BIOL 1442 Lecture Notes – Section 003 – Dr. Britton – Spring 2006, UTA - 1 - BIOL 1442 Lecture Notes: Chapter 38 Alternation of Generations Sporophyte and gametophyte generations alternate in the life cycles of plants. The life cycles of angiosperms and other plants are characterized by an alternation of generations, in which haploid (n) and diploid (2n) generations take turns producing each other. • The diploid plant (2n), the sporophyte, produces haploid spores by meiosis. • These spores then divide by mitosis, giving rise to multicellular male and female haploid plants called gametophytes. • These gametophytes produce gametes (sperm and eggs). • Fertilization of these gametes results in diploid zygotes, which divide by mitosis and form new sporophytes. Sporphytes = Big, Gametophytes = Small • In angiosperms, the sporophyte is the dominant generation, the conspicuous plant we see. • Over the course of seed plant evolution, gametophytes became reduced in size and dependent on their sporophyte parents. • Most angiosperm gametophytes consist of only a few cells. In angiosperms, the sporophyte produces a unique reproductive structure, the flower. • Male and female gametophytes develop within the anthers and ovaries, respectively, of a sporophyte flower. • Pollination by wind or animals brings a male gametophyte (pollen grain) to a female gametophyte. • Union of gametes (fertilization) takes place within the ovary. • Development of the seeds containing the sporophyte embryos also occurs in the ovary, which itself develops into the fruit around the seed. Flowers • Flowers are specialized shoots bearing the reproductive organs of the angiosperm sporophyte. • Flowers, the reproductive shoots of the angiosperm sporophyte, are typically composed of four whorls of highly modified leaves called floral organs, which are separated by very short internodes. • Unlike the indeterminate growth of vegetative shoots, flowers are determinate shoots in that they cease growing once the flower and fruit are formed. • Four Major Floral OrgansBIOL 1442 Lecture Notes – Section 003 – Dr. Britton – Spring 2006, UTA - 2 - o Sepals (non-reproductive organ)  Sepals, which enclose and protect the floral bud before it opens, are usually green and more leaf-like in appearance. o Petals (non-reproductive organ)  In many angiosperms, the petals are brightly colored and advertise the flower to insects and other pollinators o Stamens (male reproductive organ)  A stamen consists of a stalk (the filament) and a terminal anther within which are pollen sacs o Carpels (female reproductive organ)  A carpel has an ovary at the base and a slender neck (called the style)  At the top of the style is a sticky structure called the stigma that serves as a landing platform for pollen  Within the ovary are one or more ovules.  Some flowers have a single carpel, in others, several carpals are fused into a single structure, producing an ovary with two or more chambers.  • Where flowers attach to the stem is called the receptacle. Sporangia • The stamens and carpels of flowers contain sporangia, within which the spores and then gametophytes develop. • The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers. • The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries. Pollination & Fertilization • Pollination begins the process by which the male and female gametophytes are brought together so that their gametes can unite. • Pollination occurs when pollen released from anthers is carried by wind or animals to land on a stigma. • Each pollen grain produces a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, fertilizing the egg. o Fertilization only occurs when a sperm reaches an egg o Pollination can occur without fertilization, the two terms are not synonymous o A fertilized egg is called a zygote. • The zygote goes through mitosis to give rise to an embryo. • The ovule develops into a seed and the entire ovary develops into a fruit containing one or more seeds. • Fruits carried by wind or by animals disperse seeds away from the source plant where the seed germinates.BIOL 1442 Lecture Notes – Section 003 – Dr. Britton – Spring 2006, UTA - 3 - Types of Flowers • Numerous floral variations have evolved during the 130 million years of angiosperm history. • Plant biologists distinguish between… o Complete flowers, those having all four organs, and… o Incomplete flowers, those lacking one or more of the four floral parts. • A bisexual flower (in older terminology a “perfect flower) is equipped with both stamens and carpals. • All complete and many incomplete flowers are bisexual. • A unisexual flower is missing either male or female organs o Carpellate flowers lack stames o Staminate flowers lack carpels • A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant. o For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers. • A dioecious species has staminate flowers and carpellate flowers on separate plants. o For example, date palms have carpellate individuals that produce dates and staminate individuals that produce pollen. • In addition to these differences based on the presence of floral organs, flowers have many variations in size, shape, and color. • Much of this diversity represents adaptations of flowers to different animal pollinators. • The presence of animals in the environment has been a key factor in angiosperm evolution. Gametophytes • Male and female gametophytes develop within anthers and ovaries, respectively: Pollination brings them together o The male gametophyte begins its development within the sporangia (pollen sacs) of the anther. o The female gametophyte begins to develop within the ovules of the ovary • The development of angiosperm gametophytes involves meiosis and mitosis. • Male Gametogenesis: The male gametophyte begins its development within the sporangia (pollen sacs) of the anther. o Within the sporangia are microsporocytes, each of which will from four haploid microspores through meiosis.BIOL 1442