5The structure of flower Flowers are typically composed of four whorls of highly modified leaves called floral organs.From outside to inside, these four whorls of floral organs are: sepal, petal, stamen, and carpel.
6The gametophytesThe stamen and carpels of flowers contain the sporangia, the structures where first the spores and then the gametophytes develop.Male gametophytes are pollen grains; female gametophytes are embryo sacs (egg-producing structures).
7PollinationWhen pollen released from anthers and carried by wind or animals land on a stigma, pollination can occur.Each pollen grain produces a structure called a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, resulting in fertilization of the egg.
8Complete flowerPlants like Trillium bear flowers with all four organs are called complete flower or “perfect” flower. Their flowers are bisexual flower.
9Incomplete flowerPlants like Maize or Sagittaria produce flowers with one floral organ eliminated – usually stamen or carpel – are called incomplete flower. Their flowers are sometimes called “imperfect” flower or unisexual flower.
10Unisexual flower - monoecious Unisexual flowers are called staminate or carpellate, depending on which set of reproductive organs is present.If staminate and carpellate are located on the same individual plant, then that plant species is said to be monoecious.Example: Maize
11Unisexual flower: dioecious A dioecious plant has staminate flowers and carpellate flowers on separate plants.
12InflorescenceSome plants like Lupines have clusters of flowers called inflorescences.
13Composite inflorescence and ray flower Plants like sunflowers have composite inflorescence in the central disk consists of tiny complete flowers. The “petals” of sunflower is actually imperfect flowers called ray flowers.
14Development of male gametophyte Within the sporangia (pollen sacs) of an anther are numerous diploid cells called microsporocytes.Each microsporocyte undergoes meiosis, forming four haploid microspores.
15Development of male gametophyte A microspore divides once by mitosis and produces two cells, a generative cell and a tube cell. The generative cell will eventually produce sperm. The tube cell, which encloses the generative cell, will produce the pollen tube, a structure essential for sperm delivery to the egg.
16Development of male gametophyte The two-structure is encased in a thick, resistant wall that becomes sculptured into an elaborate pattern unique to the particular plant species.These two cells and their wall constitute a pollen grain.
17Development of male gametophyte A pollen grain becomes a mature male gametophyte when the generative cell divides by mitosis to form two sperm cells. In most species, this process occurs after the pollen grain lands on the stigma of a carpel and the pollen tube begins to form.
18Development of female gametophyte Ovules, each containing a single sporangium, form within the chambers of the ovary.One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores.
19Development of female gametophyte In many angiosperms, only one of the megaspores survives. This megaspore continues to grow, and its nucleus divides by mitosis three times, resulting in one large cell with eight haploid nuclei.
20Development of female gametophyte Membranes then partition this mass into a multicellular female gametophyte – the embryo sac.
21Development of female gametophyte 1 egg cell2 synergids flank the egg cell and function in attraction and guidance of the pollen tube.3 antipodal cells (function unknown).2 polar nuclei, share the cytoplasm of the large central cell of the embryo sac.
22Pollination and prevention of self-fertilization Pollination brings the male and female gametophytes together.Some flowers self-fertilize (selfing), but the majority of angiosperms have mechanisms that make it difficult or impossible for a flower to fertilize itself.
23Prevention of self-fertilization In some plants with bisexual flowers, the stamens and carpels mature at different times or are structurally arranged in such a way that it is unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower
24Self-incompatibility is the most common anti-selfing mechanism Self-incompatibility is referring to the ability of a plant to reject its own pollen and the pollen of closely related individuals.
25Self-incompatibility When a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg.
26Self-incompatibility Recognition of “self” pollen is based on genes for self-incompatibility, called S-genes. If a pollen grain and the carpel’s stigma upon which it lands have matching alleles at the S-locus, then the pollen grain fails to initiate or complete formation of a pollen tube, and thus no fertilization occurs.
27Mechanisms of self-incompatibility Different plant families have different mechanisms for blocking pollen tube growth.Some plants produce RNases that will destruct RNA of the rudimentary pollen tube.
28Possible mechanism of sporophytic self-incompatibility aquaporins?This happens in plants of mustard family
29Pollination and fertilization After pollen germinated, the tip of the pollen tube enters the ovary (possibly by chemical attractant [Ca2+]) through the micropyle, and discharges its two sperm within the embryo sac.
30Double fertilizationAfter sperms discharged, one sperm fertilizes the egg to form the zygote. The other sperm combines with the two polar nuclei to form a triploid (3n) nucleus in the center of the large central cell of the embryo sac. This large cell will give rise to the endosperm, a food-storing tissue of the seed.
31Double fertilization is a distinctive feature of the angiosperm life cycle The union of two sperm cells with different nuclei of the embryo sac is termed double fertilization.This process is almost unique to angiosperms, sharing with only a few gymnosperms. It is to ensure that the endosperm will develop only in ovules so nutrients will not be wasted.
32After double fertilization Increase in the cytoplasmic Ca2+ levels of the egg.Establishment of a block to polyspermy (fertilization of an egg by more than one sperm cell).Then endosperm development occurs, which is always precedes embryo development.
33Endosperm development After double fertilization, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. This liquid mass becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these cells produce cell walls and endosperm becomes solid.In many dicots, the food reserves of the endosperm are completely exported to the cotyledons (seed leaves) before the seed completes it development so the mature seed lacks endosperm.
34Embryo developmentThe first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell.The terminal cell eventually give rise to most of the embryo.
35Embryo developmentThe basal cell continues to divide transversely, producing a thread of cells called the suspensor, which anchors the embryo to its parent.The suspensor also functions in the transfer of nutrients to the embryo from the parent plant (or endosperm).
36Embryo developmentThe terminal cell divides several times and form a spherical proembryo attached to the suspensor.The cotyledons begin to form as bumps on the proembryo.Dicots look heart-shaped at this stage; monocots develop only one cotyledon.
37Embryo developmentDevelopment of plant embryo must establish two features: the root-shoot axis (meristems at opposite ends) and a radial pattern of protoderm (dermal), ground meristem (ground tissue) and procambium (vascular tissue).
38Embryo developmentThe last stage of embryo development involves dehydration. The seed dehydrates until its water content is only about 5~15% of its weight.
39Structure of the mature seed Above cotyledonShoot tip & leavesBelow cotyledonEmbryonic root
41Structure of the mature seed Specilized cotyledon to absorb nutrients from endospermCovers the young shootCovers the young root
42Fruit developmentPollination tirggers hormonal changes that cause the ovary to begin its transformation into a fruit.The wall of the ovary becomes the pericarp (thickened wall of the fruit).
43Seed dormancySeed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling.Many different factors can “break” the dormancy of seed.
44GerminationFor seeds to germinate, imbibition (the uptake of water due to the low water potential) must occur first.Imbibition causes seed to expand and rupture its coat and also triggers metabolic changes in the embryo.
45GerminationAfter imbibition, the embryo releases hormones called gibberellins (GA) as signals to aleurone.
46GerminationAleurone responds by synthesizing and secreting digestive enzymes (for example, a-amylase) that hydrolyze stored foods in the endosperm, producing small, soluble molecules.The first organ to emerge from the germinating seed is the radicle (embryonic root).
47GerminationThen the shoot tip must break through the soil surface.
48Many dicots will form a “hook” in the hypocotyl during germination
49Other dicots form a hook in the epicotyl during germination
50Monocots like maize have coleoptile to protect embryonic shoot