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?

Conifer Life Cycle

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?