plant reproduction

PLANT REPRODUCTION
All Materials © Cmassengale

PLANT LIFE CYCLES: 

  • A life cycle includes all of the stages of an organism’s growth and development
  • A plant’s life cycle involves two alternating multicellular stages – a Diploid (2n) sporophyte stage and a Haploid (1n) gametophyte stage
  • This type of life cycle is called Alternation of Generations

Moss Characteristics: 

  • Nonvascular (pass water cell-to-cell)
  • Seedless (reproduce by spores)
  • Low growing
  • Phylum Bryophyta (also includes liverworts & hornworts)
  • Grow on moist brick walls, in sidewalks, as thick mats on forest floors, and on the shaded side of trees

  • Can survive periodic dry spells, reviving when water becomes available
  • Require water for fertilization so sperm can swim to egg
  • Rhizoids (root like structures) anchor mosses
  • Have waxy covering called cuticle on aerial parts to prevent desiccation 

 Moss Life Cycle:

  •  Dominant form of a moss is a clump of leafy green gametophytes (photosynthetic)
  •  Moss alternates between a haploid (1n) gametophyte and diploid (2n) sporophyte
  • Gametophyte generation produces gametes (eggs & Sperm)
  • Sporophyte generation forms at the top of the gametophytes and produces spores 
  • Stalk-like sporophytes lack chlorophyll
  • Capsule at the top of the sporophyte forms haploid (1n) spores

Sexual reproduction in Moss:



  • Moss produce 2 kinds of jacketed gametes — eggs & sperm
  • Egg producing organ is called the archegonium
  • Eggs are larger and nonmotile
  • Sperm producing organ is called the antheridium
  • Sperm are smaller, flagellated cells
  • Antheridia & archegonia are both part of the gametophyte plant
  • Fertilization can occur only during or soon after RAIN  when the gametophyte is covered with Water
  • Sperm swim to the egg by following a trail of chemicals released by the egg in the water
  • Fertilization produces a zygote that becomes a sporophyte
  • Mature sporophytes produce homosporous spores (all the same type)
  • Mature capsules open & release spores spread by wind
  • Spores landing on moist places germinate into protonema that become new gametophytes

 Asexual Moss Reproduction:

  • Small pieces may break off from a gametophyte & become a new plant (fragmentation)
  • Small buds called gemmae may be washed off by rain and develop new moss plants

Fern Characteristics & Life Cycle:

  • Largest group of seedless, vascular plants
  • Grow in moist places
  • Goes through alternation of generations 
  • Sporophyte phase is the dominant stage
  • Fern gametophytes are small, flat plants anchored to the soil by root-like rhizoids
  • Antheridia & archegonia form on the underside of fern gametophytes

  • Sperm swim to egg through water droplets to form zygote (fertilized egg)
  • Zygotes form new sporophytes with roots, stems, & leaves
  • Spore cases called sori form on the underside of fern fronds (leaves)

  • Ferns are homosporous (single type of spore formed)

 
 

  • New fronds form from an underground stem called the rhizome
  • Vascular tissue carries nutrients & water between the parts of the fern
  • Fronds are compound leaves attached by a short stalk called the stipe to the underground stem or rhizome

  • Immature fronds or fiddleheads are coiled
     

Characteristics & Life Cycle of Conifers:

  • Called gymnosperms
  • Have naked seeds that develop on scales of the female cones
  • Sporophyte is the dominant stage
  • Adapted to cooler climates
  • Called evergreens (pine, cedar, spruce, fir…)
  • Giant Redwood is one of the Earth’s largest organisms
  • Bristlecone Pines are the oldest living organisms (some more than 5000 years old)

Giant Redwood Bristlecone Pine

  • Produce 2 types of spores (heterosporous)
  • Male spores called microspores grow into male gametophytes
  • Female spores called megaspores grow into female gametophytes
  • A Pine cone is the female cone on a pine tree
  • Male cones on pine trees are smaller & grow in clusters at the tips of branches
  • Both male & female cones appear on the same tree

Female Cones Male Cones

  • The pine life cycle takes 2-3 years from the formation of cones until seeds are released
  • Female cones have spirally-arranged scales with ovules at their base
  • Female cones produce sticky resin
  • Ovules contain an egg that will develop into a seed 
  • Male cones produce large amounts of pollen in the spring that is spread by wind to the female cones
  • Resin traps the pollen so pollination can occur
  • A tube from the pollen grain takes a year to grow to the ovule so a sperm can fertilize the egg and form seeds

Angiosperms or Flowering Plants:

  • Bright colors, attractive shapes, and fragrant aromas help flowering plants attract their pollinators (insects, birds, mammals…)
  • Flowers without bright colors and pleasing odors are usually wind or water pollinated (grasses)
  • Called angiosperms
  • Flowers, the reproductive part of a plant,  have a swollen base or receptacle to attach to the stem
  • Flowers have 4 whorls (modified leaves) attached to the receptacle — petals, sepals, pistils, and stamen
  • Pistils (innermost whorl) are the female part of the flower, while Stamens are the male part
  • Sepals (outermost whorl) are found below the petals and may look leaf-like (some may be the same color as petals)
  • Sepals enclose the flower bud before it opens 
  • Sepals are collectively called the calyx
  • Petals are often colorful to attract pollinators
  • Petals are collectively called the corolla

  • Monocot flower parts are arranged in multiple of THREES, while dicots are in multiples of FOUR or FIVE
  • Perfect flowers have both stamens & pistils (rose)
  • Imperfect flowers are either a male (pistillate) or female (staminate) flower (pumpkin or melons)
  • Some angiosperms have both male & female flowers on the SAME plant (monoecious)
  • Other angiosperms have entire male OR female plants (dioecious)

Female Reproductive Structures:

  • Called carpals
  • Carpals may be fused to form the pistil
  • Produce eggs
  • Composed of 3 parts — stigma, style, and ovary
  • Stigma is located at the top and may be sticky or have hairs to hold pollen grains landing there
  • Style is a stalk-like connection between the stigma and the ovary
  • Ovary is the enlarged base containing ovules with eggs


Pistil

Male Reproductive organs:

  • Called stamens
  • Produce pollen
  • Composed of 2 parts — filament & anther (pollen sac)
  • Anthers produce pollen grains containing sperm
  • Filament is stalk-like & supports the pollen sacs


Stamen

Angiosperm Life Cycle:

  • Undergo alternation of generations
  • Sporophyte is dominant phase
  • Gametophytes (flowers) form male & female gametes
  • Anthers form pollen grains from microspores 
  • Pollen grains contain 2 cells — tube cell & generative cell (sperm)
  • Two protective layers called integuments surround the megasporangium
  • The entire structure including the integuments is the ovule and becomes the seed
  • Each ovule has 4 megaspores (three disintegrate)
  • The remaining megaspore undergoes mitosis to produce a large cell & polar nuclei 
  • When pollen lands on the stigma, a pollen tube grows through the style to the ovary
  • Two sperm travel down the pollen tube — one fertilizes the egg and the other join with polar nuclei to form endosperm (stored food for Seed)
  • Called Double Fertilization

  • After fertilization, ovule becomes the seed and the ovary & surrounding tissues form a protective fruit
  • A fruit is a ripened ovary with seeds (apple, melon, cocklebur…)
  • When seed land on moist soil, they germinate (sprout) and form new sporophyte plants

Pollination:

  • Wind, water, and animals help spread pollen
  • As pollinators drink nectar or eat the fruit, pollen gets on their bodies and is spread to other flowers
  • Self pollination occurs whenever pollen from a flower lands on the stigma of that SAME flower (pea plants)
  • Cross pollination occurs whenever pollen is spread to a different flower producing hybrids (more gene combinations)

Seeds & Fruit:

  • Fruits are adaptations for dispersing seeds (coconuts float, cockleburs catch onto animal fur, some seeds eaten by birds aren’t digestible…)
  • More energy is required to produce seeds than spores because they contain stored food
  • Seeds may be dormant (inactive) for weeks or years protected by their seed coat
  • Seeds contain a plant embryo and endosperm

  • Many fruits are fleshy & their seeds aren’t digested by the animals that eat them
  • Heavy seeds have adaptations such as wing-like structures (maple) or prickly coats (cocklebur) to help them disperse

maple seeds

Cockleburs

Coconut

  • Fruits may be dry or fleshy
  • Three types of fruits exist — simple, aggregate, & multiple
  • Simple fruits (apple) form from One pistil on a flower
  • Aggregate fruits (raspberry) form from several pistils on a flower
  • Multiple fruits (pineapple) form several flowers growing close together
  • Cotyledons are leaf-like structures of the plant embryo
  • Monocot seeds have one seed leaf (Cotyledon), while dicots have two cotyledons

  • The epicotyl is the part of the plant embryo ABOVE the cotyledon & becomes the stem
  • The radicle is the part of the plant embryo BELOW the cotyledon & becomes the root
  • The hypocotyl is the part of the plant embryo BETWEEN the cotyledon &  the radicle
  • The hilium is a scar along the seed edge where it was attached to the ovary
  • In monocot seeds like corn, a sheath called the coleoptile grows out of the ground to protect the newly emerging plant

Germination:

  • Many seeds require environmental factors, such as Water, Oxygen, and Temperature to trigger germination
  • Some seeds only germinate after exposure to extreme cold or after passing through an animal’s digestive tract
  • Water must FIRST be absorbed by the seed to break the seed coat & activate enzymes to change starch in the endosperm or cotyledons into simple sugars for energy
  • The radicle emerges first

  • Once the seed coat opens, OXYGEN is needed for cellular respiration carried on by the embryo plant
  • The shoot (hypocotyl & embryonic leaves) begin to grow, synthesize chlorophyll, and carry on photosynthesis

  • After the stored food is used up in dicots, the cotyledons fall off


Dicot Seed Germination

  • In Monocots like corn, the Cotyledon remains underground and transfers nutrients to the growing Embryo.

 Asexual Reproduction in Plants:

  • Asexual reproduction is FASTER and produces well-adapted offspring 
  • Called vegetative reproduction
  • Occurs from non-reproductive parts such as roots, stem, or leaves
  • Runners, Rhizomes, Bulbs, and Tubers can be used to produce new plants
  • Cutting is taking a piece of Stem or Leaf and growing a new plant 
  • Grafting occurs whenever 2 cut ends of plant stems are fused
  • Layering occurs when aerial roots touch soil & start growing new plants