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Overview of Plants The Evolution of Plants All plants are multicellular autotrophs that make food by photosynthesis. All plants contain chlorophyll. Most plants are terrestrial. They and fungi both descended from multicellular protists. Aquatic life was protected from cosmic radiation by the water they lived in. This was not true of the organisms that began to move out onto the land. Plants and fungi first colonized the land about the same time, about 440 m.y.a. when atmospheric O2 and O3 (ozone) accumulated in large enough amounts to protect the living organisms from cosmic radiation. 1) 2) 3) Before plants could survive on land, they had to solve three serious problems: They had to be able to absorb needed minerals from the rocky surface of the land. They had to prevent water loss in the dry environment and support their bodies. They had to have a way to reproduce on land. Problems Solved! Absorbing Minerals: The first land plants did not have roots. Symbiotic relationships called mycorrhizae developed between the first land plants and early fungi. The plants make carbohydrates needed by both organisms by photosynthesis, and the fungi absorb from the rocky land the minerals needed by both organisms. Conserving Water and Supporting the Plant Body: The first land plants lived by the edges of bodies of water so they could replace lost water. As plants developed a watertight waxy cuticle, they could then move into drier environments. The plant cuticle is waterproof, but it also is airtight. Plants developed specialized holes in the cuticle called stomata for gas exchange. Guard cells open and close the stomata to exchange gases while preventing water loss. Plants do not have skeletons like animals do. Instead, individual plant cells have stiff cell walls made of cellulose for support. They also use water pressure in the cell’s central vacuole to help plant cells retain their shape. Large plants have developed special woody layers that also contribute to supporting the plant body in air. Reproducing on land: The gametes of land plants must be able to move from the male to the female plant without drying out. Primitive plants still need water to reproduce, but higher plants surround sperm in multi-layered structures called pollen grains, which are transmitted by wind or animals, rather than water. Evolution of a Vascular System The first land plants did not look different above or below the ground. As later land plants evolved, they developed specialized structures such as stems, roots, and leaves to help them adapt to their new environment. A vascular system transports materials like water and food throughout the plant body. Early plants also lacked a vascular system. All materials had to be transported by osmosis or diffusion. This limited plant size. All nonvascular plants are very small. Mosses have very simple vascular tissue, but it is not well-developed. They do not have a vascular system. The first vascular plants appeared 430 m.y.a. Today, vascular plants dominate almost every habitat. 1) 2) 3) Modern vascular plants can grow very tall, and have 3 features in common : All vascular plants have a dominant sporophyte form in their life cycle. All vascular plants have specialized conducting tissues called xylem (transports water) and phloem (transports food). The bodies of vascular plants have a central shaft from which specialized structures branch out. The roots (below ground) are different from the shoots (above ground). Alternation of Generations Plants exist in two life forms that alternate with each other: the sporophyte generation and the gametophyte generation. The sporophyte generation is diploid and The gametophyte generation is haploid and undergoes meiosis to produce haploid produces eggs or sperm by mitosis. The eggs spores, which grow up into the and sperm unite during fertilization to form gametophyte. the zygote, which grows into the diploid sporophyte and completes the cycle. (Meiosis ) Haploi d Spores Sporophyte (diploid) (Mitosis) Fertilization ! (diploid zygote) (Growth by Mitosis) Gametophyte (haploid) eggs sper Evolution of Plant Life Cycles The life cycle of non-vascular plants is dominated by a large gametophyte generation that supports a smaller, dependent sporophyte generation. The fern life cycle represents an intermediate stage in the evolution of plant life cycles. In ferns and other seedless vascular plants, the sporophyte is dominant and the gametophyte is smaller, but independent and self-sufficient. Non-vascular plants (mosses and liverworts) Large, Dominant Gametophyte Small, Dependent Sporophyte In seed plants, the gametophyte has become much smaller, and are entirely dependent on the sporophyte generation for all the necessities of life. The seed plant you see is the sporophyte generation. Gametophytes produce either eggs or sperm, which are haploid. Seedless vascular plants (ferns) Dominant Sporophyte Smaller, but Independent Gametophyte Higher vascular plants Large and Dominant Sporophyte Small, Dependent Gametophyte Evolution of Seeds – Part 1: Ferns Following the development of a vascular system, the next great advance was the development of seeds. The first vascular plants did not produce seeds. Ferns reproduce using spores.The great carboniferous fern forests of the Paleozoic Era needed a moist climate. Like nonvascular plants, ferns have swimming sperm and need some water for fertilization to occur. The fern gametophyte produces eggs in organs called archegonia and sperm in organs called antheridia. The same plant has both male and female sex organs. When a film of water is present, sperm are able to swim to the eggs and fertilize them. The fern life cycle represents an intermediate stage between the lower plants and the seed plants. The sporophyte stage is dominant, but the gametophyte, while smaller, is still able to live independently. The fern sporophyte consists of roots, underground stems called rhizomes, and long, highly divided leaves called fronds. The fern gametophyte is a small thin, heart-shaped photosynthetic plant that lives in moist places and is no more than 1 cm in diameter. Evolution of Seeds – Part 2: sporophyte generation produces two Gymnosperms The kinds of spores: microspores, which The word gymnosperm means “naked produce the male gametophyte, and seed.” Gymnosperm seeds do not develop megaspores, which produce the female inside of a fruit (a mature ovary). gametophyte. Gymnosperms first appeared about 380 m.y.a., and were the first seed plants. The Pollen grains are actually microspores, that produce the male gametophyte and its flowering plants (angiosperms) evolved sperm. from gymnosperms between 150-200 m.y.a. An ovule contains a megaspore that and are the most recently evolved plant produces the female gametophyte and its phylum. egg. In seed plants, the gametophyte is very In many gymnosperms, the fertilized egg much reduced in size, and is no longer (zygote) is retained in a cone until it is independent. Seed plants produce two mature. kinds of gametophytes: the male (microgametophyte) and the female Female gametophyte (megagametophyte). Male gametophyte sperm Pollen grain (microspore) egg Ovule (megaspore) What is a Seed? A seed is a sporophyte plant embryo surrounded by a protective seed coat, which keeps the embryo from drying out. In addition, seeds often contain a food supply for the developing embryo. Seeds have allowed plants to become adapted to life on land in three major ways: 1) 2) 3) Dispersal — many seeds have appendages that aid in dispersal by water, wind, or animals. Dispersal prevents competition between parent and offspring. Nourishment — Most kinds of seeds have lots of food stored within them, which provides the young embryo with an energy source to begin its growth. Dormancy — Seeds can lie dormant for many years, protecting the embryo from unfavorable conditions Evolution of Flowers Gymnosperms have not efficiently solved the problem of fertilization. Many pollen grains must be carried by wind in order to ensure the joining of sperm and egg within a female cone. Angiosperms often have pollen delivered directly from one individual to another, thus greatly increasing the chance of fertilization. This innovation is made possible by the evolution of the flower. Layers of a Flower What is a flower? It is the reproductive structure of angiosperms. The basic structure of a flower consists of four concentric whorls of appendages: Calyx — outermost whorl, made of sepals, which are modified leaves, and protect the bud from damage. Corolla — next layer, made of one or more petals, which are also modified leaves, but produce bright pigments or strong fragrances to attract animals. Androecium — produces the microgametophytes, or pollen grains. This layer is made of the stamens. Gynoecium — is the innermost layer of the flower and consists of one or more pistils, containing the ovary and ovules. Flowers containing all four parts are complete, while flowers missing one or more parts are incomplete. Parts of a Flower Stamen: Anther Filament Petals Sepals The outermost layer of a flower consists of sepals, which serve to protect the developing flower parts. Inside of this is the whorl of petals, which are often brightly colored to attract animals. The next layer of a flower contains the male reproductive structures. Stamens consist of two parts: the filament, a strong stalk, and the anther, Pistil: which rests on the filament and Stigma produces pollen grains. Style Ovary with ovules The inside layer of a flower contains the female reproductive structures. These may be one or more pistils. The pistil is subdivided into the ovary, where ovules develop, the sticky stigma, where pollen lands, and the style which connects them to each other.