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Objectives• Explain what a plant is• Describe what plants need to survive• Describe how plants first evolved
What Is a Plant?• Plants are members of the kingdom Plantae• Plants are multicellular eukaryotes that have cell walls made of cellulose. They develop from multicellular embryos and carry out photosynthesis using the green pigments chlorophyll a and b
The Plant Life Cycle• Plants have life cycles that are characterized by alternation of generations• the two generations are the haploid (N) gametophyte, or gamete-producing plant, and the diploid (2N) sporophyte, or spore- producing plant.
What Plants Need to Survive1. Sunlight2. Water and Minerals3. Gas Exchange4. Movement of water and nutrients
Early Plants• The first plants evolved from an organism much like the multicellular green algae living today.
The First Plants• The first true plants were still dependent on water to complete their life cycles, similar to today’s mosses in that they were simple in structure and grew close to the ground.
• From these plant pioneers,several major groups of plantsevolved.
• During at least one stage of their life cycle, bryophytes produce sperm that must swim through water to reach eggs of other individuals.• Therefore, they must live in places where there is rainfall or dew for at least part of the year
Objectives• Describe the reproductive adaptations of seed plants• Describe the evolution of seed plants• Identify the four groups of gymnosperms
• Over millions of years, plants with a single trait—the ability to form seeds—became the most dominant group of photosynthetic organisms on land.• Seed plants are divided into two groups:
Gymnosperms• Cone plants• Bear their seeds directly on the surfaces of conesEx.) conifers, pines, spruces, cycads, ancient ginkgoes and gnetophytes
Angiosperms• Flowering plants• Bear their seeds within a layer of tissue that protects the seedEx.) grasses, flowering trees shrubs, wild flowers
Reproduction Free From Water• Unlike mosses and ferns, however, seed plants do not require water for fertilization of gametes.Q: Why was this an advantage?A: They could live further away from water• Adaptations that allow seed plants to reproduce without water include flowers or cones, the transfer of sperm by pollination, and the protection of embryos in seeds.
seed• An embryo of a plant that is encased in a protective covering and surrounded by a food supply
embryo• Early development stage of a sporophyte plant• The seed’s food supply provides nutrients to the embryo as it grows
Seed Coat• Surrounds and protects the embryo and keeps the contents of the seed from drying out• Can be specialized for dispersal
Evolution of Seed Plants• Over a period of millions of years, continents became much drier, making it harder for seedless plants to survive and reproduce.• For that reason, many moss and fern species became extinct. They were replaced by seed plants with adaptations that equipped them to deal with drier conditions.
Gymnosperms—Cone Bearers• The most ancient surviving seed plants are the gymnosperms.• Gymnosperms include:
Annuals• Angiosperms that complete a life cycle within one growing season
Biennials• Angiosperms that complete their life cycle in two years• In the first year, biennials germinate and grow roots, stems, leaves• During their second year, biennials grow new stems and leaves and then produce flowers and seeds
Perennials• Flowering plants that live for more than two years