Reproductive Life Cycles of Vascular Plants. Plant life cycles are characterized by alternate sporophytic and gametophytic generations.

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1 Reproductive Life Cycles of Vascular Plants Plant life cycles are characterized by alternate sporophytic and gametophytic generations.

2 Reproductive Life Cycles of Vascular Plants The sporophyte produces specialized reproductive structures that facilitate gamete production through meiosis. This initiates the gametophytic generation.

3 Reproductive Life Cycles of Vascular Plants Male and female gametes have a haploid genetic composition. Fusion of these gametes (fertilization) results in a reproductive zygote (embryo) that restarts the sporophytic generation.

4 Reproductive Life Cycles of Vascular Plants Plants The most primitive nonvascular and vascular plants sexually reproduce by spores. Non-vascular Vascular Mosses 400 million years ago The more advanced vascular plants produce seeds. Spores Seedless plants Lycopods Horsetails Ferns 300 million years ago Seeds Seed plants Cycads Ginkgo Conifers 200 million years ago 100 million years ago Angiosperms 55 million years ago

5 Seed plants Seed plants are separated into gymnosperms and angiosperms. The seed habit is characterized by several anatomical features that differentiate them from spore-producing plants: (1) Rather than producing a single spore type (homospory), seed plants produce a separate female megaspore and male microspore (heterospory). (2) The female gametophyte is retained on the mother plant (sporophyte) and is enclosed within a protective maternal seed coat. (3) The ovule has an opening designed to receive pollen that does not depend on water for male gamete transfer.

6 Reproductive Life Cycles of Vascular Plants Seed Plants - Extinct seed producing plants The evolution of the seed habit began during the Devonian period about 350 to 385 million years ago in progymnosperms. Fossil leaf Progymnosperms were sporeproducing, but showed heterospory. Heterospory is a first step in the evolution of a true seed. Progymnosperms are considered the common ancestor of all seed plants. Archaeopteris

7 Reproductive Life Cycles of Vascular Plants The seed ferns in the late Devonian were the first plants to produce seedlike structures enclosed in female tissue called cupules. The cupules contained a single seed (megaspore) within protective coverings. Cupules Cupules Alethopteris

8 Seed Plants - Gymnosperms Gymnosperms are the oldest living seed producing plants. The term gymnosperm means naked seeds and refers to the absence of ovary tissue covering the seeds, which is a characteristic of angiosperms (flowering plants). Gymnosperms include the cycads, ginkgo, gnetophytes (Ephedra, Gnetum) and the conifers (like pine, fir, and hemlock). Fir (Abies) cone

9 Cycads Cycads appear in the fossil record 320 million years ago. Encephalartos

10 Cycads The living cycads are palm-like in character and mostly tropical and subtropical. Cycas Encephalartos

11 Cycads Cycads produce male and female cone-like sporangia on separate plants. Male Zamia integrifolia Female

12 Cycads Females in other cycads are more cone-like. Encephalartos Lepidozamia

13 Cycads Cycas nicely shows that the female is a modified leaf called a megasporophyll with attached sporangia. Cycas

14 Cycads Cycad seeds are covered with a fleshy aril that resembles and functions like a true fruit. Encephalartos

15 Ginkgo Only extant member from a family developed over 120 million years ago. Ginkgo is a unique gymnosperm because it has a broad leaf and is deciduous.

16 Ginkgo Female ovule in Ginkgo are produced in pairs at the tips of reproductive short shoots. There is a small opening at the tip of the ovule to permit sperm to enter. Reproductive short shoot Pair of ovules Ovule opening

17 Ginkgo Males are produced on separate trees and are clusters of sperm containing microsporangia that release pollen. Microsporangia

18 Ginkgo Ginkgo seeds are produced in autumn and have an outer fleshy and inner hard seed coat. Endosperm Intact seed with fleshy covering. Seed with outer coat removed. Embryo Cut seed showing embryo and endosperm.

19 Gnetophytes Gnetophytes include Gnetum, Ephedra, and Welwitschia. Gnetophytes appear to be the closest living relatives to the Angiosperms. Ephedra female (left) and male (right) plants.

20 Gnetophytes Welwitschia is an unusual desert plant that produces only two opposite strap-like leaves on either side of a central disc (stem).

21 Gnetophytes Welwitschia Welwitschia reproductive structures are produced on the margin of the central disc. There are separate male and female plants. Male Female Welwitschia

22 Gnetophytes Welwitschia Welwitschia seeds have a papery outer seed coat.

23 Conifers Conifers (cone-bearing) represent the largest group of genera in the gynmosperms dating back to 290 million years ago. Abies Picea Keteleeria

24 Conifers Conifers include: Araucariaceae Agathis and Araucaria. Pinaceae Abies, Cedrus, Larix, Picea, Pinus, Pseudolarix, Pseudotsuga, Tsuga, and Sciadopitys. Cupressaceae Chamaecyparis, Cupressus, Juniperus, Microbiota,Thuja, Platycladus, and Thujopsis. Taxodiaceae Taxodium, Metasequoia, Cryptomeria, Cunninghamia, Taiwania, Sequoia, and Sequoiadendron. Taxaceae Cephalotaxus, Taxus, and Torreya.

25 Taxaceae Members of the Taxaceae produce males and females on separate plants. The female is cone-like and produces a drop of fluid at the tip of the ovules to capture air borne pollen. Taxus Cephalotaxus

26 Taxaceae Males shed wind-borne pollen. Cephalotaxus Taxus Wind-borne pollen

27 Taxaceae Unlike many of the other conifers, members of the Taxaceae produce seeds with a fleshy outer coat. Taxus Cephalotaxus

28 Conifer life cycle Conifer Reproductive Cycle - Pine

29 Conifer life cycle Pollen formation The male gametophyte is produced in a staminate cone. Staminate cone

30 Conifer life cycle Pollen formation Four haploid pollen grains are produced through meiosis. The male gametophyte is a winged pollen grain spread by the wind.

31 Conifer life cycle Ovule formation In many gymnosperms, the female gametophyte is produced in the axils of the ovulate cone between protective scales. The ovulate cone consists of many spirally arranged ovuliferous scales subtended by a cone bract. Ovulate cone Each ovuliferous scale has a pair of ovules on its surface. The ovuliferous scale will form the seed wing that covers the mature seed. Spruce (Picea) Ovuliferous scale

32 Conifer life cycle Ovule formation The haploid female gametophyte is developed within the nucellus (megasporangium) from the megaspore mother cell by meiosis. Nucellus Megaspore mother cell Cone bract Ovuliferous scale Nucellus Megaspore mother cell Ovuliferous scale Cone bract

33 Conifer life cycle Fertilization Within the female gametophyte, two archegonia are formed each with one haploid egg cell. Only one egg cell will be fertilized and develop into an embryo within the ovule. Micropyle Archegonia Integuments Ovule

34 Conifer life cycle Fertilization The pollen grain contains two nuclei one is the tube cell and one is the generative cell. Following pollen germination, the generative nucleus divides into additional sperm nuclei. Germinating pollen Sperm nuclei Pollen grain Pollen tube Generative cell Tube cell Tube nucleus

35 Conifer life cycle Fertilization The pollen germinates and the pollen tube enters the ovule and deposits the sperm nuclei. Pollen Micropyle Ovule The sperm nucleus and egg nucleus fuse to complete fertilization and form the 2n zygote. However, fertilization in most conifers does not occur until months after the pollen tube enters the ovule. Egg nucleus In pines, it can take over a year between pollination and egg cell fertilization.

36 Conifer life cycle Fertilization Fusion of the egg and sperm cells completes fertilization and results in a new zygote. Pollen tube Sperm nucleus Egg nucleus Archegonium

37 Conifer life cycle Fertilization Following gamete fusion, cells organize to form an embryo tier of cells and a suspensor tier. The suspensor cells elongate and several multiple embryos (polyembryos) are formed inside a single ovule. Suspensors Embryos Initially, there can be as many as 12 developing embryos. However, it is usual that one embryo becomes dominate and continues to develop as the seed matures.

38 Conifer life cycle Fertilization In most conifers, the mature seed is attached to a wing derived from the ovuliferous scale and the embryo that will be the next sporophytic generation. Seed coat Embryo A pair of mature seeds in pine Cone bract Wing Endosperm (1n) Female gametophyte Wing Spruce (Picea) seed Seed Seed

39 Conifer life cycle Fertilization In gymnosperms, there is only a single fertilization event between the sperm and egg nuclei. Therefore, the endosperm is not triploid as in most angiosperms, but is derived from the female gametophyte (megasporangia) that is haploid. It is still often referred to as the endosperm or as the female gametophyte storage tissue. Endosperm Female gametophyte (1n) Mature pine seed Seed coat Embryo

40 Angiosperm life cycle Angiosperms are true flowering plants. The term angiosperm means enclosed seeds and refers to the female ovary tissue (carpels) that forms the fruit surrounding angiosperm seeds. Angiosperms are the dominant plant type on Earth with approximately 250,000 species, compared with only about 8,000 living species of gymnosperms. Akebia quinata

41 One reason for angiosperm success and diversity is the mutualistic co-evolution of animals (especially insects) as pollinators and seed dispersers. Angiosperm life cycle

42 Angiosperm life cycle Angiosperms present an incredible diversity of flower forms and colors.

43 Angiosperm life cycle Flowers are the sexual organs in angiosperms. The male organ is the stamen and the female is the pistil. Stamen Anther Filament Pollen Stigma Petals Stigma Style Anthers Pistil Style Petal Ovule Ovary Ovary Sepal Receptacle Pedicel Receptacle Pedicel

44 Angiosperm life cycle

45 Angiosperm life cycle Pollen development (Microsporogenesis) Male gametes are formed in the pollen grains (microspores) that are produced within the stamen of the flower. Stamen Pollen grains Anther Anther Pollen grain Filament

46 Angiosperm life cycle Pollen development (Microsporogenesis) There are four pollen sacs in each lily anther. Microsporangia (Pollen sacs) Microspore mother cells Cross-section lily stamen

47 Angiosperm life cycle Pollen development (Microsporogenesis) Microspore mother cells (Microsporocytes) will divide via meiosis to become the pollen grains. The tapetum is a layer of nutritive cells surrounding the developing pollen. Microsporangia (Pollen sacs) Microspore mother cells initiating division Tapetum

48 Angiosperm life cycle Pollen development (Microsporogenesis) Many pollen grains matures within each pollen sac. Eventually the anther will open to release the pollen. Pollen sacs Pollen

49 Angiosperm life cycle Pollen development (Microsporogenesis) The outer layer of the pollen grain is called the exine. The exine provides protection for the pollen grain. The exine tends to be smooth in wind-pollinated plants and rough or spiked in insect-pollinated plants. Hibiscus pollen with a rough exine indicating that the pollen is carried by insects.

50 Angiosperm life cycle Pollen development (Microsporogenesis) A mature pollen grain typically contains two nuclei; one generative nucleus and one tube nucleus. The outer surface of the pollen grain has an outer exine and inner intine interrupted by several pores. The pollen tube will exit (germinate) through one of the pores. Tube nucleus Pore Pore Tube nucleus Intine Exine Pore Generative nucleus Generative nucleus

51 Angiosperm life cycle Pollen germination Generative nuclei Tube nucleus Pore Hydration of the pollen grain enables the pollen tube to emerge through a pore on the pollen surface. Pollen tube Soon after the pollen tube elongates, the tube nucleus followed by the generative nucleus enters the tube. As the nuclei move down the tube, the generative nucleus divides to form two male sperm nuclei that will unite with female egg cells during fertilization.

52 Angiosperm life cycle Pollen germination The pollen tube moves down the style towards the ovule. The journey may be short, less than ½ an inch in beet; or it might be a long distance (over 5 to 15 inches) as in lily or corn. Style Pollen Stigma Pollen tube Generative nuclei Tube nucleus

53 Angiosperm life cycle Pollen germination The tube nucleus acts to guide the pollen tube, while the generative nuclei will eventually fuse with female egg cells. The pollen tube enters the micropyle (a natural opening between the integuments) releasing the generative nuclei into the embryo sac.

54 Angiosperm life cycle Pollen germination The interaction between the pollen and stigmatic surface is important for pollen germination and tube growth. This interaction is a way to force cross-pollination.

55 Angiosperm life cycle Pollen germination Sporophytic self-incompatibility. Each pollen contains genes of both S 1 and S 2 alleles, and the pollen tube will only grow down a style with a different genotype.

56 Angiosperm life cycle Pollen germination Gametophytic self-incompatibility. Each pollen grain has a single S allele. Pollen tube will not grow down a style where that allele is represented.

57 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) In the angiosperm flower, the female gametophyte consists of nucellar tissue that is surrounded by either a single or a double outer tissue layer called the integuments. The integuments will become the seed coat. Integuments Integuments Nucellus Nucellus

58 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) In the nucellus, a megaspore mother cell forms that will undergo meiosis and become the female egg cells within the egg sac. Outer Integument Inner Integuments Megaspore mother cell Nucellus

59 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Soon after the completion of meiosis, the egg sac is formed and haploid (1n) nuclei organized according to their future function. Ovary Funiculus Ovule A gap is retained between the enveloping integuments called the micropyle. This is the opening where the pollen tube will enter the embryo sac. Micropyle Integuments Egg sac Nucellus

60 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) The integuments grow to cover the nucellus and continues to enclose the embryo sac creating the ovule. The ovule eventually will become the seed. Megaspore mother cell forms in the nucellus. Megaspore mother cell undergoes meiosis. Haploid cells form in the embryo sac.

61 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis)

62 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Ovule development over time. A common form of ovule development has the ovule turn along the placenta (funiculus) and become inverted. Funiculus Nucellus Integuments Micropyle

63 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Ovules vary in their orientation and shape in the ovary. Three common types include orthotropous, anatropous, and hemianatropous. Stigma Style Ovary Integuments Ovule Egg sac orthotropous anatropous hemianatropous

64 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Nuclei in the embryo sac from by meiosis. In meiosis I, there is an initial cell division to give two cells with diploid nuclei. At the end of meiosis II, there are four linear haploid nuclei formed. Only one nucleus survives to duplicates to form the archegonia in gymnosperms or the contents of the embryo sac in angiosperms.

65 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) In angiosperms, the most common arrangement of cells in the embryo sac is called the Polygonum type and occurs in about two-thirds of flowering plants

66 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) The Polygonum type of embryo sac has seven cells (eight nuclei) that occupy specific locations that dictate their function. Egg sac

67 Angiosperm life cycle Different forms of ovule formation

68 Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis)

69 Angiosperm life cycle Double fertilization In angiosperms, sexual reproduction involves double fertilization. One male and one female egg nuclei fuse to form the zygote. The second male and two female egg nuclei fuse to form the triploid endosperm.

70 Angiosperm life cycle Double fertilization The pollen tube enters the ovule through the micropyle and deposits the two male nuclei into the embryo sac.

71 Angiosperm life cycle Double fertilization Synergid function Synergids produce a chemical that attracts the pollen tube to the micropyle. Arrests tube growth. Ensures the proper release of the sperm cells.

72 Angiosperm life cycle Double fertilization The exact function of antipodal cells is not completely understood, but they disintegrate soon after fertilization of the egg cell.

73 Angiosperm life cycle Double fertilization The pollen tube enters the synergid and deposits the two male nuclei. Female egg nuclei Pollen tube entering the embryo sac Embryo sac

74 Angiosperm life cycle Double fertilization The egg cell to form the zygote (2n embryo). The central cell and its two polar nuclei to form the 3n endosperm. Embryo sac Central cell Egg cell Double fertilization in lily.

75 Angiosperm life cycle Double fertilization Most seeds of angiosperms mature to include three basic parts. The diploid embryo from the fusion of male and female gametes. The triploid endosperm from the fusion of one male and two female gametes. The diploid seed coat developed from the integuments derived from diploid maternal tissue. Seed coat Embryo Endosperm