Question

31. Identify the major differences between the sexual life cycles of mosses, ferns, gymnosperms, and angiosperms.

long detail answer

Answer 1

Mosses

The sexual reproduction of the moss (bryophyte) life cycle alternates between diploid sporophyte and haploid gametophyte phases. In a nutshell, haploid gametophytes produce haploid gametes, which can be sperm or eggs. When egg and sperm merge, they form a diploid zygote which grows into a diploid sporophyte. Sporophytes produce haploid spores, containing genetic information from both haploid gametophyte parents. A spore gives rise to a haploid gametophyte, completing the cycle.

Ferns

Ferns are leafy vascular plants. While they have veins that permit the flow of water and nutrients like conifers and flowering plants, their life cycle is very different. Conifers and flowering plants evolved to survive hostile, dry conditions. Ferns require water for sexual reproduction.Ferns are leafy vascular plants. While they have veins that permit the flow of water and nutrients like conifers and flowering plants, their life cycle is very different. Conifers and flowering plants evolved to survive hostile, dry conditions. Ferns require water for sexual reproduction.Ferns are leafy vascular plants. While they have veins that permit the flow of water and nutrients like conifers and flowering plants, their life cycle is very different. Conifers and flowering plants evolved to survive hostile, dry conditions. Ferns require water for sexual reproduction.The fern life cycle requires two generations of plants to complete itself. This is called alternation of generations.Ferns use both sexual and asexual reproduction methods.In sexual reproduction, a haploid spore grows into a haploid gametophyte. If there is enough moisture, the gametophyte is fertilized and grows into a diploid sporophyte. The sporophyte produces spores, completing the life cycle.Asexual methods of reproduction include apogamy, poliferous frond tips, and rhizome spreading.

Gymnosperms

Gymnosperms are vascular plants that produce seeds in cones. Examples include conifers such as pine and spruce trees. The gymnosperm life cycle has a dominant sporophyte generation. Both gametophytes and the next generation’s new sporophytes develop on the sporophyte parent plant. ones form on a mature sporophyte plant. Inside male cones, male spores develop into male gametophytes. Each male gametophyte consists of several cells enclosed within a grain of pollen. Inside female cones, female spores develop into female gametophytes. Each female gametophyte produces an egg inside an ovule.ones form on a mature sporophyte plant. Inside male cones, male spores develop into male gametophytes. Each male gametophyte consists of several cells enclosed within a grain of pollen. Inside female cones, female spores develop into female gametophytes. Each female gametophyte produces an egg inside an ovule.In gymnosperms, the gametophyte generation takes place in a cone, which forms on the mature sporophyte plant.Each male gametophyte is just a few cells inside a grain of pollen.

Angiosprms

Flowers are modified leaves containing the reproductive organs of angiospems; their pollination is usually accomplished by animals or wind.They are seed-producing plants that generate male and female gametophytes, which allow them to carry out double fertilization.The adult, or sporophyte, phase is the main phase of an angiosperm’s life cycle. As with gymnosperms, angiosperms are heterosporous. Therefore, they generate microspores, which will produce pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Inside the anthers’ microsporangia, male gametophytes divide by meiosis to generate haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall. Within each megasporangium, a megasporocyte undergoes meiosis, generating four megaspores: three small and one large. Only the large megaspore survives; it produces the female gametophyte referred to as the embryo sac. The megaspore divides three times to form an eight-cell stage. Four of these cells migrate to each pole of the embryo sac; two come to the equator and will eventually fuse to form a 2n polar nucleus. The three cells away from the egg form antipodals while the two cells closest to the egg become the synergids.he mature embryo sac contains one egg cell, two synergids (“helper” cells), three antipodal cells, and two polar nuclei in a central cell. When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle, an opening in the integuments of the ovule. The two sperm cells are deposited in the embryo sac.A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote, the future embryo. The other sperm fuses with the 2n polar nuclei, forming a triploid cell that will develop into the endosperm, which is tissue that serves as a food reserve. The zygote develops into an embryo with a radicle, or small root, and one ( monocot ) or two (dicot) leaf-like organs called cotyledons. This difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots and the eudicots. Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and the well-protected embryo at the center.Some species of angiosperms are hermaphroditic (stamens and pistils are contained on a single flower), some species are monoecious (stamens and pistils occur on separate flowers, but the same plant), and some are dioecious (staminate and pistillate flowers occur on separate plants). Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent (wind or water) or an animal, such as an insect or bird. Cross-pollination increases genetic diversity in a species.