2. Origin of Land Plants
• All green algae and the land plants shared
a common ancestor a little over 1 BYA
– Kingdom Viridiplantae
– Not all photoautotrophs are plants
• Red and brown algae are photoautotrophs but not
classified as plants
• A single species of freshwater green algae
gave rise to the entire terrestrial plant
lineage
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4. Focus on Land Plants
•Adaptations to terrestrial life
– Protection from desiccation (“drying out”)
• all land plants have at least some adaptations for
this!
– Alternation of generations
• all land plants have this lifecycle!
– Vascular tissue allows plants to grow taller
• Xylem and phloem to conduct water and food
• Only “vascular” plants have this!
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6. Alternation of generations (also called
the haplodiplontic cycle)
• Multicellular diploid stage
– The whole plant in this stage is called the sporophyte
– All the cells in the sporophyte plant are diploid at first.
– A few cells called sporocytes are located in the organ
called the sporangium
– Only the sporocytes go through meiosis to produce
haploid cells called spores
– The sporocytes are found in the sporangium. When
meiosis is complete, the spores (daughter cells of
meiosis) are still in the sporangium.
– Then the sporangium cracks open and releases the
spores 6
8. • Multicellular haploid stage
– The plant in this stage is called the gametophyte
– When a haploid spore is released from the sporangium, it
lands on moist soil and germinates (ie it divides by
mitosis) until the whole gametophyte is grown
– A few cells in the organ called the gametangium go
through a “special” mitosis to form gametes.
– Sperm will leave their gametangium and swim to the eggs
in the female gametangium
– Gametes fuse to form diploid zygote
– Zygotes grows by mitosis into the next sporophyte plant
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Alternation of generations (also called
the haplodiplontic cycle)
11. • Simple, but highly adapted to diverse terrestrial
environments
• 24,700 species in 3 clades
– Liverworts
– Mosses
– Hornworts
• Gametophyte – the most noticeable part, and
the photosynthetic part
– Sporophytes are small and dependent
• Require water for sexual reproduction 11
13. Vascular tissues
• Xylem
– Consists of dead cells called tracheids that form a
tube
– Conducts water and dissolved minerals upward from
the roots
• Phloem
– Consists of living cells
– Conducts sucrose and hormones throughout the plant
• Vascular tissues develop in sporophyte but not
gametophyte, so gametophyte is smaller
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14. Tracheophytes
• Vascular plants include seven extant phyla
grouped in three clades
1. Lycophytes (club mosses)
2. Pterophytes (ferns, whisk ferns, and horsetails)
3. Seed plants
• Gametophyte has been reduced in size relative
to the sporophyte during the evolution of
tracheophytes
14
16. • 40 million years between appearance of
vascular tissue and true leaves
– Stems, then roots
• Provide transport and support
– Then leaves
• Increase surface area for photosynthesis
• Seeds - didn’t evolve until Mesozoic Era
– Highly resistant to desiccation
– Contain an embryo plus a food supply
– Lycophytes and pterophytes do not have seeds
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18. Lycophytes – misnamed “club moss”
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Pterophytes
•Whisk ferns
•Horsetails
•Ferns (see next slide)
Seedless Tracheophytes
19. Ferns
• Most abundant group of
seedless vascular plants
– About 11,000 species
• Coal formed from forests
300 mya
• The larger sporophyte
and much smaller
gametophyte are both
photosynthetic
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21. Fern reproduction
• Produce distinctive sporangia in clusters
called “sori” on the back of the fronds
• Diploid spore mother cells (also called
sporocytes) in sporangia produce haploid
spores by meiosis
• Every spore germinates into bisexual
gametophyte (contains archegonia and
antheridia on same gametophyte plant)
• Flagellated sperm
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