Download Vascular Plant Systematics - Fall 2001 Lecture #10

Vascular Plant Systematics - Fall 2001
Lecture #10 - The Origin & Classification of the Magnoliophyta (Angiosperms)
Note: In plants, Kingdom, Division, Class, Order, Family, Genus, species
Magnoliopsida (Dicots) and Liliopsida (Monocots) are class names. Both Dicots and Monocots
are in the Division Magnoliophyta.
** - Among all plants, angiosperms (the flowering plants), include the most species today
and occupy the greatest number of habitats. Angiosperms range from tiny duck weeds to
giant redwood trees; adaptive radiation has resulted in parasites, saprophytes (plants that feed on
dead organic matter), epiphytes, insectivorous plants and typical green, photosynthetic plants.
Characteristics of Flowering Plants:
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Angiosperms possess flowers and produce a fruit that protects and surrounds the
seed (versus gymnosperms which produce naked seeds & lack flowers ..... and ferns
that produce spores .... and also lack flowers)
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Angiosperms possess vesse l members in the water conducting tissue (xylem) ....
along with tracheids; they also possess companion cells along with the sieve tube
members in the food conducting tissue (phloem). No other group of plants possess
vessel members and companion cells.
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Angiosperms have an embryo sac that contains 8 nuclei and they undergo double
fertilization; they also possess closed, modified carpels that collectively make up the
ovary (which will eventually form the fruit).
Fossil Record: Time of Origin and Diversification The best available evidence from the fossil record indicates that the flowering plants likely
evolved during the early Cretaceous Period about 135 million years ago (see:
http://www.flmnh.ufl.edu/natsci/paleobotany/angio.htm).
About 100 million years ago during the upper Cretaceous Period, during a period of about 10
million years, the flowering plants spread over the earth from the equator to the poles (largely
undeterred by ocean barriers and competition).
** - During this period of time, ferns, cycads and gymnosperms dominated. Ferns evolved about
400 million years ago (Devonian Period) and gymnosperms began to appear about 325 million
years ago (Carboniferous Period).
** - The first primitive vascular plants (plants that possess xylem & phloem) horse-tail like
plants appeared about 440 million years ago during the Ordovician Period.
Paleobotanists, including David Dilcher (http://www.flmnh.ufl.edu/natsci/paleobotany/dilcher.htm)
working on middle Cretaceous fossils, found that: (1) some early angiosperms were unisexual
and others were bisexual, (2) most were wind pollinated
but others were generalists that
were pollinated by insects (mostly beetles at that time), and that (3) seeds lacked dispersal
mechanisms. The record though indicates an extremely rapid radiation of types of angiosperm
reproduction during the early Cretaceous Period.
** - Dilcher found a fossil bisexual flower that dates back 95 million years
fossil of a flowering plant.
the oldest such
** - Early flowering plants were generalists and advanced specialized families didn t evolve
until much later. For example, members of the family Compositae (based on pollen studies)
didn t appear until the end of the Oligocene some 38 million years ago.
Ancestors of the Flowering Plants - Angiosperms are believed to have arisen from Mesozoic
gymnosperms ...... probably from the seed ferns. Within the Division Pinophyta, there are three
subdivisions: (a) seed ferns & cycads, (b) conifers and ginkgo, and (c) gnetophytina.
** Caytoniales and Glossopteridales are orders of seed ferns
and in the former, the ovules are
semi-enclosed in pouches; in the latter, the seeds are born on a partially enrolled megasporophyll
(leaf like structure). Some botanists think that the cycad line could have been the angiosperm
forerunner again based on the morphology of the ovule bearing unit.
Early Flowering Plants
some hypotheses
** - I. The first angiosperms were Magnolia-like evergreen trees in tropical uplands and were
pollinated by insects. The theory holds that the primitive angiosperm flower was a solitary,
terminal, bisexual, actinomorphic, and with numerous sepals and petals.
** - II. Stebbins (1974) - proposed that the first angiosperms were small woody plants inhabiting
pioneer habitats exposed to seasonal drought and other environmental stresses. Such conditions
would be favorable to the rapid development of flowers and seeds. Dilcher s findings support
this theory ...... that the first angiosperms were woody, small leaved plants in the rift valley
system joining Africa and South America.
** - In the transition from the pro-angiosperms to the first flowering plants, the
megasporophylls were folded along the midrib with the ovules inside. Upon reduction, the
megasporophylls gave rise to the carpels (a modified leaf like structure bearing ovules).
** - The Fossil Record of Early Angiosperms:
** - Dilcher argues that the first angiosperms were generalists pollinated by both insects and
through wind-born pollen. Angiosperm numbers increased rapidly (essentially they underwent
an adaptive radiation). Apparently, early angiosperms had few free, floral parts, had radially
symmetrical floral parts, and had small seeds.
** - Interrelationships of Angiosperms & Animals ** - Flowering plants have been closely interrelated with animals in their evolution. Plants and
animals developed by a process of reciprocal interactions .... a process called co-evolution.
A. Pollination - early seed plants were largely wind pollinated; insect pollination probably came
about fortuitously brought on by pollen-eating beetles. Insect activities would ultimately
influence the development of edible floral parts, protein-rich pollen, nectaries and bisexual floral
parts.
** - Bees, butterflies etc.. hadn t evolved by the time the first angiosperms appeared.
** - The ovules then had to be protected from being eaten (beetles)
such protection afforded
by closed carpels allowed the ovules to become smaller and faster developing (more enduring
compared to ferns and gymnosperms).
** - By the beginning of the Cenozoic Era (65-70 mya) - bees, moths, and butterflies were
diversifying and many plant species developed floral features to accommodate their insect
visitors. Birds and bats also began to act as pollinators.
Characteristics of Plants Pollinated by Various Groups
** Beetle Pollination - flowers are dull or white, spicy, fermenting or fruity orders; ovules are
heavily protected (beetles have a good sense of smell ... those that visit flowers). Magnolias and
other flowers with primitive features are often beetle pollinated.
** Bee Pollination - 20,000 bee species pollinate flowers. They see in the ultraviolet spectrum
(red appears black to a bee); thus flowers that are pollinated by bees are generally blue, yellow or
white (but typically not red) and are often more derived with landing platforms.
** Moth & Butterfly Pollination - flowers are often tubular red, blue or yellow. Most moths
are nocturnal and often pollinate white or dull-colored flowers that possess a heavy fragrance.
Often flowers visited by butterflies are actinopmorphic (more primitive state).
** - Bird Pollination - ornithophilous - red, tubular flowers with abundant nectar and little
fragrance (hummingbirds and honeyguides)
birds generally rely more on vision and have
little or no sense of smell.
** - Bat Pollination - chirophilous - mostly a tropical phenomenon (New and Old World);
dull-colored flowers that open at night which possess fermenting or fruity orders bats lap the
nectar.
** - Rodents also are involved to some extent in pollinating flowers
Africa.
particularly in southern
** - Seed Dispersal in Angiosperms - in the first angiosperms was likely accomplished by water
and/or the wind; reptiles were dominant and they had few adaptations for seed dispersal. Birds
and mammals appeared in the late Mesozoic and seed dispersal mechanisms began to evolve.
** Other Co-evolution plant examples = ants and acacia plants (tropical regions);
monarch/milkweeds; grasses & herbivores; cacti & ants