39Reproduction in Flowering

Plants

39 Sexual Reproduction in Plants

Flowers contain the sex organs of plants. They have four groups of organs: carpels, stamens, petals, and sepals.

39 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.

39 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.

39 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)

39 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

Figure 39.1 Development of Gametophytes and Nuclear Fusion (Part 1)

Figure 39.1 Development of Gametophytes and Nuclear Fusion (Part 2)

39 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.

39 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.

39 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.

39 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.

39 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.

39 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”).

39 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.

39 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.