Sexual Reproduction in Plants Flowers contain the sex organs of plants. They have four groups of organs: carpels, stamens, petals, and sepals.
Sexual Reproduction in Plants • Carpels are female sex organs. A pistil is a structure composed of one or more carpels. • The base of the pistil is the ovary, which contains one or more ovules. • Each ovule contains a megasporangium. • The stalk of the pistil is the style, and the end of the style is the stigma.
Sexual Reproduction in Plants • Stamens are male sex organs. • Each stamen is composed of a filament bearing a two-lobed anther, which consists of four microsporangia fused together. • Petals and sepals of many flowers are arranged in whorls (circles) around the carpels and stamens. • All parts of the flower are borne on a stem tip, the receptacle.
Sexual Reproduction in Plants • The multicellular, diploid plant is called the sporophyte. • In angiosperms (flowering plants), the diploid sporophyte generation is the larger and more conspicuous one. • Cells contained in sporangia undergo meiosis to produce haploid spores. • Mitosis produces the haploid plant (gametophyte)
Sexual Reproduction in Plants • Female gametophytes, the megagametophytes, are called embryo sacs and develop in megasporangia. • Male gametophytes, the microgametophytes, are called pollen grains and develop in microsporangia. Mature embryo sac of lily Pollen grains of an eudicot
Sexual Reproduction in Plants • In angiosperms, the transfer of pollen from the anther to the stigma is called pollination. • In some plants self-fertilization occurs by direct contact of anther and stigma before the flower bud opens. Pollen grains adhere to sticky stigma.
Sexual Reproduction in Plants • The pollen of many species is carried from plant to plant by wind. These plants produce pollen grains in great numbers. • Water carries pollen to some aquatic plants. • Animals such as insects, birds, and bats carry pollen among the flowers of many plants.
Sexual Reproduction in Plants • Plants can cross-pollinate or self-pollinate. • Many plants are self-incompatible; their stigma rejects the pollen from their own flowers. • The stigma can also reject pollen from other species. Pollen from the same species binds strongly to the stigma; foreign pollen falls off.
Sexual Reproduction in Plants • After a pollen grain lands on the stigma of a compatible pistil, a pollen tube develops from the grain. • The pollen tube traverses the style until it reaches an ovule. Pollen tube germinated in vitro.
Sexual Reproduction in Plants • During transport through the pollen tube, the pollen grain cell undergoes one mitotic division to produce two haploid sperm cells. • One sperm cell unites with the two polar nuclei, forming the 3n endosperm. • The other sperm cell fuses with the egg cell, forming the diploid zygote.
Sexual Reproduction in Plants • After fertilization, the zygote divides and the two daughter cells have different fates. • One daughter cell produces the embryo and the other produces a supporting structure. • The embryo is called a Cotyledon – an embryonic organ that stores and digests reserve materials (a “seed leaf”).
Sexual Reproduction in Plants • In some species the cotyledons absorb the nutrient reserves from the endosperm. • The seed will lose as much as 95 percent of its water content. • The embryo remains quiescent in this desiccated state until conditions are right for germination.
Sexual Reproduction in Plants • In flowering plants, the ovary wall together with its seeds develops into a fruit. • Fruits serve to help seed dispersal. • Winged fruit can be blown by the wind. • Coconuts have spread from island to island by floating in the ocean. • Some seeds hitch rides on animals. • Fleshy, edible fruits may be eaten by birds and other animals and the seeds pass through the digestive tract.