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Honors Biology
Table of Contents
Unit 14 Overview: Plant Structure, Function & Reproduction
14.1 Plant Tissues and Organs
14.2 Plant Transport Systems
14.3 Plant Reproduction and Life Cycles
14.4 Plant Responses to Environmental Stimuli

Plants have a unique life cycle that alternates between two generations. This process, called alternation of generations, involves a diploid sporophyte stage and a haploid gametophyte stage. It's a key adaptation that allows plants to thrive on land.

Sexual reproduction in plants involves pollination and fertilization. In flowering plants, pollen carries male gametes to the female parts. This process leads to seed formation, which is crucial for plant dispersal and survival in various environments.

Alternation of Generations

Sporophyte and Gametophyte Generations

  • Plants undergo alternation of generations, a life cycle that alternates between a diploid sporophyte generation and a haploid gametophyte generation
  • The sporophyte generation is the dominant stage in vascular plants (ferns, gymnosperms, angiosperms) and produces spores through meiosis
  • Spores develop into the gametophyte generation, which is reduced and produces gametes (eggs and sperm) through mitosis
  • In non-vascular plants like mosses, the gametophyte generation is dominant and photosynthetic, while the sporophyte is nutritionally dependent on the gametophyte

Alternation and Plant Evolution

  • Alternation of generations evolved as plants adapted to life on land, allowing them to disperse and reproduce in the absence of water
  • Over the course of plant evolution, the sporophyte generation became increasingly dominant and complex, while the gametophyte generation became reduced
  • In angiosperms (flowering plants), the female gametophyte is reduced to just a few cells within the ovule, and the male gametophyte is the pollen grain

Sexual Reproduction

Pollination and Fertilization

  • Sexual reproduction in angiosperms involves pollination, the transfer of pollen (male gametophytes) from the anther to the stigma of a flower
  • Pollination can occur through various agents like wind, water, animals (bees, birds, bats), or self-pollination within the same flower
  • Once pollen lands on the stigma, it germinates and grows a pollen tube that delivers sperm to the ovule
  • Angiosperms undergo double fertilization, where one sperm fertilizes the egg to form a zygote, and another fuses with two polar nuclei to form the triploid endosperm

Seed and Fruit Formation

  • The fertilized ovule develops into a seed, which consists of an embryo (new sporophyte), endosperm (nutritive tissue), and a seed coat for protection
  • As seeds mature, the ovary wall surrounding them develops into a fruit, which aids in seed dispersal and can take various forms (berries, drupes, nuts, legumes)
  • Fruits can be fleshy (apples, tomatoes) or dry (nuts, grains) and are often adapted to specific dispersal mechanisms like animal consumption, wind, or explosive dehiscence
  • Seeds remain dormant until suitable conditions trigger germination, allowing the embryo to emerge as a seedling and grow into a mature sporophyte

Asexual Reproduction

Vegetative Propagation Methods

  • Many plants can reproduce asexually through vegetative propagation, which involves the growth of new individuals from vegetative parts of the parent plant
  • Common methods of vegetative propagation include fragmentation (breaking off of stems or leaves), layering (inducing roots to form on stems while still attached), and grafting (joining two plant parts)
  • Some plants form specialized structures for vegetative propagation, such as bulbs (onions), corms (gladiolus), tubers (potatoes), and runners (strawberries)
  • Advantages of vegetative propagation include faster growth, preservation of desirable traits, and avoidance of the energy costs of sexual reproduction

Asexual Reproduction and Agriculture

  • Asexual reproduction is widely used in agriculture and horticulture to propagate crops and ornamental plants with desirable characteristics
  • Grafting is commonly used to join disease-resistant rootstocks with high-yielding cultivars in fruit trees (apples, citrus) and grapevines
  • Many crops, such as bananas, pineapples, and sugarcane, are propagated almost entirely through vegetative means due to sterility or seedlessness
  • However, asexual reproduction can also lead to reduced genetic diversity and increased vulnerability to pests and diseases, as seen in the historical overreliance on a single potato variety (Lumper) that contributed to the Irish Potato Famine

Key Terms to Review (32)

Pollinator attraction: Pollinator attraction refers to the mechanisms and traits that plants use to entice pollinators, such as bees, butterflies, and birds, to visit their flowers for the purpose of transferring pollen. This process is crucial for plant reproduction, as successful pollination leads to the production of seeds and fruits. Plants have evolved various strategies like colorful blooms, enticing scents, and nectar rewards to attract specific pollinators, ensuring their reproductive success and genetic diversity.
Seed dormancy: Seed dormancy is a physiological state in which seeds enter a period of suspended growth, preventing germination even under favorable environmental conditions. This mechanism is crucial for plant survival, allowing seeds to remain viable until the timing is right for germination, ensuring that they do not sprout prematurely and are better suited to face the challenges of their environment.
Grafting: Grafting is a horticultural technique that involves joining two plants together so that they grow as a single plant. This method allows for the transfer of desirable traits from one plant, known as the scion, to another plant, called the rootstock. It plays a crucial role in plant reproduction and life cycles by enabling the propagation of specific varieties with improved characteristics such as disease resistance, growth habit, or fruit quality.
Bulbs: Bulbs are specialized underground storage organs of certain plants that serve as a means of vegetative reproduction. They consist of a short stem surrounded by fleshy leaves or scales, which store nutrients and energy to help the plant survive adverse conditions, like winter. This structure allows the plant to produce new growth in the spring, enabling it to flourish in various environments.
Layering: Layering is a horticultural technique used to propagate plants by encouraging roots to form on a stem while it is still attached to the parent plant. This method takes advantage of the plant's natural growth processes, allowing for the development of new plants without needing to separate them from the original plant immediately. It's often used for woody plants and can result in healthy, robust offspring that retain desirable traits from the parent.
Corms: Corms are underground storage organs of certain plants, consisting of a swollen stem base that serves as a food reserve to support the growth and development of the plant. These structures allow plants to survive unfavorable environmental conditions, facilitating regeneration and asexual reproduction. They play a crucial role in the life cycles of various flowering plants, particularly in regions with seasonal climates.
Runners: Runners are specialized stems that grow horizontally above or just below the ground, allowing plants to propagate and spread. This vegetative reproduction strategy enables plants to produce new offspring quickly and effectively, promoting their survival in various environments and enhancing their ability to colonize areas.
Tubers: Tubers are swollen, fleshy parts of a stem or root that serve as storage organs for nutrients and energy in certain plants. They play a crucial role in plant reproduction and survival, allowing the plant to store food during periods of dormancy and to produce new growth in favorable conditions. Tubers can also facilitate vegetative reproduction, enabling plants to propagate without relying solely on seeds.
Vegetative propagation: Vegetative propagation is a form of asexual reproduction in plants where new individuals are produced from vegetative parts like roots, stems, or leaves, rather than through seeds. This method allows for the rapid and efficient reproduction of plants, ensuring that desirable traits are passed on to the offspring. It is a common technique used in horticulture and agriculture to propagate clones of plants with specific characteristics.
Fragmentation: Fragmentation is a form of asexual reproduction in plants where a parent plant breaks into distinct pieces, each capable of growing into a new individual. This method allows plants to reproduce and spread rapidly, making it an effective survival strategy in various environments. Fragmentation can occur naturally, such as when a branch or stem breaks off, or it can be induced artificially through horticultural practices.
Endosperm: Endosperm is a tissue that provides nourishment to the developing embryo in seeds of flowering plants. This tissue is formed during double fertilization, where one sperm cell fertilizes the egg cell to form the zygote, and another sperm cell fuses with two polar nuclei to create the endosperm. The endosperm serves as a food reserve, supporting the growth of the embryo until it can begin photosynthesis on its own.
Fruit: Fruit is the mature ovary of a flowering plant, usually containing seeds, and is a key part of the plant's reproductive process. It develops from the fertilized ovule after pollination and serves to protect and disperse seeds, facilitating the continuation of the plant species. The structure and characteristics of fruit can vary widely among different plants, playing an essential role in the life cycles of flowering plants by aiding in reproduction and seed dispersal.
Seed: A seed is a small, embryonic plant enclosed in a protective outer covering, capable of developing into a new plant. It serves as a vital component in the reproductive cycle of plants, ensuring the continuation of the species. Seeds come in various shapes and sizes, often containing stored nutrients to support the young plant during germination and early growth.
Asexual reproduction: Asexual reproduction is a mode of reproduction where an organism can reproduce without the involvement of gametes or sexual processes, resulting in offspring that are genetically identical to the parent. This type of reproduction allows organisms to quickly increase their population size and is commonly observed in various species, contributing to their survival and adaptation in different environments.
Seed coat: The seed coat is the protective outer layer of a seed that encases the embryo and stored food, ensuring its safety during development and until germination. This structure is crucial for protecting the delicate embryo from physical damage, pathogens, and environmental stresses while providing some regulation over water absorption, which is essential for the seed's awakening process.
Embryo: An embryo is an early stage of development in a multicellular organism, particularly after fertilization when the zygote undergoes rapid cell division and differentiation. In plants, the embryo develops from the fertilized ovule and is crucial for the formation of the new plant, as it will eventually grow into the mature structure. The embryo contains the essential genetic information from both parent plants and serves as a foundation for the plant's growth and development.
Zygote: A zygote is the initial cell formed when a sperm fertilizes an egg, marking the beginning of a new organism's development. In plants, the zygote plays a crucial role in sexual reproduction, as it develops into the embryo after fertilization. This process is vital for genetic variation and propagation in plant species.
Germination: Germination is the process by which a seed develops into a new plant, beginning when the seed absorbs water and ends with the emergence of the seedling. This crucial phase in a plant's life cycle involves various physiological changes, including the activation of metabolic processes and growth of the embryo. Germination is influenced by environmental factors like temperature, moisture, and light, which together help determine whether a seed will sprout successfully.
Seed dispersal: Seed dispersal is the process by which seeds are spread away from the parent plant to new locations where they can germinate and grow. This mechanism is crucial for the reproduction of many plants, as it helps to reduce competition between seedlings and allows colonization of new areas, thus contributing to plant diversity and ecosystem stability.
Double fertilization: Double fertilization is a unique reproductive process in flowering plants where two sperm cells from a pollen grain fertilize different parts of the female gametophyte. This process leads to the formation of both a zygote, which develops into the embryo, and a triploid endosperm that nourishes the developing seed. It is an essential mechanism that ensures the successful development of seeds in angiosperms.
Pollen grain: A pollen grain is a microscopic structure produced by seed plants that contains the male gametes, or sperm cells, necessary for fertilization. These grains are crucial for sexual reproduction in flowering plants and are responsible for the transfer of genetic material from the male part of a flower to the female part during pollination.
Angiosperms: Angiosperms, commonly known as flowering plants, are a diverse group of plants that produce seeds enclosed within a fruit. They represent the largest and most varied group of land plants, characterized by their reproductive structures called flowers, which facilitate pollination and seed dispersal. The presence of fruit, which aids in protecting and dispersing seeds, is a key feature that sets angiosperms apart from other plant groups.
Sporophyte: A sporophyte is the diploid multicellular stage in the life cycle of plants and some algae that produces spores through the process of meiosis. This phase follows the gametophyte generation, where haploid gametes are formed. Sporophytes are critical in plant reproduction and life cycles, contributing to genetic diversity by generating spores that can develop into new gametophytes.
Pollination: Pollination is the process by which pollen is transferred from the male anthers of a flower to the female stigma, leading to fertilization and the production of seeds. This critical mechanism not only facilitates reproduction in flowering plants but also plays a significant role in maintaining biodiversity and supporting ecosystems through interactions with various species.
Fertilization: Fertilization is the biological process where male and female gametes unite to form a zygote, initiating the development of a new organism. This event is crucial in both plant and animal reproduction as it leads to genetic diversity and the continuation of species. In plants, fertilization often occurs after pollination, while in animals, it usually takes place within the reproductive tract of the female, setting the stage for subsequent developmental processes.
Gametophyte: A gametophyte is the haploid stage in the life cycle of plants and some algae, which produces gametes (sperm and eggs) through mitosis. This stage plays a crucial role in plant reproduction, as it allows for the formation of gametes that will eventually fuse during fertilization to form a diploid zygote, marking the beginning of the sporophyte generation. Gametophytes can be free-living or dependent on the sporophyte for nutrition, depending on the type of plant.
Alternation of generations: Alternation of generations is a biological life cycle characteristic of many plants, where organisms alternate between two distinct multicellular stages: the haploid gametophyte and the diploid sporophyte. This process involves both sexual and asexual reproduction, allowing for genetic diversity and adaptation to different environments.
Pollen tube: A pollen tube is a slender, elongated structure that grows from a pollen grain following pollination, facilitating the transfer of male gametes (sperm cells) to the ovule for fertilization. This process is crucial in the reproductive cycle of seed plants, as it ensures successful fertilization and subsequent seed development. The growth of the pollen tube occurs through the style of the flower and ultimately penetrates the ovule in the ovary, allowing for the fusion of gametes.
Mitosis: Mitosis is the process of cell division that results in two genetically identical daughter cells from a single parent cell. This essential mechanism allows for growth, tissue repair, and asexual reproduction in organisms, maintaining the chromosome number of the original cell. It plays a crucial role in life cycles and can be compared to meiosis, which involves different processes and outcomes.
Meiosis: Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the formation of four genetically diverse gametes. This process is crucial for sexual reproduction, as it ensures genetic variation and maintains the species' chromosome number across generations.
Diploid: Diploid refers to a cell or organism that contains two complete sets of chromosomes, one inherited from each parent. This genetic arrangement is crucial for sexual reproduction, allowing for genetic variation through the combination of alleles during the formation of gametes. Diploid cells are typically represented as 2n, where 'n' is the number of unique chromosomes.
Haploid: Haploid refers to a cell or organism that has a single set of chromosomes, which is half the number of chromosomes found in diploid cells. This condition is crucial in sexual reproduction, as it ensures that when two haploid gametes fuse during fertilization, they create a diploid zygote. In organisms that undergo meiosis, haploid cells are produced as gametes, playing a significant role in genetic diversity and evolution.