Plant Life Cycles Alternate • Plants produce gametes, but their life cycle includes a few extra steps. • Plants complete their life cycle by alternating between two phases. • One phase involves a Diploid (2n) plant body that produces spores. • The other phase involves a Haploid (n) plant body that produces gametes. • This type of life cycle that alternates between diploid and haploid phases is called Alternation of Generation.
General Overview of Alternation of Generation • The diploid phase begins with a fertilized egg (zygote). • This divides by mitosis and grows into a mature Sporophyte. • A mature sporophyte has cells that divide by meiosis to produce haploid spores. • A Spore marks the beginning of the Haploid phase of the plant life cycle. • A spore divides by mitosis and grows into a mature Gametophyte (gamete producing plant). • Specialized parts of a mature gametophyte produce gametes (sperm and egg). • When sperm meets egg, fertilization takes place and the cycle continues with a new sporophyte. • Why must gametophyte cells divide by mitosis?
Life Cycles in Different Plant Groups • The sporophyte and gametophyte stages look different in nonvascular plants, vascular plants, and seed plants.
Moss Life Cycle • The gametophyte stage is dominant. • The green carpet-like plants that you recognize as moss are gametophytes. • Moss sporophytes look like a brown stem topped with a tiny cup called a capsule. • These capsules contain spore producing sacs called. sporangia. • Male structures produce sperm with flagella and female structures produce eggs. • The sperm can get to the egg when water is present.
Fern Life Cycle • The sporophyte is the dominant stage. • The plants that you recognize as ferns. • On the underside of a fern leaf (frond) you will see clusters of sporangia called sori. • Spores are released from the sori when they are mature. • If they land in a good spot, they develop into a gametophyte (prothallus) • The gametophyte anchors using rhizoids. • When water is present, male structures release sperm that swim towards the egg. • After fertilization, Fiddleheads form and slowly uncurl as they grow. • Eventually the sporophyte will grow into a new fern.
Conifer Life Cycle • The sporophyte is the dominant stage. • Seed plants produce two types of spores that develop into male and female gametophytes. • Gametophytes of seed plants are also microscopic. • Confers like Pine trees produce 2 different cones (male and female) . • Female cones are usually larger and more scaly than male cones. • Male spores develop into pollen and female spores develop into eggs. • Pollination occurs in a cone-bearing plant when a pollen grain reaches the small opening of an ovule. • A pollen tube will then form down to the egg. • If sperm fertilizes egg a zygote forms and grows into a new plant. • What is the difference between how seedless plants and seed plants disperse to new areas?
Flowers • Flower parts are arranged in layers. • Sepals are modified leaves that protect the developing flowers. • Petals are also modified leaves that are usually bright colors to attract animal pollinators. • Flowering plants that are not pollinated by animals usually have very small sepals and petals, or none at all.
Flowers • Most flowers have both male and female structures. • A Stamen is the male structure of a flower. • It is made up of the filament (stalk) and the anther (produces pollen grains). • The Carpel (Pistil) is the female structure made up of three parts: • The Stigma is the sticky top that serves as the landing spot for pollen. • The Style is the tube connecting the stigma to the ovary. • The Ovary is found at the base of the flower and produces gametophytes. • What parts of conifers have functions similar to stamens?
Pollination • When a pollen grain reaches the stigma of the same plant species, pollination has occurred. • You can often tell how a flowering plant is pollinated by looking at its flowers. • Wind pollinators are usually small and produce large amounts of pollen. • Those that use insects, birds or other animals are specialized. • While drinking nectar, animals get pollen stuck to them and transport the pollen elsewhere. • Why is pollination more reliable by animals than by wind?
Fertilization • The sporophyte is the dominant stage.
Male Gametophytes • Anthers produce pollen grains. • Cell within the anthers divide by meiosis to produce four male spores. • Each spore divides by mitosis, producing two haploid cells. • The two cells together produce one pollen grain.
Female Gametophytes • One female gametophyte can form in each ovule of a flower’s ovary. • One cell in the ovule divides by meiosis to produce four female spores. (3 usually die) • The nucleus of the last spore grows, diving by mitosis three times (1 spore with 8 nuclei). • The end product is an embryo sac and an egg.
Double Fertilization • After pollination, one cell in the pollen grain grows into a pollen tube. • The tube grows to the ovule. • Two sperm travel down the pollen tube. (one fertilizes the egg). • The other sperm combines with the polar nuclei in the embryo sac and will become the endosperm. • The Endosperm is the food supply for the developing plant embryo. • Double Fertilization is the process in which one sperm fertilizes an egg and the other form a triploid cell. • Happen ONLY in flowering plants. • What is the function of each sperm during double fertilization?
Seeds and Fruit • At fertilization the ovule becomes a seed (contains an embryo and a nutritious endosperm in a seed coat). • While the seed develops, the surrounding ovary grows into a fruit. • Fruit is the mature ovary of a flowering plant. • Ex: Apples, watermelons, and cherries are fruits. • Sweet peppers, tomatoes, cucumbers, peanut shells are also considered “fruits” • Flowering plants that produce more seeds, produce larger fruits. • What is the major difference between seeds of flowering plants and seeds of cone-bearing plants?
Seed Dispersal • Seed dispersal is important because a plant that grows right next to its parent may compete with it for space, sunlight, water and nutrients. • Fruits come in many shapes and sizes, each adapted to spread seed to new areas. • Fleshy fruits are designed to be eaten and digested with the seeds passing through the digestive system. • Some plants have burrs, which are designed to stick to fur. • Seeds dispersed by wind often have fruits that act like parachutes or wings. • Some plants grow by water and have seeds that float (like coconuts). • Why is it important for a fruit to ripen when its seeds are mature?
Seeds Grow • When a seed is released, it may be days, months, or years until the seeds begin to grow into new plants. • Dormancy • Dormancy is the period of time when the embryo has stopped growing. • This can take place for years, until conditions are right for growth. • Germination • During Germination, the embryo breaks out of the seed coat and being sto grow into a seedling. • Germination begins when the embryo starts to take up water. • As the embryo grows, the embryonic root (radicle) breaks through the cracks. • Germination continues until the plant can begin photosynthesis and make its own food. • Which emerges first from a seed, a root, or a shoot?
Plants can Reproduce Asexually • Plants can reproduce sexually and asexually. • Sexual reproduction gives rise to genetic diversity. • Asexual reproduction allows a well-adapted plant to make many copies of itself. • Plants that can grow a new individual from a fragment of a stem, leaf, or root are reproducing by Regeneration. • Ex: prickly pear cactus have a jointed stem that looks like teardrop-shaped pads stuck together. • If one of these pads fall off it can take root and form a new plant.
Plants can Reproduce Asexually • Vegetative Reproduction is a type of asexual reproduction in which stems, leaves, or roots attached to the parent plant produce new individuals. • Ex: The Aspen Tree Forrest in Utah is actually 47,000 trunks growing from the roots of one parent plant. • Many plants have structures specifically designed for vegetative reproduction: • Stolons – Horizontal stems, also called runners. • Rhizomes – Horizontal underground stems. • Tubers – Large underground stem modified for storage. • Ex: the eyes of a potato can sprout new plants. • Bulbs – Bulbs are underground stems surrounded by modified leaves adapted for storage, covered with a papery skin. • What distinguishes regeneration from vegetative reproduction?
Humans can also use Vegetative Propagation • Plant growers use a process called vegetative propagation to grow plants with desirable qualities, such as seedless fruits or tolerance to frost. • Many plants are created using cuttings from stems or leaves. • Fruit and nut trees growers usually use trees that have been produced by grafting, or joining vegetative structures from two or more plants together. • What is a benefit of producing houseplants through asexual reproduction?
Plant Hormones • A Hormone is a chemical messenger produced in one part of an organism that stimulates or suppresses the activity of cells in another part. • Hormones regulate many of the functions in cells. • Gibberellins • Plant hormones that produce dramatic increases in size. • Ethylene • A plant hormone that promotes ripening of fruits. • Cytokinins • A plant hormone that stimulate Cytokinesis (the final stage of cell division). • Auxins • Plant hormones involved in the lengthening of plants cells that promotes growth of new branches or stems. • The lengthening of cells caused by Auxins also controls some forms of Tropisms – the movement of plant to environmental stimulus. • If you started your own plant nursery, explain two way in which you could use different plant hormones to your advantage.
Tropisms • There are 3 major types of Tropisms: • Phototropism, • Thigmotropism, and • Gravitropsim
Tropisms • Phototropism • Is the tendency of a plant to grow towards light. • Thigmotropism • The response of many plants towards touch. • Ex: Ivy and other vines wrap themselves around structures they come in contact with. • Gravitropism • The up and down growth of a plant in response to Gravity. • Downward growth (roots) show Positive gravitropism. • Upward growth (stems) show Negative gravitropism. • Rapid Response • Some plants have rapid response that have nothing to do with tropisms. • Ex: The mimosa quickly folds its leaves together a few second after being touched. • Ex: The Venus Flytrap quickly closes its leaves on prey. • Photoperiodism • A response in plants to the signals from the changing lengths of day and night throughout the year. • Ex: Leaves shedding in the fall. • What stimulus causes each of the following tropisms: phototropism, gravitropism, thigmotropism?