Section 20.1 KEY CONCEPT Plant life began in the water and

Section 20.1
KEY CONCEPT
Plant life began in the water and became adapted to land.
Where did life begin?
What did life begin as?
Land plants evolved from green algae.
• Plants and green algae have many common traits.
– both are photosynthetic eukaryotes
– both have the same types of chlorophyll
– both use starch as a storage product
– both have cell walls with cellulose
Land plants evolved from green algae.
• Genetic analysis points to the common ancestor of all
plants.
– extinct green algae species
–modern green algae common in lakes and ponds
Land plants evolved from green algae.
• Important plant characteristics likely originated in green algae
– multicellular body allowing for specialization of cells and tissues
– cell division that allows for chemical communication between
cells
– reproduction involving sperm swimming to egg
Land plants evolved from green algae.
• True plants evolved through natural selection.
– Ancestral green algae lived in areas of shallow water.
– Those that could survive longer dry periods were
favored.
– True plants have embryos that develop while attached
to female parent.
Land plants evolved from green algae.
• True plants evolved through natural selection.
Plants have adaptations that allow them to live on land.
• Life in the ocean had plentiful water
• Plants needed some adaptations to survive the dry
environment of land
Challenge 1: Land is dry compared to the ocean.
• A cuticle allows plants to retain moisture.
– waxy, waterproof layer
– holds moisture in
• Stomata are tiny holes in the cuticle.
– can open and close
– allow air to move in and out
stoma
Challenge 2: Sunlight comes from above so plants needs to
be as tall as possible.
• Plants develop vascular systems to transport materials
around plant
– brings water and mineral nutrients up from roots
– disperses sugars from the leaves
– allows plants to grow higher off the ground
water
and
mineral
nutrients
sugars
Challenge 3: Very tall plants need a tough material to hold
them upright.
• Lignin allows plants to grow upright.
lignin
plant cells
– hardens cell walls of some vascular tissues
– provides stiffness to stems
Challenge 4: Plants need to reproduce without using water.
• Pollen grains protect plant gametes.
– pollen grains contain a cell that
divides to form sperm
• A seed is a storage device for a
plant embryo.
– seed coats protect embryos
from drying wind and sunlight
Plants evolve with other organisms in their environment.
• Plants and other organisms can share a mutualistic
relationship.
– a mutualism is an interaction in which two species benefit
– plant roots and certain fungi and bacteria (nitrogen fixation)
– flowering plants and their animal pollinators
Plants evolve with other organisms in their environment.
• Plants have adaptations that prevent animals from eating them
(Plant-herbivore interactions).
– spines and thorns
– defensive chemicals
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