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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 • True plants evolved through natural selection. – Ancestral charophyceans lived in areas of shallow water. – 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. – 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 water and mineral nutrients sugars 20.1 Origins of Plant Life • Lignin allows plants to grow upright. lignin plant cells – hardens cell walls of some vascular tissues – provides stiffness to stems 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. – spines and thorns – defensive chemicals