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Overview of Green PlantsChapter 30
2
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
3
Defining Plants
4
Defining Plants
• Land plants have two major features
1. Protected embryos
2. Multicellular haploid and diploid phases
5
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
6
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.
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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
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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
10
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
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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
13
MITOSIS
Ulva life cycle
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Land plants
Charophytes
• Charophytes are green algae related to land plants
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Charophytes
• Charales (300 species) – Macroscopic– Plant-like plasmodesmata– Sister clade to land plants
• Choleocaetales (30 species) – Microscopic– Plant-like mitosis– Next closest plant relatives
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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
17
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
19
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
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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)
22
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
23
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
24
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
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Pterophytes• The phylogenetic relationships among ferns
and their relatives is still being sorted out
27
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
31
Pterophytes
Uncoiled fronds are called “fiddleheads” and are a delicacy among northern First Nation peoples
32
Pterophytes
33
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
34
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
35
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
36
• 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
Seed Plants
37
• There are four living groups– Cycadophytes– Gnetophytes– Ginkgophytes– Coniferophytes
Gymnosperms
38
Gymnosperms
• Cycads (phylum Cycadophyta)– Slow-growing gymnosperms of tropical and
subtropical regions
– Sporophytes resemble palm trees
– Have largest sperm cells of all organisms!
39
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
40
Gymnosperms
• Ginkgophytes (phylum Ginkgophyta)– Only one living species remains
• Ginkgo biloba
– Dioecious• Male and female
reproductive structures form on different trees
41
• 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)
Gymnosperms
42
• 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
Gymnosperms
43
Pine reproduction
44
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
45
Angiosperms
46
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
47
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
48
Angiosperms
The ovary eventually develops into the plant’s fruit
49
Angiosperm Reproduction