Chapter 32: Plant Reproduction 32-1 Plant Life Cycles 32-2 Sexual Reproduction in Flowering Plants 32-3 Dispersal and Propagation
32-1 Plant Life Cycles I. The Life Cycle of Mosses (e.g., homosporous alternation of generations) • Includes a relatively large, leafy green GAMETOPHYTE (haploid), which produces GAMETES, and a smaller SPOROPHYTE (diploid), which grows from the tip and the gametophyte and produces one type of SPORE.
(1) Antheridium (on MALE moss gametophyte) • Male reproductive structure that produces hundreds of flagellated sperm by MITOSIS. (2) Archegonium (on FEMALE moss gametophyte) • Female reproductive structure that produces a single egg by mitosis (3) Homospory (mass production of ONE type of spore) • Cells in the capsule (sporangium) undergo MEIOSIS to from haploid spores, which are all identical and will be carried out by wind.
II. The Life Cycle of a Fern (homosporous alternation of generations) • Includes a large SPOROPHYTE, which produces only one type of spore, and a small GAMETOPHYTE, which produces gametes. BOTH the EGGS and SPERM are produced on the same gametophyte—as with mosses, the flagellated sperm MUST swim to the egg.
(1) Sorus (sori) • In most ferns, certain cells on the underside of the fronds develop into sporgangi, of which get clustered into sori.
III. The Life Cycle of Conifers (heterosporous alternation of generations) • Includes a large sporophyte which produces TWO types of spores, and microscopic gametophytes, which produce gametes. • NOTE: Female gametophytes produce eggs, and male gametophytes produce sperm (pollen). The NON-flagellated sperm reach the eggs through a pollen tube.
(1) Microspores (male) and Megaspores (female) [HETEROSPORY] • Develop into the male and female gametophytes inside the cones, AND remain INSIDE the sporophyte. (2) Microsporangia (male cones) and Megasporangia (female cones) • Cells undergo meiosis and divide to produce the microspores and megaspores. (3) Microgametophytes (male) and Megagametophytes (female) • Develop into the unflagellated pollen grain (male) and the ovules (female)
(4) Integument and Micropyle • Thick layer of cells surrounds each megasporangium (integument), containing a small opening (micropyle). (5) Ovule (female gamete) and Pollen Grain (male gamete) • Gametes produced in one of two types of cones (staminate or ovulate) (6) Pollen Tube (after pollination) • A slender extension of the pollen grain that enables sperm to reach the egg via the micropyle. NOTE: Enclosure of the male gametophyte produced pollen, which greatly increased sperm mobility. (i.e., making cross-fertilization more likely)
32-2 Sexual Reproduction in Flowering Plants I. Parts of a Flower • Flowers are considered to be highly specialized branches and the parts of a flower to be specialized leaves.
(1) Receptacle • The swollen tip of a floral “branch” where the specialized leaves of a flower develop. (2) Sepals and Petals • Sepals surround and protect the other parts of a developing flower before it opens; petals (of animal-pollinated species) tend to be brightly colored—in wind-pollinated flowers, they are very small or absent.
(3) Stamen (Anther and Filament—MALE) • Contains microsporangia (i.e., microspores that become pollen) and is elevated by the stalk-like filament. (4) Pistil (Ovary, Style, and Stigma—FEMALE) • The style rises from the ovary (at the base), and at the top, is a sticky stigma which is designed to trap pollen. (5) Carpels (makes up the pistil) • One of the FOUR whorls of flower parts—including sepals, petals, stamens, and carpels.
II. Ovule Formation (female gamete in angiosperms) • Ovules form in the ovary of the pistil and consist of a megaporangium surrounded by two integuments. (1) Megaspore Mother Cell (the initial large diploid cell in the ovule) • Undergoes meiosis and yield four haploid megaspores. (2) Polar Nuclei (two cells adjacent to the egg) • Fertilized by a SECOND sperm, forming the ENDOSPERM of a seed. (3) Embryo Sac (contains 7 cells and 8 nuclei) • Mature female gametophyte (megagametophyte) that becomes the ovule in the flowering ovary. (NOT found in gymnosperms) NOTE: In gymnosperms, ovules form on the scales of cones, NO embryo sac ever forms.
II. Pollen Grain Formation (similar in BOTH gymnosperms and angiosperms) • Pollen grains form in the anthers of stamens inside the flower; each anther contains FOUR microsporangia (pollen sacs) (1) Microspore Mother Cells (diploid cells found in the four pollen sacs) • Undergo meiosis to yield four haploid microspores, of which develop into mature male gametophytes or microgametophytes (two-celled pollen grain) (2) Tube Cell and Generative Cell (TWO cells of pollen grain) • When a pollen grain germinates, the tube nucleus causes the tube cell to grow through the style, forming a pollen tube; the smaller generative cell divides by mitosis to form TWO SPERM.