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Monday, May 22, 2017 THE LIFE CYCLE OF ANGIOSPERMS 1 Packet #75 Chapter #38 Monday, May 22, 2017 INTRODUCTION Angiosperms are the predominant plant species on the planet. Unlike gymnosperms, these plants produce seeds that have a covering. 2 Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM Figure 38.2 (More Detail) Page 772 In the angiosperm, the sporophyte generation is dominant Gametophytes are extremely reduced in size and nutritionally dependent on the sporophyte generation Flowering plants are hetereosporous 3 Monday, May 22, 2017 FIGURE 38.4 THE DEVELOPMENT OF ANGIOSPERM GAMEOPHYTES 4 Pollen sac (microsporangium) (a) Development of a male gametophyte (pollen grain) 1 Each one of the microsporangia contains diploid microsporocytes (microspore mother cells). 2 Each microsporocyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain. 3 Figure 38.4a Monday, May 22, 2017 FIGURE 38.4 DEVELOPMENT OF POLLEN GRAINS A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.) Microsporocyte MEIOSIS Microspores (4) Each of 4 microspores MITOSIS Generative cell (will form 2 sperm) Male Gametophyte (pollen grain) 20 75 m Nucleus of tube cell m Ragweed pollen grain KEY to labels Haploid (2n) Diploid (2n) 5 Monday, May 22, 2017 FIGURE 38.4 DEVELOPMENT OF EMBRYO SACS (b) Development of a female gametophyte (embryo sac) Megasporangium Ovule MEIOSIS Megasporocyte Integuments Micropyle Surviving megaspore Female gametophyte (embryo sac) MITOSIS Ovule Antipodel Cells (3) Polar Nuclei (2) Egg (1) Integuments 100 Haploid (2n) Diploid (2n) m Key to labels Synergids (2) Embryo sac 1 Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother cell). 2 The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore. 3 Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue). Figure 38.4b 6 Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM Each microspore develop into a pollen grain. Immature male gametophytes are contained within pollen grains. Each pollen grain has two haploid (n) cells. One of each four megaspores produced during meiosis develops into an embryo sac (female gametophyte). Embryo sac contains seven cells with eight nuclei The egg cell and the central cell, which contains two polar nuclei, participate in fertilization. 7 Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM Pollen released from the anther Pollen is carried to the sticky stigma Flowers can selfpollinate or crosspollinate. Transfer of pollen from flowers of one plant to flowers of another plant is called cross-pollination 8 Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM Pollen grain germinates after it sticks to the stigma. Pollen grain now contains the mature male gametophyte and extends a tube down through the style. After it reaches the ovary, the pollen tube penetrates the microphyle and discharges two sperm cells into the female gametophyte (embryo sac) Results in double fertilization 9 Monday, May 22, 2017 POLLEN TUBE 10 Double Fertilization Results in the formation of a diploid zygote and triploid endosperm One sperm nucleus fuses with the egg to form a diploid zygote. The other sperm nucleus fuses with the two nuclei if the female gametophyte Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM Cell is now triploid (3n). Characteristic of flowering plants. 11 Double fertilization ensures the endosperm will develop only in ovules where the egg has been fertilized Monday, May 22, 2017 FIGURE 38.6 DOUBLE FERTILIZATION Prevents the angiosperms from wasting resources 12 After double fertilization, the ovule matures into a seed. The zygote develops into the sporophyte embryo with a rudimentary root and either one or two seed leaves (cotyledons— monocots vs. dicots) Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM 13 Seed consists of the embryo, endosperm, sporangium and a seed coat. An ovary develops into a fruit as its ovules develop into seeds. Dispersal occurs via wind or animals Seed germinates if the environmental conditions are favorable. Seed coat ruptures, the embryo emerges as a seedling and uses the stored food in the endosperm and cotyledons to begin growth. Monday, May 22, 2017 LIFE CYCLE OF ANGIOSPERM 14 Monday, May 22, 2017 MECHANISMS TO PREVENT SELF-POLLINATION (SELFING!) 15 Chapter 38 Some flowers selffertilize but most angiosperms have mechanism that prevent “selfing.” Dioecious plants Unisexual Monday, May 22, 2017 INTRODUCTION I Ackee {Barbados}— Ginnup {Jamacia} Bisexual flowers Stamens and carpels mature at different times Stamens and carpels are arranged in a way that makes self-pollination unlikely 16 AKA Self-incompatibility Most common method that plants use to prevent “selfing” Some plants recognize their own pollen and that of closely related individuals Monday, May 22, 2017 INTRODUCTION II BIOCHEMICAL BLOCKING Hmmm! Biochemical block prevents pollen from completing its development and fertilizing the egg 17 Monday, May 22, 2017 REVIEW 18 Imagine that…plants which are more simplistic organisms appear to have more complex reproductive cycles. Students List potential examination questions and/or here, and on following slides, based on the packet. Monday, May 22, 2017 REVIEW 19