Chapter 32: Plant Reproduction

32-1 Plant Life Cycles

32-2 Sexual Reproduction in Flowering Plants32-3 Dispersal and Propagation

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.

32-1 Plant Life Cycles

(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)

I. Parts of a Flower• Flowers are considered to be highly specialized branches and the parts of a flower to be specialized leaves.

32-2 Sexual Reproduction in Flowering Plants

(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.

IV. Pollination (self-pollination vs. cross-pollination)• Before a sperm can fertilize the egg contained in the embryo sac, pollen must be transferred from an anther to a stigma. (1) Nectar (animal-pollination)• Nutritional food source of pollen and sugars designed to attract animal pollinators.

V. Fertilization (union of gametes, FOLLOWS pollination)• In order for fertilization to occur, a pollen tube must grow to an egg, and sperm must form. (pollen tubes take about a year to reach an egg in gymnosperms, a day or two for angiosperms)

(1) Double-Fertilization (1st make the zygote, the 2nd makes the endosperm)• Following pollination, a pollen grain germinates and forms a pollen tube, which grows through the style and enters an ovule though its micropyle.• TWO sperm travel down the pollen tube. The FIRST sperm fertilizes the egg in the ovule’s embryo sac, forming a zygote. The SECOND sperm fertilizes the TWO polar nuclei, forming the endosperm which nourishes the embryo. (this double-fertilization is UNIQUE to angiosperms)

I. Dispersal and Propagation• Fruits and seeds result from sexual reproduction in flowering plants. (e.g., fruits are adapted for dispersing seeds, while seeds function in the dispersal and propagation of plants)

32-3 Dispersal and Propagation

II. Types of Fruits (ripened ovaries)

• Fruits protect seeds, aid in their dispersal, and often delay their germination. (NOTE: Fruits are classified based on how many pistils or flowers form the fruit and whether it is DRY or FLESHY.)

III. Structure of Seeds• Differs among the major groups of seed plants—gymnosperms and angiosperms (monocots—maintain endosperm and dicots—burn up endosperm).

(1) Seed Coat• Protective coat that surrounds the plant embryo and is shed after germination.

(2) Radicle• Embryonic ROOT, the first structure to emerge from the seed.(3) Hypocotyl• Embryonic STEM that is BETWEEN the cotyledons and the radicle.(4) Epicotyl• Embyronic STEM that is ABOVE the cotyledons.

(5) Plumule and Hilum• Epicotyl and embryonic leaves (plumule); The hilum is a SCAR that marks where the seed was attached to the ovary wall.

IV. Seed Germination• Although its embryo is alive, a seed will not germinate until it is exposed to certain environmental conditions.

(1) Dormancy (an evolutionary advantage, but what are bad times to sprout?)• A state of reduced metabolism that some seeds enter even when exposed to ideal conditions. (e.g., a botanist germinated a lotus seed that was estimated to be close to 1,000 years old)

(A) Conditions Needed for Germination• Water, oxygen, temperature are all factors that can influence the rate of germination for a seed. (e.g., softening the seed coat)

(B) Process of Germination• Emergence of the radicle, breaking of the seed coat, growth of the shoot.

EX: BEAN seed (dicot): The hypocotyl curves and becomes hook-shaped, breaking through the soil. The CORN seed (monocot) does NOT hook, and the cotyledon remains UNDERGROUND. Instead the corn plumule is protected by a sheath as it pushes through the soil. When the shoot breaks through the soil surface, the leaves of the plumule unfold.

V. Asexual Reproduction (reproduction WITHOUT union of gametes)• Fast reproductive rate to fill up an unoccupied area (benefit), lack of genetic variation among the offspring (detriment).(1) Clones• Identical offspring that result from asexual methods.

(2) Vegetative Reproduction• Reproduction with usually nonreproductive parts (i.e., leaves, stems, or roots)—e.g., Offspring yielded from the runners (or stolons) of an airplane plant (spider plant) represent vegetative reproduction.

VI. Vegetative Propagation (asexual reproduction in plant propagation)• Many species of plants are vegetatively propagated from specialized structures called RUNNERS, RHIZOMES, BULBS (monocots), and TUBERS.

(Newer methods have more recently become employed—layering, grafting, cuttings, and tissue cultures)

(1) Cuttings• Roots will form on a cut piece of a stem or shoots will form on a piece of a root. (e.g., African violets, grapes, figs, and olives)

(2) Layering• When roots form on stems where they make contact with the soil.

(3) Grafting• The joining of two or more plant parts to form a single plant. (e.g., a bud or small stem of one plant is attached to the roots or stems of a second plant—however, the vascular cambium of BOTH parts must be aligned for a successful graft)

(4) Tissue Culture• The production of new plants from pieces of tissue placed on a sterile nutrient medium.

Non-Vascular Plants

Mosses 10,000

Liverworts 6,500

Hornworts 100

Vascular Plants - Seedless

Whisk ferns 10 - 13

Club-mosses 1,000

Horsetails 15

Ferns 12,000

Seed Plants - Gymnosperms

Conifers 550

Cycads 100

Gingko 1

Gnetae 70

Angiosperms

Flowering plants 235,000

Revisiting Evolution• Because plants are not mobile, they must rely on wind, water, or animals to disperse their sperm cells (and in many cases, disperse their offspring)

Assessing Prior Knowledge• How does the production of sex cells (gametes) take place?• What forces will determine if natural selection favors or opposes a type of dispersal mechanism?