Angiosperms same strobilus

    Darwin called the origin of the angiosperms an "abominable mystery".

    • The evolution of angiosperms remains a mystery to this day, although great progress has been made in recent years solving this mystery using a combination of fossil evidence, molecular data, and the discovery of the primitive angiosperm Amborella.

    • Flowering plants evolved sometime during the Cretaceous, approximately 140 million years ago, while the dinosaurs were at their peak.


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    • However, no fossils showing a transition from gymnosperm to angiosperm have been discovered. This makes the origin of the angiosperms mysterious.

    • Angiosperms quickly became the dominant plants, although gymnosperms continued to rule in cold, dry, or sandy habitats, as they still do today.

    • Regardless of the origin of the angiosperms, by the end of the Cretaceous (65-70 mya) most flowering plant families had evolved.


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    Pollination and Seed Dispersal angiosperm have been discovered. This makes the origin of the angiosperms mysterious.

    • Coevolution occurs when an evolutionary change in one organism leads to an evolutionary change in another organism that interacts with it.

    • Flowering plants show two great examples of coevolution: evolution of animal pollination and evolution of fruit dispersal.

    • Flowers that rely on wind pollination are tiny and inconspicuous (like oak trees, maple trees, corn, grasses).

    • Flowers that are pollinated by animals have showy petals to attract the pollinators.


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    Monocots or eudicots l.jpg
    Monocots or Eudicots? attract pollinators.

    • Some flowering plants are neither monocots or dicots.

    • Magnolia


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    Evolution of the flower attract pollinators.

    • What were the flowers of the earliest angiosperms like?

    • Deduce their nature form what we know of certain living plants and from the fossil record.

    • In general flowers were diverse in the number of floral parts and in their arrangements.


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    Parts of the flower provide clues to evolution attract pollinators.

    • The perianth of early angiosperms did not have distinct sepals and petals

    • Sepals and petals were identical or there was a gradual transition in appearance between these whorls (magnolias and water lilies).

    • i.e. petals can be viewed as modified leaves that have become specialized for attracting pollinators.



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    In most angiosperms attract pollinators.

    • Petals were probably derived originally from stamens that lost their sporangia- becoming sterile and modified to new role

    • Most petals like stamens are supplied by one vascular strand

    • In contrast sepals are normally supplied by the same number of vascular strands in a leaf



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    The Stamens attract pollinators.

    • Magnoliids- broad, colored, and scented role in attracting floral visitors

    • In others- small greenish, fleshy

    • Many living angiosperm in contrast have thin filaments and thick terminal anthers

    • In stamens of monocots and eudicots are less diverse than Magnoliids


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    Stamens continue attract pollinators.

    • In some specialized flowers the stamens are fused together.

    • Form columnar structure i.e pea, melon and mallow fig 20-8d and sunflower 20-9d

    • Some stamens fused with corolla i.e. snapdragon, phlox, and mint families.


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    Stamens can become nectaries attract pollinators.

    • In some families stamens become sterile losing their sporangia and becoming specialized nectaries.

    • Nectaries are glands that secrete nectar- sugary fluids tha tattract pollinators and provides food for them.

    • Most nectaries are not modified stamens but arose other ways.


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    The Carpels attract pollinators.

    • The carpels of many early angiosperms were unspecialized

    • Carpels with no specialized areas for the entrapment of pollen grains comparable to specialized stigmas of most living andiosperms.

    • Magnoliids- carpels are free from one another unlike most contemporary angiosperms.


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    Four evolutionary trends among flowers are evident attract pollinators.

    • Evolved toward having few parts that are definite in number

    • Floral whorls have been reduced four to one in more advance ones and the floral parts often have become fused.

    • Ovary has become inferior in position and the perianth has become differentiated into a distinct calyx and corolla

    • The radial symmetry (regularity) or actinomorphy of early flowers has given way to bilateral symmetry (irregularity)or zygomorphy in more advance ones.


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    The Asteraceae and Orchidaceae are examples of specialized families

    Two largest families of angiosperms

    • Asteraceae- compositae which are eudicots

    • Orchidacea- monocots


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    The flower of the Asteraceae are closely bunched together into a head

    • The epigynous flowers are relatively small and bunched together into a head

    • Each flower have an inferior ovary composed of two fused carpels with a single ovule in one locule


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    Composite flowers into a head

    • Stamens are reduced to five in number

    • Usually fused to one another (connate)

    • And fused to the corolla (adnate)

    • The petals also five are fused to one another and to the ovary

    • The sepals are absent or reduced to a series of bristles or scales (pappus)


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    Pappus into a head

    • Often serves as an aid to dispersal by wind


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    Orchidaceae is the largest Angiosperm family into a head

    • 24, 000 species Orchids

    • Unlike composites are monocots

    • individual species rarely abundant

    • Most are tropical

    • 140 native to US and Canada


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    Orchids into a head


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    Orchids into a head

    • Like the composites:

      • The carpels are fused (The three carpels)

  • Ovary is inferior

  • Unlike the composites:

  • Ovaries contain many thousands of minute ovules

  • Each pollination event may result in huge number of seeds


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    Animals serve as the primary agents of floral evolution into a head

    • Flowers and insects have coevolved



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    Fruit is a Mature ovary into a head

    • Accessory fruit- fruit which some additional parts are retained (strawberry)

    • Simple fruits develop from one carpel or from several united carpels.

    • Aggregate fruits, such as those magnolias, raspberries and strawberries consist of a number of separate carpels of one gynoecium

    • Multiple fruits consist of the gynoecia of more than one flower- the pineapple


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    Simple fruits into a head

    • May be

      • soft and fleshy,

      • dry and woody, or

      • papery


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    Simple fruit fleshy fruits into a head

    • Berries- tomatoes, dates, and grapes

    • Drupes- one to several carpels but only 1 seed- peaches, cherries, olives, plums

    • Pomes- example of an accessory fruit- apples, pears


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    Honeysuckle, into a headLonicera hispidula


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    Dry simple fruit into a head

    • Dehiscent- tissue of the mature ovary wall (the pericarp) break open freeing seeds

    • Indehiscent- the seeds remain in the fruit after the fruit has been shed from the parent plant.




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    Poison Ivy into a head



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    Cannabis sativa into a head


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    Quinine tx malaria into a head


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    Erythroxylum coca into a head


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