Evolution of the Angiosperms

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Chapter 20

• Evolution of the Angiosperms

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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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• Cut into the pistil and you will see one or more
  tiny chambers, each chamber holding one or more
  sporangia on tiny stalks.
• These sporangia are the ovules - each carpel can
  hold one or several ovules
• Ovules in the ovary develop into seeds
• 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.

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Evolution of the Carpel

• 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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• Pistils probably formed by the fusion of several
  carpels along the midrib of the modified leaves.
  
• Goethe's "foliar theory of the carpel" is still
  the best hypothesis for explaining the evolution
  of the carpel.

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Derived Features of Angiosperms

• 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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• Bisexual Flowers microsporangia and
  megasporangia in same strobilus
• Advanced pollination syndromes - insects, birds,
  etc.
• Fruits to protect and diserse seeds
• Extreme diversity in secondary metabolism

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Origin of the Angiosperms

• 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

• 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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• Flowers advertise their reward of nectar, sugar
  water, to attract pollinators.
• Fruits function to disperse seeds.
• Animals eat fruit, but don't digest seeds.
• Tiny hooks and spines to attach to animal.
• Also dispersed by wind, water (coconuts).

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Monocots or Eudicots?

• Some flowering plants are neither monocots or
  dicots.
• Magnolia

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Evolution of the flower

• 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

• 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

• 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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• Petal fusion resulting in a tubular corolla
  figure 20-8c

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The Stamens

• 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

• 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

• 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

• 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

• 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
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Orchidaceae is the largest Angiosperm family