23 Reproduction in Plants section 1 Introduction to Plant Reproduction Before You Read On the lines below, describe the characteristics you use to recognize friends and family members in old photos. In this section, you will learn that the way a young plant looks can be quite different from the way it will look when it is mature. The life cycle of mosses, ferns, and conifers includes alternation of generations. What You ll Learn forms of vegetative reproduction stages of alternation of generations reproduction of mosses, ferns, and conifers Read to Learn Vegetative Reproduction Vegetative reproduction is a form of asexual reproduction in which new plants grow from parts of an existing plant. The new plants are genetically identical to the original plant. It is often faster to grow plants by vegetative reproduction than to grow plants from seeds. Plants grown by vegetative reproduction are more similar than plants grown from seeds. Vegetative reproduction is the only way to reproduce fruits that do not produce seeds. Does natural vegetative reproduction occur? When there is little water, some mosses dry out and are easily broken and scattered by animals and wind. When it rains, some of these pieces will grow. New strawberry plants can grow at the end of horizontal stems, called stolons. If a stolon is cut, the new plant continues to grow. How do humans use vegetative reproduction? Farmers and scientists have used leaves, roots, and stems cut from some plants to grow new plants. A potato can be cut into pieces. Each piece with an eye (bud) can be planted and grown into a new potato plant. A few cells of plant tissue can also be used to grow some plants, using a technique called tissue culture. Create a Quiz After you read this section, create a fivequestion quiz from what you have learned. Then, exchange quizzes with another student. After taking the quizzes, review your answers together. 1. Draw Conclusions Would a piece of potato without an eye grow into a new potato plant? Explain. Reading Essentials Chapter 23 Reproduction in Plants 269
2. Identify Which stage produces haploid spores? Picture This 3. Explain to a partner the alternation of generations using the figure. Fertilization forms zygote (2n) Male gamete (sperm) Alternation of Generations Recall that the life cycle of a plant includes an alternation of generations. One generation is a diploid (2n) sporophyte stage. The other generation is a haploid (n) gametophyte stage. As shown below, the sporophyte stage produces haploid spores. The spores divide by mitosis to form the gametophyte generation. The size of the gametophyte generation depends on the plant species. As plants have become more complex, smaller gametophytes have evolved. The gametophyte stage produces gametes eggs and sperm. Nonvascular plants and some vascular plants need water for sperm to reach an egg. Flowering plants do not need water for sperm to reach an egg. Fertilization of an egg by a sperm forms a zygote. A zygote is the first cell of the sporophyte stage. As plants have become more complex, larger sporophytes have evolved. The growth pattern of the sporophytes has also evolved. Most nonvascular plants have sporophytes that depend on the gametophyte for support and food. Flowering plants and other vascular plants have sporophytes that do not depend on the gametophyte for support and food. In these types of plants, the sporophytes live apart from the gametophyte. Female gamete (egg) Sporophyte Diploid cells (2n) Haploid cells (n) Mature gametophyte Reproductive cells develop Meiosis forms spores (n) 270 Chapter 23 Reproduction in Plants Reading Essentials
Moss Reproduction and Life Cycle The life cycle of mosses includes an alternation of generations. As shown below, the gametophyte stage is the dominant generation. The gametophyte grows in damp shady places or on rocks along a stream. Gametophytes can produce archegonia and antheridia. These structures can be on the same moss plant but often are on separate moss plants. The archegonium of some moss species produces one egg. The archegonium of other moss species produces many eggs. The tissues of the archegonium surround the egg or eggs with a protective layer. Antheridia produce sperm that have flagella. These sperm need water to move to the archegonium. Sperm exhibit chemotaxis (kee moh TAK sus) because they respond to chemicals produced by archegonia. The fertilized egg forms the first cell of the sporophyte stage, called the zygote. Tissues of the archegonium protect the zygote. The new sporophyte gets nutrients from the archegonium as it grows and matures. The sporophyte does not undergo photosynthesis. Instead, it depends on the gametophyte for nutrition. A mature sporophyte consists of a stalk with a capsule at its tip. Some cells within the capsule undergo meiosis and produce spores. If a spore lands in a suitable place, mitosis begins. A small, threadlike structure called a protonema forms. A protonema can develop into the gametophyte plant, and the cycle repeats. 4. Name the dominant stage in the life cycle of a moss plant. Picture This 5. Highlight the area where eggs are produced. Circle the area where sperm are produced. Reading Essentials Chapter 23 Reproduction in Plants 271
Picture This 6. Identify Circle the name of the first cell of the sporophyte stage. 7. Predict What would happen if a fern spore landed on dry, nutrient-poor soil? Fern Reproduction and Life Cycle Spore-producing structures called sori are found on a fern s lacy fronds. Each sorus consists of sporangia. Cells in the sporangium undergo meiosis. The resulting spores begin a new gametophyte generation, as shown in the figure above. If a fern spore lands on damp, nutrient-rich soil, it can grow and form a tiny heart-shaped gametophyte called a prothallus (pro THA lus) (plural, prothalli). Because cells of the prothallus contain chloroplasts, photosynthesis can occur. Most prothalli develop both antheridia and archegonia. Antheridia produce sperm that have flagella. These sperm need water to move to the archegonia. Each archegonium contains one egg. When a sperm fertilizes an egg, a zygote is formed. The zygote is the first cell of the sporophyte stage. The zygote undergoes mitosis and forms a photosynthetic, multicellular sporophyte. Initially the sporophyte grows on the prothallus and receives nutrients and support. Later the prothallus dissolves and the sporophyte develops fronds and a rhizome. A rhizome is a thick underground stem that produces roots and supports the fronds. Photosynthesis occurs in the fronds. 272 Chapter 23 Reproduction in Plants Reading Essentials
Conifer Reproduction and Life Cycle Pine trees are examples of the sporophyte generation of conifer plants. Conifers are heterosporous. Heterosporous (he tuh roh SPOR us) plants make two types of spores that develop into male and female gametophytes. What are the differences in conifer cones? Each scale of a female cone has two ovules at its base. Within each ovule, meiosis produces four megaspores. Three megaspores dissolve. One megaspore undergoes mitosis and becomes the female gametophyte. A fully developed female gametophyte consists of hundreds of cells and contains two to six archegonia. Each archegonium contains one egg. Male cones have many small scales. The scales have hundreds of sporangia. Cells in these sporangia undergo meiosis and form microspores. A pollen grain is made of four cells and develops from microspores. Pollen grains are carried by air. How does pollination occur? Pollination occurs when a pollen grain from one species of seed plant lands on the female reproductive structure of a plant of the same species. The opening of a female cone s ovule is called the micropyle. A conifer pollen grain that lands near the micropyle is trapped in a sticky substance called a pollen drop. As the pollen drop is absorbed into the ovule, the pollen grain is pulled closer to the micropyle. The pollen grain will continue to develop. How do seeds develop? Seed development can take as long as three years. After pollination, the pollen grain produces a pollen tube. The pollen tube grows through the micropyle and into the ovule. One of the four cells in the pollen grain undergoes mitosis, forming two sperm. The sperm travel through the pollen tube to an egg. When one sperm fertilizes an egg, a zygote is formed. The other sperm and the pollen tube dissolve. The zygote depends on the female gametophyte for nutrition as it undergoes mitosis. After mitosis, an embryo with one or more cotyledons has formed. Cotyledons undergo photosynthesis and provide nutrition for the embryo when the seed sprouts. The outside layer of the ovule forms a seed coat. The female cone opens and releases mature seeds. 8. Compare What is the male gametophyte of a conifer? 9. Summarize pollination in conifers. Reading Essentials Chapter 23 Reproduction in Plants 273