Plants: The Land Invasion

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Plants The Land Invasion
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Challenges that Land Plants Face

• Problem 1
• No water to support the mass of plant
• Solution
• Tissues become stronger to hold up plant
• Development of roots in higher plants allows
  anchoring in soil

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Challenges that Land Plants Face

• Problem 2
• Obtaining and transporting water other
  nutrients to different parts of plant
• Solution
• Development of roots allows nutrients and water
  to be more easily absorbed
• In more advanced plants (discussed later), the
  development of vascular tissues greatly increases
  a plants transporting abilities.

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Challenges that Land Plants Face

• Problem 3
• Minimizing water loss due to evaporation while
  maintaining efficient gas exchange
• Solution
• Waxy covering, cuticle, prevents water loss from
  evaporation,
• Microscopic pores, stomata, (not found in mosses)
  allows gas exchange between the plants and air
  (usually in leaves)

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Challenges that Land Plants Face

• Problem 4
• Dispersal of seeds (spores)
• Solution
• Various modes of reproduction that increases
  chance of seed dispersal

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Challenges that Land Plants Face

• Although Bryophytes (ex. mosses) have adapted to
  live on land, they have not completely solved all
  these problems.
• They need to live close to a water source and
  must remain wet most of the time.

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The First Land Plants

• Adaptation to live out of water began
  450 to 500 million years ago

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Algae-like groups of land plants

• One group that developed from algae now in the
  phylum Bryophyta
• Includes mosses, liverworts, hornworts
• Another group evolved into the phylum
  Tracheophyta
• Includes ferns rest of higher plants

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Phylum Bryophyta
Moss
Liverwort
Hornwort
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General characteristics of Bryophytes

• Low-growing in tightly knit clumps
• Resemble small green carpets

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General characteristics of Bryophytes

• Lack advanced modifications for terrestrial life
  ? restricted to areas that are very moist
• Non-vascular (no internal transport system for
  fluid nutrients) ? no true leaves or stems
• Lack transport tissues ? transport of water,
  nutrients, and food up down plant achieved
  through simple diffusion/ osmosis

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General characteristics of Bryophytes

• Have root-like structures called rhizoids ?
  function like true roots to anchor the plant ?
  but only a minor role in absorption transport
  of water nutrients from the soil
• Like algae, mosses go through an alternation of
  generations (2 life stages)
• Need water for reproduction (sperm cells swim
  through water to fertilize eggs)

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• Mosses and other similar plants are often called
  pioneer plants because they can inhabit on land
  habitats with minimal amount of soil. As these
  plants grow and die, some rocky materials get
  broken down into new soil, and the dead plants
  themselves are recycled, releasing nutrients into
  the newly formed soil so future plants can grow.

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Reproduction in Mosses

• Life cycle involves alternation of generations
  between haploid gametophyte and a diploid
  sporophyte.

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Gametophytes (n)

• Leafy dominant stage
• Vaguely resembles small advanced plant

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Sperm transfer

• Female gametophyte produces eggs in ARCHEGONIUM
• Male gametophyte produces sperm in ANTHERIDIUM
• Q How does the sperm get to the egg?

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Sperm meets egg (n n)

• What happens?
• FERTILIZATION
• What is formed?
• ZYGOTE (2n)

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Zygote grows into Sporophyte (2n)

• A dependent structure called the sporangium grows
  on top of mature gametophyte

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Sporophyte (2n)

• Spores (n) are made and released from
  sporophytes sporangium

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Spores (n)

• Spores disperse in wind, but may lie dormant for
  long time
• When conditions are right, will germinate into
  protonema (n)

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Protonema (n)

• Will grow into? GAMETOPHYTE (n)
• Sexes may be separate or the two sexes may be
  together side to side

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Quiz 2 Life Stages

• Gametophytes are
• and they produce
• Sporophytes are
• and they produce

HAPLOID
GAMETES (egg sperm)
DIPLOID
SPORES that are HAPLOID
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Limitations of moss

• Lack of vascular tissue
• Need free water for sperm transfer
• Vulnerable protonema stage