Transcript An Introduction to Plants

An Introduction to Plants
Evolution and
Classification
Overview of the Plant Kingdom
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Evolutionary Relationships Among Plants
Flowering plants
Cone-bearing plants
Ferns and
their relatives
Flowers; Seeds
enclosed in fruit
Mosses and
their relatives
Seeds
Water-conducting (vascular)
tissue
Green algae
ancestor
Evolution and Classification
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The organisms we call plants are assigned to a
single clade; that is, a natural grouping based on the
belief that they have all evolved from a common
ancestor more recent than any shared with other
organisms.
Among the criteria for doing this are:
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their shared use of the photosynthetic pigments
chlorophyll a and chlorophyll b
the similarities in the nucleotide sequences of both their
small subunit (18S) and large subunit (28S) ribosomal
RNA (rRNA) genes
their shared cellulose cell wall.
Green Algae (Division Chlorophyta)
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The ancestors of these organisms were the most primitive
members of the clade. In other words, organisms that we would
put in this division were probably the ancestors of all the other
plants.
There are some 7000 species living today. They include:
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microscopic, unicellular forms like Chlorella and
Chlamydomonas (pronounced CLAMMY-doe-moan-us)
colonial forms like the filamentous Spirogyra
multicellular forms like Ulva, the sea lettuce.
Green Algae (cont)
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Although some of the multicellular forms are large,
they never develop more than a few differentiated
types of cells and their fertilized eggs do not develop
into an embryo.
Green algae are an important source of food for
many aquatic animals. When lakes and ponds are
"fertilized" with phosphates and nitrates (e.g., from
sewage and the runoff from fertilized fields and
lawns), green algae often form extensive algal
"blooms".
Bryophytes
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The three
groups of
bryophytes
are:
mosses
liverworts
hornworts
Liverworts and Mosses
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These are fairly simple plants that do produce a
number of differentiated cell types and whose
fertilized egg develops into a distinct embryo.
However, they have neither vascular tissue (xylem
and phloem) nor woody tissue and thus never grow
very large.
Some 16,000 living species are known. Most grow in
moist places.
Lycopsids (Division Lycopsida)
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The members of this group are often called club
mosses. They are not mosses at all, but vascular
plants with xylem and phloem running through their
roots, stems, and leaves. The leaves are quite
simple and small with their vascular tissue in a
single, unbranched vein.
The "club" of their name comes from the appearance
of their spore-forming structures called strobili.
Lycopsids (cont)
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Club mosses are also
sometimes called "ground
pines", but they are not pines
either. The photo shows
Lycopodium obscurum.
About 1000 species of
lycopsids exist today. All are
small (those in the photo stand
about 8 in. [20 cm] tall), but it
was not always so. Fossil
lycopsids in the Mississippian
and Pennsylvanian periods (the
so-called Carboniferous era)
reached heights of 100 feet.
Their remains contributed to the
formation of coal.
Chloroplast Genes
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Chloroplasts (as
well as
mitochondria) have
their own genome.
Chloroplast Genes (cont)
The diagram (based on the work
of Ohyama, K. et al., Nature
322:572, 1986 and Linda A.
Raubeson and R. K. Jansen,
Science 225:1697, 1992) shows
the genome of the first chloroplast
DNA to be sequenced, that of the
liverwort Marchantia
polymorpha. It contains 121,024
base pairs encoding 128 genes.
The short lines indicate a few of
the tRNA genes, some of which
are labeled.
Chloroplast Genes (cont)
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The order of the genes between the arrows (~6:30 to
~10:00) is also found in the lycopsids. But in all
other vascular plants, this region is inverted and
the order of the genes is precisely reversed. This
provides further evidence that the other vascular
plants we shall examine below, the
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horsetails
ferns
gymnosperms and
angiosperms
belong to a separate clade.
Horsetails (Division Equisetopsida)
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The common name comes from the characteristic pattern of
branching: whorls or rings of branchlets arising from an aboveground shoot. The shoot develops each season from an
underground stem (rhizome).
Horsetails often grow in sandy places and incorporate silica in
their stems. This gives them an abrasive quality which caused
them to once be used for cleaning pots and pans, which gave
rise to another common name: scouring rush.
Only one genus, Equisetum, containing about 25 species,
survives today. However, many other, much larger, species
were dominant features of the Carboniferous and, like the early
lycopsids, contributed to the formation of coal.
Horsetails (cont)
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The drawing is of
Equisetum
palustre, a
common
horsetail. Spores
are formed in the
strobilus.
Ferns (Division Filicopsida)
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Over 10,000 species of ferns live on earth today.
Many of these are found in the tropics where some
— the "tree ferns" — may grow to heights of 40 ft (13
m) or more.
The ferns of temperate regions are smaller. They are
usually found in damp, shady locations.
Their stems — called rhizomes — as well as their
roots grow underground and are perennial.
Their leaves, called fronds, grow up from the
rhizome each spring.
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The Underside of a Fern Frond
Seed Plants
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Seed plants are divided into two groups:
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Gymnosperms bear seeds directly on the surfaces of
cones.
Angiosperms, or flowering plants, bear seeds within a
layer of tissue that protects the seed.
Seed Plants: Gymnosperms
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Gymnosperms—Cone Bearers
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The four groups of gymnosperms are:
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Gnetophytes (nee-toe-fites)
cycads
ginkgoes
conifers
Gymnosperms: Conifers
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These gymnosperms get their name from
their cones:
male cones in which the microspores
develop;
female cones in which megaspores develop.
Gymnosperms: Conifers (cont)
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The microspores develop into pollen grains that are
carried by the wind to the female cones. Here each
germinates into a pollen tube which grows into the
tissues of the female cone until it reaches the vicinity
of the egg. (In pines, this may take a year.) Then the
tube ruptures and a sperm nucleus fuses with the
egg to form the zygote.
After fertilization, the zygote develops into a tiny
embryo sporophyte plant.
Gymnosperms: Conifers (cont)
Gymnosperms: Conifers (cont)
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There are approximately 550 species of living conifers. They include the
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pines
spruces and
firs.
Conifers include the largest and the oldest of all living organisms. One redwood
(genus Sequoia) growing in California is almost 400 feet high. Bristlecone
pines growing in the mountains of eastern California are more than 4000 years
old.
Although most conifers are evergreen, their leaves are modified as "needles",
and these reduce snow load and transpiration during the winter in the harsh
high-latitude climates where conifers are the dominant species of plants. But by
retaining their needles during the winter, conifers are ready to begin
photosynthesis immediately upon the return of spring.
Coniferous forests are of great economic importance producing lumber for
building and pulp for paper making.
Angiosperms
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Although angiosperms appear in the fossil
record in Jurassic deposits, it was not until
the end of the Mesozoic era that
angiosperms became the dominant plants of
the landscape. That they dominate the
earth's flora today is clear: there are some
240,000 species of living angiosperms; the
rest of the plant kingdom includes only some
34,000 species.
Monocots and Dicots
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Of over 400 families of angiosperms, some
80 of them fall into a single clade, called
monocots because their seeds have only a
single cotyledon. The remainder are the
dicots whose seeds have two cotyledons.
Monocot Traits
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a single cotyledon in their seed
parallel venation in their leaves
petals and sepals in 3s or some multiple
thereof
vascular bundles scattered randomly
throughout the stem
Monocots (cont)
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Monocots include:
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palms (Arecaceae)
orchids (Orchidaceae)
yams, sweet potatoes (Dioscoreaceae)
lilies, onion, asparagus (Liliaceae)
bananas (Musaceae)
and all the grasses (Poaceae), which include many of our most
important plants such as
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corn (maize)
wheat
rice
and all the other cereal grains upon which we depend so heavily for
food as well as
sugar cane and bamboo
Dicot Traits
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two cotyledons in their seeds
netted venation in their leaves
petals and sepals in 4s, 5s, or some multiple
thereof
vascular bundles in the stem arranged in a
radial pattern like spokes of a wheel.
Examples of Dicots
Family
Examples
Anacardiaceae
poison ivy, cashews, pistachios
Asteraceae
asters and all the other composite flowers
Brassicaceae
cabbage, turnip; Arabidopsis, and other mustards
Cactaceae
cacti
Cucurbitaceae
squashes
Euphorbiaceae
cassava (manioc)
Fabaceae
beans and all the other legumes
Fagaceae
oaks
Linaceae
flax (source of linen)
Malvaceae
cotton
Oleaceae
olives, ashes, lilacs
Rosaceae
roses, apples, peaches, strawberries, almonds
Rubiaceae
coffee
Rutaceae
oranges and other citrus fruits
Solanaceae
potato, tomato, tobacco
Theaceae
tea
Vitaceae
grapes
Major
Differences
in Monocots
and Dicots:
Source:
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http://users.rcn.com/jkimball.ma.ultranet/Biol
ogyPages/P/Plants.html