Alternation of Generations
Sporophyte - diploid - produces spores by meiosis
Gametophyte
- haploid - produced by mitotic division(s) of spores
Flowers
Complete -
has all four floral parts (sepals, petals, stamens & carpels
Incomplete -
lacks one or more floral parts
Perfect -
has both stamens and carpels
Imperfect -
missing either stamens or carpels
Flowers
on Plants
Monoecious - staminate and carpellate
flowers on same plant
Dioecious - staminate and carpellate
flowers on separate plants
Development
of
Pollen Grain
Microsporocyte
(diploid) in pollen sac divides by meiosis - gives rise to 4 microspores
(haploid)
Microspore
undergoes mitosis - gives rise to pollen grain - consists of generative cell
and tube cell
Generative cell
divides to form 2 sperm - becomes male gametophyte
Development
of
Embryo Sac
Megasporocyte undergoes meiosis - gives rise to 4 hapoid cells - only 1 survives - becomes megaspore
Megaspore
undergoes 3 mitotic divisions - forms embryo sac (female gametophyte) -
consists of egg, two synergid cells, 3 antipodal
cells & 1 large central cell with 2 nuclei
Pollination
& Fertilization
Pollen grain attaches to stigma
Tube extends through style - enters micropyle & injects 2 sperm cells
One sperm fertilizes egg - the other unites with 2
nuclei in central cell - called double fertilization - gives rise to
zygote and endosperm (triploid) tissue
Endosperm
Development
Usually begins before embryo development
Triploid nucleus divides mitotically
- forms multinucleate super cell (endosperm)
Cytokinesis forms membranes & walls between nuclei
Rich in nutrients - provides nourishment to embryo
Embryo
Development
Zygote undergoes mitosis - forms terminal cell and
basal cell
Basal cell divides to form suspensor - anchors
embryo & transfers nutrients
Terminal cell gives rise to most of embryo -
establishes root-shoot axis - produces radial pattern of 3 primary meristems (protoderm, ground meristem & procambium)
Structure
of Seed
Embryo consists of cotyledon(s), epicotyl (above attachment point of cotyledons), hypocotyl
(below point of attachment)
Surrounded by endosperm - covered with seed
coat
Epicotyl - embryonic shoot
Hypocotyl - embryonic
root
Fruit
Mature ovary - surrounded by pericarp
Simple fruit
- derived from single ovary
Multiple -
derived from inflourescence (cluster of flowers)
Aggregate -
derived from single flower with multiple carpels
Commonly attractive to animals (for dispersal)
Germination
Depends on imbibition
- absorption of water - ruptures coat & causes metabolic changes
Radicle (embryonic root) emerges first
Shoot
exhibits growth patterns during emergence - light main cue that ground has been
broken
Evolutionary
Adaptations -
Seed Dormancy
Dormancy -
very low metabolic rate - no growth or development
Increases probability that germination will occur at
right time & place (survival)
May remain dormant for long periods
Breaking dormancy may require certain environmental
conditions
Soil has pool of ungerminated
seeds
Asexual
Reproduction
Vegetative reproduction - called cloning - an extension of the capacity of
plants for indeterminate growth - clones genetically identical
Fragmentation
- separation of plant into parts that reform whole plants - most common form
Apomixis - production of seeds without fertilization
Dilemma
of Monoculture
Plant breeders have selected for self-pollinating
varieties of exceptional plants
Genetic uniformity allows for predictable growth,
size, fruiting patterns & maturity
Monoculture
- large areas of genetically uniform crops - extremely vulnerable to diseases
and other pests
Cellular
Mechanisms of Growth -
Role of Cytoskeleton
Cytoskeleton
guides geometry of cell division and expansion
Preprophase band of microtubules
in cortex determines plane of division
Uptake of water accounts for >90% of plant cells
expansion
Microfibrils nonelastic - expansion
occurs perpendicular to grain of microfibrils
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