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Chapter 21 The Evolution of Plants: • Primary Endosymbiosis Produced the First Photosynthetic Eukaryotes • Key Adaptations Permitted Plants to Colonize Land • Vascular Tissues Led to Rapid Diversification of Land Plants • Seeds Protect Plant Embryos • Flowers and Fruits Increase the Reproductive Success of Angiosperms • Trends in Plant Evolution o Glaucophytes, red algae and green algae are ancestors of the land plants o Early land plants:  have no plumbing  gametophyte (haploid) is main part of plant o Later land plants  All have plumbing  In most, sporophyte (diploid) is main part of plant  Many are seed plants o Gymnosperms (“naked seeds”) o Angiosperms (“enclosed seeds”)  Charophytes are the closest relatives of land plants  Algae: o Two algal groups in Plantae: Red and Green Algae o Red algae – Most are marine. o Green algae – Have chlorophyll a and b (green) in chloroplasts; store energy as starch. o Charophytes are most closely related to land plants.  Some Key Adaptations of Land Plants: 1. Cuticle, a waxy coating 2. Stomata, closable openings 3. Gametangia, organs that enclose gametes and prevent them from drying out. 4. Embryos, young plants contained within a protective structure. 5. A mutually beneficial association with fungi that promotes nutrient uptake and water from the soil.  Land plants: o Non-vascular (non-tracheophytes) ex. mosses o Vascular (tracheopythes):  No seeds ex. ferns  With seeds, but no flowers ex. pines  With seeds and flowers ex. orchids  Land plants all have alternation of generations: o Includes a multicellular diploid stage and a multicellular haploid stage. o Gametes are produced by mitosis, spores are produced by meiosis. o Spores develop into multicellular haploid organisms.  Sporophyte – the multicellular diploid plant o Cells in specialized structures called sporangia produce haploid spores by meiosis.  Gametophye – the multicellular haploid plant. Spores develop into gametophytes. o Gametophyte produces haploid gametes by mitosis. o Fusion of gametes produces a diploid zygote, which develops into the sporophyte.  Nonvascular plants: o Live in moist habitats; have thin cuticles. o Mostly small; they have no well-developed vascular system to transport water, so size is restricted. o Small enough that minerals can be distributed throughout their bodies by diffusion o Have stomata  Mosses: o Gametophyte is photosynthetic, nutritionally independent of sporophyte. o The sporophyte, however, is always nutritionally dependent on the gametophyte and remains attached to it. o Gametes are produced in specialized structures called gametangia. The sperm must swim or be splashed by water to reach the egg. o In mosses, liquid water is still necessary for fertilization!  Vascular Tissues Led to Rapid Diversification of Land Plants o A vascular system consists of tissues specialized for transport of water and materials from one part of the plant to another. o Evolution of vascular tissue allowed land plants to spread to new environments and diversify rapidly.  Ferns: Vascular plants that are largely terrestrial o Still require liquid water for movement of sperm, so most inhabit moist woodlands and swamps o Gametophyte is small and short-lived; it is independent of sporophyte! Sporophyte can be very large (note difference from mosses) and can sometimes survive for hundreds of years.  Pollen: o Seed plants evolved independence from water for getting the sperm to the egg, allowing them to colonize drier habitats. o The male gametophyte is a pollen grain. o Seeds Protect Plant Embryos o Seeds viable for long periods. o Seed coat protects the embryo. o Many seeds have adaptations to aid in dispersal. o When the embryo begins to grow, it draws on nutrients stored in the seed. o Seeds are one of the major reasons for the evolutionary success of seed plants.  Non-flowering Seed Plants o Gymnosperms—seed plants that do not form flowers or fruits. o Ovules and seeds are not protected by ovary or fruit tissue. “Gymnosperm” means naked seed. o Has simpler plumbing than in flowering plants. o Most are woody plants. Our example of a non-flowering seed plant is a conifer (cone-bearing) tree-a pine tree.  Seeds Protect Plant Embryos o Conifers: dominate forests at high latitudes and high altitudes.  Cones contain the reproductive structures.  female cones in which seeds are protected by woody scales  male pollen-bearing cones  Flowering Plants- Angiosperms o Angiosperms—reproductive organs in flowers; seeds develop in ovaries and are enclosed in fruits. o Female gametophyte very reduced—usually only seven cells. o Male gametophyte is the pollen grain. o Largest clades of angiosperms:  Monocots (1 cotyledon); grasses, lilies, orchids  Eudicots (2 cotyledons); most trees and many shrubs and annual plants. o Synapomorphies of the angiosperms: • Double fertilization • Nutritive tissue called the endosperm • Ovules and seeds enclosed in a carpel • Germination of pollen on a stigma • Flowers • Fruits • Phloem with companion cells • Reduced gametophytes  Fruits: o Fruits develop from the ovaries after fertilization. o Fruits protect seeds and aid in dispersal. Most survive passage through the digestive organs of animals.