Evolution of the Flower
Chapter 20
• 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".
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.
– 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.
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.
• 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.
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
• 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
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.
• 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.
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.
• 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).
Monocots or Eudicots?
• Some flowering plants are neither monocots
or dicots.
• Magnolia
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.
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.
Wintergreen Chimphila umbellata
In most angiosperms
• Petals were probably derived originally
from stamens that lost their sporangiabecoming 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
• Petal fusion resulting in a tubular corolla
figure 20-8c
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
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.
Stamens can become nectaries
• In some families stamens become sterile
losing their sporangia and becoming
specialized nectaries.
• Nectaries are glands that secrete nectarsugary fluids tha tattract pollinators and
provides food for them.
• Most nectaries are not modified stamens but
arose other ways.
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.
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.
The Asteraceae and Orchidaceae are
examples of specialized families
Two largest families of angiosperms
• Asteraceae- compositae which are eudicots
• Orchidacea- monocots
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
Composite flowers
•
•
•
•
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)
Pappus
• Often serves as an aid to dispersal by wind
Orchidaceae is the largest
Angiosperm family
•
•
•
•
•
24, 000 species Orchids
Unlike composites are monocots
individual species rarely abundant
Most are tropical
140 native to US and Canada
Orchids
Orchids
Like the composites:
• The
carpels
are
fused
(The
three
carpels)
• Ovary is inferior
Unlike the
composites:
•Ovaries contain
Animals serve as the primary agents
of floral evolution
• Flowers and insects have coevolved
Wind pollination flowers produce no
nectar
Fruit is a Mature ovary
• 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
Simple fruits
• May be
– soft and fleshy,
– dry and woody, or
– papery
Simple fruit fleshy fruits
• Berries- tomatoes, dates, and grapes
• Drupes- one to several carpels but only 1
seed- peaches, cherries, olives, plums
• Pomes- example of an accessory fruitapples, pears
Honeysuckle, Lonicera hispidula
Dry simple fruit
• 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.
Dehiscent fruit, Legumes
Cypselas, modified calyx (the Pappus)
Poison Ivy
Mescaline from the peyote cactus
Cannabis sativa
Quinine tx malaria
Erythroxylum coca