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BIOL 321 (1)

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University of Waterloo
BIOL 321
Susan Lolle

 Alternation of generations: o refers to the alternation of sporophyte and gametophyte life forms during sexual reproduction in all plants  Seedless Vascular Plants: Fern Life Cycle: o Sporophyte is dominant o Gametophyte is significantly reduced in physical size o Sporophyte and gametophyte are independent at maturity (unlike bryophytes and seed plants) o Alternation of generations:  Most SVPS are homosporous (1 type of spore)  Gametophytes can be female, male, or bisexual • Gametophytes are short lived and small • Have archegonia (egg) and antheridia (sperm)  Heterosporous SVPs produce two types of spores: megaspores (large), microspores (small)  SVps re ly on water for fertilizations  Seed Plants: Life cycle of gymnosperms and Angiosperms o Sporophyte is dominant  Diploid phase, recognizable as plant  Produces spores by meiosis • is hetersporous; female gametophyte develops from megaspore, male gametophyte develops from a microspore o Gametophyte is significantly reduced in physical size  Is haploid phase  Pollen is male gametophyte, ovule contains female gametophyte  Produces gametes by mitosis o Sporophyte is independent at maturity, while gametophyte is dependant  Seed Plants: Angiosperm Life cycle: Gametophyte development o Female gametophyte (embryo sac) develops within the carpel (ovule)  Structure • The carpel consists of an ovary, style(filament), stigma(sticky receptacle) • Ovary holds one more ovules; site of embryo sac development • Each ovule contains a nucelleus; a roughly egg shaped mass of cells o Each nucleus has a megaspore mother cell • Surrounding the nucellus is the integuments; two outer layers of cells, • do not join at tip of ovule, gap is referred to as microphyle  Development: • Begins with the meiosis of a single cell in the nucellus just below the microphyle ; results in a strand of four haploid megaspores • Three of the megaspores degenerate but the fourth ( farthest from the microphyle) enlarges and undergoes three rounds of mitosis to create an embryo sac containing eight haploid cells distributed among seven cells • Three of the seven cells cluster around the end of the embryo sac closest to the microphyle; egg cell and synergids • At the other end of the embryo sac there is a group of three cells called the antipodal cells , in the middle is the singlebinucleate central cell ( polar nuclei) o Male Gametophyte:  Structure: • Develops in anther at the top of the stamen ; consists of four pollens sacs; consists of a column of diploid microspore mother cells ; surrounded by tapetum; nutritive tissue  Development: • Each mother cell in the pollen grain undergoes meiosis to form four haploid microspores • Each microspore divides mitotically to produce a tube cell and a generative cell(pollen grain)  NOTE: in flowering plants the division of generative cell to 2 sperm cells, takes place in the pollen tube which develops once the pollen grain germinates o Fertilization: in order for fertilization to occur, the pollen grain must germinate on a compatible stigma 1. Pollen tube grows into the stigma, through the style to the ovary and enters the ovule through the microphyle 2. 2. as the tube grows, the tube cell nucleus stays near to the tip and the two sperm cells follow behind 3. When the pollen tube reaches the embryo sac, it penetrates one of the syner gids and then releases both sperm cells . Note, when or even before this happens, the synergids degenerate allowing the sperm cells access to both the egg cell and the central cell. 4. Double fertilization occurs; one sperm cell fuses with the egg cell to produce the zygote. The other sperm cell fuses with two polar nuclei in the central cell to create a triploid nucleus from wich the endosperm ( nutritive tissue develops) 5. Post fertilization, remaining haploid cells of embryo sac degenerate.  Embryogenesis: o -zygote divides in a plan perpendicular to the long axis of the embryo sac to produce a large basal cell near to the microphyle and a terminal cell ( close to the central cell, now developing triploid endosperm) o Basal cell divides to form the suspensor ( filament that connects the embryo to maternal tissue near micropyle o Terminal cell divides and develop into embryo proper (responsible for plant body) o Shoot apical meristem develops at the end of Procambium strand furthest from suspense , root apical meristem forms at the end of Procambium strand nearest to suspensor o 3 stages: 1) Globular embryo: ball of cells; Early development: cell divisions occur with little or no increase in the embryo’s total size. Different tissues appear ; Protoderm, Ground meristem, Procambium, Suspensor . This stage ends when : one or two cotyledons (first leaves) begin to form near the site of the shoot apical meristems precursors of root and shoot apical meristems appear at this stage 2) Heart Shaped embryo: shape of embryo in dicots when two cotyledons develop . 3) Torpedo stage Embryo: cotyledons elongate, same pattern of tissues and organs retained . Depending on size of embryo and seed, embryo may fold over 4) FINAL FORM: Suspensor, RAM, radicle ( embryonic root), SAM< cotyledons, Epidermis, Vascular strand, Seed coat NOTE: growth of most angiosperms pauses between the end of embryogenesis and the beginning of germination  Sporophyte Development: Germination and Seedling Development o Germination: A) Epigeal Germination (arabidopsis) ( above ground) :  if elongation takes place in the hypocotyl ( part of seedling stem below cotyledons and above radicle), cotyledons are lifted above the ground B) Hypogeal (below ground ) (broad bean): if elongation takes place in the epicotyl ( part of seedling stem above the cotyledons) . Cotyledons remain in seed case and below ground BOTH WAYS: germinating shoot grows upwards in a hook shape and pushes the shoot tip out of the soil, in the light the hook straightens out, cotyledons green up and expands  Shoot development post germination: o Consists of juvenile phase followed by an adult phase  Juvenile phase: vegetative phase prior to sexual maturity  Adult phase: vegetative and reproductive development • In an annual this phase is determinate • In perennial it is indeterminate • Only adult phase can produce flowers o juvenile shoot has lobed leaves, alternate Phyllotaxy, adventitious roots and grows as a climbing tree o b) Adult phase: adult shoot has entire leaves, spiral phyllotaxy, does not form adventitious roots and grows as a semi erect shrub  Primary Plant development: o Consists of primary meristems ; primary tissues  Protoderm: epidermis (dermal tissue)  Ground meristem: ground tissues  Procambium: primary xylem, primary phloem (vascular tissues) o Primary Development;  Post-embryonic phase where shoots and root lengthen and branch  Elongation is due to cell division and cell expansion  Growth can be due mainly to cell expansion  Organs are generated at the poles (near the tips) by the shoot and root apical meristems (SAM and RAM)  Branching is due to the activity of axillary meristems  Apical Meristems: o Are formed during embryogenesis  Therefore embryo development does not require apical meristems  Precursor of root and shoot appear at globular embryo stage o Found at the growing tips of the shoot and root o Give rise to primary meristem/tissues o Are made of initial ( undifferentiated cells)  Initial divide to give rise to daughter cells  Daughter cells become either derivatives (primary meristem or primary tissue) OR remain initials  Primary root o developing root consists of 3 distinct zones: a) zone cell division: at the apex b) zone of cell expansion: behind zone of cell division, division has vitually ceased c) maturation zone: final stage of cell differentiation takes place o Root Growth:  RAM consists of initials ( dividing cells)  Initials reside within the quiescent center:  inactive overtime. Do not lose ability to divide rapidly, can be activated after damage to the apical meristem 1) When each initial divides ; 1 daughter cell remains in the quiescent center while the other becomes a derivative 2) The derivative undergo differentiation and growth 3) RAM generates root cap ( protective shield) • Cell divisions in most apical layers of meristem add cells to the root cap of meristem replacing those worn away by friction between the growing root and soil o Root Branching:  Lateral roots develop in the region behind root hair formation  These roots are NOT derived from the RAM but are initiated in the pericycle close to the protoxylem poles o From circumferential inwards, the main tissues are: cross section  Epidermis  Cortex: beneath the epidermis and made of one or more layers of parenchyma  Endodermis: single layer inside cortex, lay down casparian strip  Regulates passage of solutes into the vascular system and up the shoot. Blocks passage of water through walls of endodermal cells. Water and minerals absorbed through soil must pass through the cytoplasm of endodermal cell before reaching the vascular tissue.  Pericycle: surrounds vascular tissue, contains a single layer of parenchyma cells, conducts moisture and nutrient around plant  Central core of vascular tissue : xylem (meta and proto) phloem (meta, and proto)  Stele consists of pericycle and Procambium , at center of roo
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