Overview of Green Plants
Chapter 30
Defining Plants
• The kingdom Viridiplantae includes land
plants and green algae
– Red and brown algae are excluded
• The green algae split into two major clades
– Chlorophytes
– Charophytes
• Streptophyta includes the Charophytes
and all land plants
• All green plants arose from a single species
of freshwater algae
2
Defining Plants
3
Defining Plants
• Land plants have two major features
1. Protected embryos
2. Multicellular haploid and diploid phases
4
Defining Plants
Adaptations to terrestrial life
• Evolution of leaves that increase
photosynthetic surface area
• Protection from desiccation by a waxy cuticle
and stomata
• Shift to a dominant vertical diploid generation
5
Plant Life Cycles
• Humans have a diplontic life cycle
– Only the diploid stage is multicellular
Plants have a haplodiplontic life cycle
– Multicellular diploid stage = Sporophyte
– Multicellular haploid stage = Gametophyte
• Plants have an alternation of generation
– sporophyte → gametophyte → sporophyte → etc.
6
Plant Life Cycles
• Sporophyte (2N) produces haploid spores
(N) by meiosis
• Spores (N) divide by mitosis producing the
gametophyte (N)
• Gametophyte (N) produces gametes (N) by
mitosis
• Gametes (N) fuse to form the diploid
sporophyte (2N)
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The General Plant Life Cycle
process
process
process
8
Plant Life Cycles
As more complex plants evolved:
• Diploid stage (sporophyte) became the
dominant portion of the life cycle
• Gametophyte became more limited in size
• Sporophyte became nutritionally
independent
9
Chlorophytes
• Chlorophytes, sister taxa of the Streptophytes,
are a diverse group including:
• Chlamydomonas
– Unicellular chlorophyte
with two flagella
– Have eyespots to direct
swimming
– Reproduces asexually as
well as sexually
10
Chlorophytes
• Volvox
– Colonial chlorophyte
– Hollow sphere of
a single layer of
500-60,000 cells
– A few cells are
specialized for
reproduction
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Chlorophytes
• Ulva
– Multicellular chlorophyte
– True haplodiplontic life
cycle
– Gametophyte and
sporophyte have identical
appearance
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Ulva life cycle
MITOSIS
13
Charophytes
• Charophytes are green algae related to
land plants
Land plants
14
Charophytes
• Charales (300 species)
– Macroscopic
– Plant-like plasmodesmata
– Sister clade to land plants
• Choleocaetales (30 species)
– Microscopic
– Plant-like mitosis
– Next closest plant relatives
15
Bryophytes
• Bryophytes are the closest living
descendants of the first land plants
– Called nontracheophytes because they lack
tracheids (specialized transport cells)
– Simple, but highly adapted to diverse terrestrial
environments
– Non-photosynthetic sporophyte is nutritionally
dependent on the gametophyte
– 3 groups: liverworts, hornworts and mosses
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Bryophytes
• Liverworts (phylum Hepaticophyta)
– Have flattened gametophytes with liver-like
lobes
– Form gametangia
in umbrella-shaped
structures
– Also undergo
asexual
reproduction
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Bryophytes
• Hornworts (phylum Anthocerotophyta)
– Sporophyte has stomata
– Sporophyte is
photosynthetic
– Cells have a
single large
chloroplast
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Bryophytes
• Mosses (phylum Bryophyta)
– Gametophytes consist of small, leaflike
structures around a stemlike axis
– Anchored to substrate by rhizoids
– Multicellular gametangia form at the tips of
gametophytes
• Archegonia – Female gametangia
• Antheridia – Male gametangia
– Mosses withstand drought, but not air pollution
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Moss Reproduction
20
Tracheophyte Plants
• Cooksonia, the first vascular land plant,
appeared about 420 MYA
– Only a few
centimeters tall
– No roots or leaves
– Homosporous
(spores are the
same size and
type)
21
Tracheophyte Plants
• Vascular tissues are of two types
– Xylem – Conducts water and dissolved
minerals upward from the roots
• contains tracheids
– Phloem – Conducts sucrose and hormones
throughout the plant
• These enable enhanced height and size in
the tracheophytes
• Tracheophytes are also characterized by
the presence of a cuticle and stomata
22
Tracheophyte Plants
• Vascular plants have gametophytes reduced
in size and complexity relative to sporophytes
• Seeds
– Highly-resistant structures that protect the plant
embryo
– Occur only in heterosporous plants
• Fruits in flowering plants add a layer of
protection to seeds
– Also attract animals that disperse seeds
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Tracheophytes
• Vascular plants (tracheophytes) include
seven extant phyla grouped in three clades
– Lycophytes (club mosses)
– Pterophytes (ferns and their relatives)
– Seed plants
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Lycophytes
• Club mosses are the earliest vascular plants
– They lack seeds
– Superficially
resemble true
mosses but they
are not related
– Homosporous or
heterosporous
25
Pterophytes
• The phylogenetic relationships among ferns
and their relatives is still being sorted out
26
Pterophytes
• Whisk ferns
– Saprophyte consists of evenly forking green
stems without leaves or roots
– Some gametophytes
develop elements of
vascular tissue
– Often symbionts with
fungi
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Pterophytes
• Horsetails
– All 15 living species are homosporous
– Constitute a single genus, Equisetum
– Consist of ribbed,
jointed photosynthetic
stems that arise from
branching rhizomes
– High silica content in
stems made them
useful as “scouring
rushes”
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Pterophytes
• Ferns
– The most abundant group of seedless vascular
plants with about 11,000 species
– The conspicuous
sporophyte and much
smaller gametophyte
are both photosynthetic
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Pterophytes
• The fern life cycle differs from that of a moss
– Much greater development, independence and
dominance of the fern’s sporophyte
• Fern morphology
– Sporophytes have rhizomes
– Fronds (leaves) develop at the tip of the
rhizome as tightly rolled-up coils then uncoil and
expand
30
Pterophytes
Uncoiled fronds are called “fiddleheads” and are a delicacy
among northern First Nation peoples
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Pterophytes
32
Pterophytes
• Fern reproduction
– Most fern are homosporous
– Produce distinctive sporangia in clusters called
sori on the back of the fronds
– Diploid spore mother cells in sporangia produce
haploid spores by meiosis
– At maturity, the spores are catapulted by
snapping action
33
Seed Plants
• Seed plants first appeared 305-465 MYA
– Evolved from spore-bearing plants known as
progymnosperms
• The seed represents an important advance
– Protects the embryo
– Easily dispersed
– Introduces a dormant phase in the life cycle
34
Seed Plants
• Seed plants produce 2 kinds of gametophytes
– Male gametophytes
• Pollen grains
• Dispersed by wind or a pollinator
– Female gametophytes
• Develop within an ovule
• Enclosed within diploid sporophyte tissue
35
Seed Plants
• There are two types of seed plants
– Gymnosperms are plants with “naked seeds”
• Ovule is exposed on a scale at pollination
• All lack flowers and fruits of angiosperms
– Angiosperms are flowering plants
• Ovules are enclosed in diploid tissue at pollination
• The carpel (modified leaf) covers seeds and
develops into fruit
36
Gymnosperms
• There are four living groups
– Cycadophytes
– Gnetophytes
– Ginkgophytes
– Coniferophytes
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Gymnosperms
• Cycads (phylum Cycadophyta)
– Slow-growing gymnosperms of tropical and
subtropical regions
– Sporophytes resemble
palm trees
– Have largest sperm
cells of all organisms!
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Gymnosperms
• Gnetophytes (phylum Gnetophyta)
– Only gymnosperms with vessels in their xylem
– Contain three
(unusual) genera
• Welwitschia
• Gnetum
• Ephedra
– ephedrine can be
extracted from species
of this genus
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Gymnosperms
• Ginkgophytes (phylum Ginkgophyta)
– Only one living species remains
• Ginkgo biloba
– Dioecious
• Male and female
reproductive
structures form on
different trees
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Gymnosperms
• Conifers (phylum Coniferophyta) are the
largest gymnosperm phylum and include:
– Pines, spruces, firs, cedars and others
– Coastal redwood – Tallest tree
– Bristlecone pine – Oldest living tree
• Conifers are sources of important products
• Timber, paper, resin and taxol (anti-cancer)
41
Gymnosperms
• Pines
– More than 100 species, all
in the Northern
hemisphere
– Produce tough needlelike
leaves in clusters
– Leaves have:
• Thick cuticle and recessed
stomata
• Canals into which cells
secrete resin
42
Pine reproduction
43
Angiosperms
• Angiosperm origins are a mystery
• The oldest known
angiosperm in the
fossil record is
Archaefructus
• The closest living
relative to the original
angiosperm is
Amborella
44
Angiosperms
45
Angiosperms
• Flower morphology
– Primordium develops into a bud at the end of
a stalk called the pedicel
– Pedicel expands at the tip to form a
receptacle, to which other parts attach
– Flower parts are organized in circles called
whorls
46
Angiosperms
• Flower morphology
– Outermost whorl = Sepals
– Second whorl = Petals
– Third whorl = Stamens (androecium)
• Each stamen has a pollen-bearing anther and a
filament (stalk)
– Innermost whorl = Gynoecium
• Consists of one or more carpels that house the
female gametophyte
47
Angiosperms
The ovary eventually develops into the plant’s fruit
48
Angiosperm Reproduction
49