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20.1 Origins of Plant Life
KEY CONCEPT Plant life began in the water and became adapted to land.
20.1 Origins of Plant Life
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
20.1 Origins of Plant Life
• Genetic analysis points to the common ancestor of all plants.– extinct green algae species in class Charophyceae– modern charophyceans common in lakes and ponds
20.1 Origins of Plant Life
• Important plant characteristics likely originated in charophyceans.– multicellular body allowing for specialization of
cells and tissues
– cell division that allows for chemical communication between cells
– reproduction involving sperm swimming to egg
20.1 Origins of Plant Life
– Ancestral charophyceans lived in areas of shallow water.
• True plants evolved through natural selection.
– Those that could survive longer dry periods were favored.
– First true plants probably grew at edges of water.– True plants have embryos that develop while attached
to female parent.
20.1 Origins of Plant Life
• True plants evolved through natural selection.
20.1 Origins of Plant Life
Plants have adaptations that allow them to live on land.• Challenges of living on land have selected for certain
plant adaptations.• A cuticle allows plants to retain moisture.
– waxy, waterproof layer– holds moisture in
20.1 Origins of Plant Life
• Stomata are tiny holes in the cuticle.
stoma
– can open and close– allow air to move in and out
20.1 Origins of Plant Life
• A vascular system allows resources to move to different parts of the plant.
sugars
water and mineral nutrients
– collection of specialized tissues– brings water and mineral nutrients up from roots – disperses sugars from the leaves – allows plants to grow higher off the ground
20.1 Origins of Plant Life
• Lignin allows plants to grow upright.
– hardens cell walls of some vascular tissues– provides stiffness to stems
plant cells
lignin
20.1 Origins of Plant Life
• Pollen grains allow for reproduction without free-standing water. – pollen grains contain a cell
that divides to form sperm – pollen can be carried by
wind or animals to female structures
20.1 Origins of Plant Life
• A seed is a storage device for a plant embryo. – seed coats protect
embryos from drying wind and sunlight
– embryo develops when environment is favorable
20.1 Origins of Plant Life
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 – flowering plants and their animal pollinators
20.1 Origins of Plant Life
• Plants have adaptations that prevent animals from eating them.
– defensive chemicals
– spines and thorns
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