Alternation of Generations and Plant Life Cycles Haploid and Diploid generations alternate in plant life cycles. Mosses have a dominant gametophyte. Ferns, like most plants, have a dominate sporophyte. A pine tree is a sporophyte with tiny gametophytes in its cones. Cones are a significant adaptation to land. They harbor all of a conifer tree’s reproductive structures. Diploid sporangia, which produce haploid spores by meiosis. Haploid female and male gametophytes.

Alternation of Generations Gymnosperms- Pine tree Female cone has many hard, radiating scales, each bearing a pair of ovules = (sporangium and a covering) Males cones are generally much smaller than females cones. Each scale on a male cone produces many sporangia, each of which makes numerous haploid spores by meiosis. Male gametophytes or pollen grains, develop from the spores. When the male cone mature, the scales open and release a cloud of pollen.

Alternation of Generations Gymnosperms- Pine tree Pollen grains house the cells that will develop into sperm. Pollination occurs when a pollen grain lands on and enters an ovule. After pollination, meiosis occurs in the ovule. A haploid spore cell begins developing into the female gametophyte. Not until months later do eggs appear within the female gametophyte. It also takes months for sperm to develop in the pollen grain.

Alternation of Generations Gymnosperms- Pine tree A tiny tube grows out of the pollen grain and eventually releases a sperm into the egg. Fertilization does not occur until more than a year after pollination. Following fertilization, the zygote develops into a sporophyte embryo and the whole ovule transforms into the seed. In a typical pine, seeds are shed from cones about two years after pollination.

Alternation of Generations Gymnosperms- Pine tree The seed falls to the ground, or is dispersed by wind or animals, and when conditions are favorable, it germinates and grows into a tree.

Alternation of Generations Gymnosperms- Pine tree Summary All the reproductive stages of conifers are housed in cones borne on sporophytes. The ovule is a key adaptation- a protective device fro all the female stages in the life cycle, as well as the site of pollination, fertilization, and embryonic development. The ovule becomes the seed, a major factor in the success of the conifers and flowering plants on land.

Angiosperms Dominate most landscapes today, except the northern parts of the globe. 400 times more species of angiosperms than gymnosperms. Nearly 80% of all plants are angiosperms. Gymnosperms supply most of our lumber. Angiosperms supply nearly all our food and much of our fiber for textiles. Cereal grains, including wheat, corn, oats, and barely, are flowering plants, as are citrus and other fruit trees, garden vegetables and cotton. Fine hardwoods from flowering plants, such as oak, cherry, and walnut trees, supplement the lumber from conifers.

Angiosperms The flower is the centerpiece of angiosperm reproduction A flower is actually a short stem with four kinds of modified leaves called sepals, petals, stamens, and carpels. Sepals Bottom of the flower and are usually green. They enclose the flower before it opens. Petals Above the sepals, which are usually the most striking part of the flower and are often important in attracting animal pollinators.

Angiosperms The flower is the centerpiece of angiosperm reproduction Stamen Reproductive structure Consists of a stalk bearing a sac called anther. The male organ in which pollen grains develop. Carpal Consist of a stalk with an ovary at the base and a sticky tip known as the stigma, which traps pollen. Ovary is a protective chamber containing one or more ovules, in which the eggs develop.

Angiosperms The angiosperm plant is a sporophyte with gametophytes in its flowers Meiosis occurring in the anthers of the flower leads to the haploid spores that undergo mitosis and form the male gametophytes, or pollen grains. Meiosis in the ovules leads to haploid spores that undergo mitosis and form the female gametophytes, each produce an egg. Pollination occurs when a pollen grain lands on the stigma. As in gymnosperms, a tube grows from the pollen grain to an egg and a sperm fertilizes the egg creating a zygote. Fertilization occurs about 12 hrs after pollination.

Angiosperms Structure of a fruit reflects its function in seed dispersal Dandelion fruit acts like a kite. Carried away by wind currents. Cockleburs attach to the fur of animals May be carried for miles before they open and release their seeds. Fleshy, edible fruits eats by animals. Seeds pass through the digestive tract and deposited some distance from where they were eaten. Fruit of flowering plants usually develop and ripens quickly, so seeds can be produced and dispersed in a single growing season.

Angiosperms The angiosperm plant is a sporophyte with gametophytes in its flowers A seed develops from each ovule. Each seed consists of an embryo (a new sporophyte) surrounded by a store of food and a seed coat. While the seeds develop, the ovary’s wall thickens, forming the fruit that encloses the seeds. When conditions are favorable, the seed germinates and the embryo grows into a mature sporophyte, completing the life cycle.

Fertilization in Angiosperms Pollen lands on the sticky stigma. Pollen grain contains tube nucleus and generative nucleus Pollen tube grows down to the ovule Generative nucleus divides into 2 sperm cells in the pollen tube Ovules are within the ovary. Embryo sac Micropyle end: 1 egg and 2 synergids End opposite of the micropyle: 3 antipodal cells In the middle: 2 haploid nuclei, the polar nuclei

Fertilization in Angiosperms When the pollen tube enters the embryo sac One sperm cell fertilizes the egg, forming a diploid zygote Second sperm fuses with both polar nuclei, forming a triploid nucleus Divides by mitosis to produce the endosperm Double fertilization