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