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Plant Reproduction © Lisa Michalek Reproductive Structures of Seed Plants The tiny gametophytes (gametes) of seed plants consist of only a few cells. An immature male gametophyte of a seed plant is a pollen grain, which has a thick protective wall. A female gametophyte of a seed plant develops inside an ovule. Following fertilization, the ovule and its contents develop into a seed. Reproductive Structures of Seed Plants Because the gametophytes of seed plants are very small, seed plants are able to reproduce sexually without water. • Wind and animals transport pollen grains to the structures that contain ovules. The transfer of pollen grains from the male reproductive structures of a plant to the female reproductive structures of a plant is called pollination. Reproductive Structures of Seed Plants When a pollen grain reaches a compatible female reproductive structure, a tube emerges from the pollen grain. This tube, called a pollen tube, grows from a pollen grain to an ovule and enables a sperm to pass directly to an egg. Seeds Seeds contain the embryos of seed plants. A seed forms from an ovule after the egg within it has been fertilized. The outer cell layers of an ovule harden to form the seed coat as a seed matures. The tough seed coat protects the embryo in a seed from mechanical injury and from a harsh environment. Seeds Different kinds of seeds germinate under different conditions. Some seeds must dry out before they germinate. Other seeds must be exposed to light. In other seeds, chemicals that inhibit germination must be washed away by rain. Often, a seed must be exposed to cold temperatures, or the seed coat must be damaged before the seed can germinate. Seeds enable the embryos of seed plants to survive conditions that are unfavorable for plant growth for long periods of time. Seeds Seeds contain tissue that provides nutrients to plant embryos. The seeds of angiosperms develop a nutritious tissue called endosperm. Endosperm originates at the same time an egg is fertilized. • In some angiosperms (such as corn and wheat) endosperm is still present in mature seeds. • In other angiosperms (such as beans and peas) the nutrients in the endosperm have already been transferred to the embryo by the time a seed is mature. Seeds Leaf-like structures called cotyledons, or seed leaves, are part of a plant embryo. Cotyledons function in the transfer of nutrients to the embryo. In the flowering plants, the embryos of monocots have one cotyledon, and the embryos of dicots have two cotyledons. Cones The gametophytes of gymnosperms develop in cones, which consist of whorls (circles) of modified leaves called scales. Gymnosperms produce two types of cones. • Male cones (pollen cones) produce pollen grains within sacs that develop on the surface of their scales. • Female cones (seed cones) produce ovules on the surface of their scales. Many gymnosperms produce both male and female cones on the same plant. In some gymnosperms, male and female cones form on separate plants. Cones Pollen cones produce large quantities of pollen grains that are carried by wind to female cones. At the time of pollination, the scales of a female cone are open, exposing the ovules. When a pollen grain lands near an ovule, a slender pollen tube grows out of the pollen grain and into the ovule. The sperm moves through the pollen tube and enters the ovule. Seed cones close up after pollination and remain closed until the seeds within them are mature. • This process can take up to two years. Flowers In angiosperms, gametophytes develop within flowers. Flower parts are arranged in concentric whorls. The outermost whorl consists of one or more sepals, which protect a flower from damage while it is a bud. The second whorl consists of one or more petals, which attract pollinators. Flowers The third whorl consists of one or more stamens which produce pollen. • Each stamen is made of a threadlike filament that is topped by a pollen producing sac called an anther. The fourth and innermost whorl consists of one or more pistils, which produce ovules. • Ovules develop in a pistil’s swollen lower portion, which is called the ovary. • Usually, a stalk, called the style, rises from the ovary. Pollen lands on and sticks to the stigma, the swollen, sticky tip of the style. Flowers Flowers may or may not have all four of the basic flower parts. A flower that has all four parts is called a complete flower. Flowers that lack any one of the four types of parts are called incomplete flowers. If a flower has both stamens and pistils, it is called a perfect flower. Flowers that lack either stamens or pistils are called imperfect flowers. Flowers and Their Pollinators Many flowers have brightly colored petals, sugary nectar, strong odors, and shapes that attract animal pollinators. Flowers are a source of food for pollinators such as insects, birds, and bats. A bee gets coated with pollen as it visits a flower and then carries that pollen to other flowers. Bees locate flowers by scent first and then by color and shape. Bee-pollinated flowers are usually blue or yellow and often have markings that show the location of nectar. Flowers and Their Pollinators Moths, which feed at night, tend to visit heavily scented white flowers which are easy to find in dim light. Flies may pollinate flowers that smell like rotten meat. Many flowers are not pollinated primarily by insects. Red flowers may be pollinated by hummingbirds. Some large white flowers that open at night are pollinated by bats. Many flowers, such as those of grasses and oaks, are pollinated by wind. • Wind pollinated flowers are usually small and lack bright colors, strong odors, and nectar. Asexual Reproduction Some plants are able to reproduce asexually. The new individuals that result from asexual reproduction are genetically the same as the parent plant. Plants reproduce asexually in a variety of ways that involve nonreproductive parts, such as stems, roots, and leaves. • The reproduction of plants from these parts is called vegetative reproduction (or vegetative propogation). Many of the structures by which plants reproduce vegetatively are modified stems, such as runners, bulbs, corms, rhizomes, and tubers. Stems Modified for Vegetative Reproduction Name Description Examples Runner Horizontal, above ground stem Airplane plant, Bermuda grass Bulb Very short, stem with Onion, Daffodil, thick, fleshy leaves; only Tulip in monocots Corm Very short, thickened, underground stem with thin, scaly leaves Gladiolus, Crocus Rhizome Horizontal underground stem Iris, Fern, Sugar cane Tuber Potato, Caladium Swollen, fleshy, underground stem Asexual Reproduction Vegetative reproduction is faster than sexual reproduction in most plants. A single plant can spread rapidly in a habitat that is ideal for its growth by reproducing vegetatively. A mass of hundreds or even thousands of individuals, such as a stand of grasses or ferns, may have come from one individual.