subject guide notes

D3.1.8—Sexual reproduction in flowering plants

stamens are the male structures of a flower

  • they consist of the anther & filament

pistil/carpel is the female reproductive part

  • consists of a swollen base (ovary)

if 4 petals aren’t present, flower is incomplete

flowers can be hermaphroditic/bisexual - consist of both male & female reproductive parts

  • perfect flower = has both

  • imperfect flower = only has one

remember - reproduction in flowering plants is sexual, even if the plant is hermaphroditic

pre- fertilization

before fertilization occurs, pollen grains are transferred from stamen to pistil

  • process is known as pollination

pollination

when pollen grain reaches stigma, it begins to germinate

tube cell elongates, forming pollen tube

pollen tube grows down the length of the style into the ovary, in response to chemicals released by synergids

generative cells divide mitotically to form sperm cells

sperm cells move through micropyle (small opening) into ovule

fertilisation

sperm nuclei will fuse with the egg to form the diploid zygote

  • this develops into embryo

second sperm nucleus fuses with central cell

  • central cells consists of 2 haploid polar nuclei

  • process is known as triple fusion & results in triploid cell

triploid cell gives rise to endosperm

process of fertilisation & triple fusion is known as double fertilisation

  • is seen only in flowering plants

D3.1.9—Features of an insect-pollinated flower

anther contains pollen grains

  • when mature, they’re released from the anther

each pollen cell consists of a generative cell & a larger tube cell

  • tube cell is surrounded by an outer tough exine & an inner intine (made of cellulose)

pistil consists of: swollen base (ovary), stalk (style) & stigma

ovary contains the ovules

  • each ovule contains an embryo-sac with 7 cells & 8 haploid nuclei

*check kognity for diagram of pollen grain & ovule

for flowers to reproduce, rate of pollination must be high

  • means lots of pollen must land on stigma

insect pollination is when pollen grains are carried from one flower to another through insects

flowers hv some adaptations to attract pollinators

  • nectar - as pollinator drinks nectar, anther brushes against its leg, transferring the pollen

    • when insect visits another flower, some pollen are transferred to stigma, resulting in pollination

  • strong scents

  • white or brightly colored

  • pollen is sticky to ensure it sticks to the insect’s body

flowers also hv ways to attract specific pollinators, so that the pollen of the plant isnt wasted on another species

D3.1.10—Methods of promoting cross-pollination

cross-pollination is transfer of pollen from from one flower on one plant, to the stigma of another flower on another plant

  • pollen grains transferred by animals

  • introduces genetic diversity, cuz male & female gametes are from 2 different plants

wind-pollinated flowers rely on wind to spread pollen grains

  • dont produce nectar

  • hv small petals

  • pollen grains are light & produced in large numbers

self-pollination is transfer of pollen grains to stigma of same flower or to stigma of another flower on same plant

plants hv some adaptations that facilitate cross-pollination & prevent self-pollination

  • some plants hv male & female females located on different plants

  • if male & female flowers located on same plant, their maturation time differs

  • in plants with hermaphroditic flowers, time at which anthers & pistils mature varies

D3.1.11—Self-incompatibility mechanisms to increase genetic variation within a species

self-pollination is form of inbreeding or mating amongst close relatives

genetic makeup of offspring is similar to parents, reducing genetic diversity

self-incompatibility is the inability of hermaphroditic plants to produce zygotes after self-pollination

  • characterized by failure at any of these stages: pollen germination, growth of pollen tube, embryo development & fertilisation

variations in length of stamen & style are governed by self-incompatibility genes

there are also genes for self-sterility, which can contribute to the incompatibility

genetically determined self-incompatibility facilitates cross-pollination & encourages variability in population

D3.1.12—Dispersal and germination of seeds

soon after fertilisation, changes occur

in ovule, zygote divides mitotically to form ovule

triploid cell develops into endosperm

wall of ovule becomes seed coat, enclosing embryo & endosperm

the ovary forms the fruit

seed dispersal

if all seeds fall close to parent plant, there’ll be intense competition for resources

to prevent this, seeds hv to be dispersed to other areas

seed dispersal can happen via wind, animals or water

dispersal by wind

wind-dispersed seeds are light & hv adaptations (wings or hair) to help them be carried by the wind

dispersal by water

seeds are buoyant

dispersal by animals

animals can eat fleshy fruits & toss the seeds a different direction

animals can eat seeds along with fruit

  • seeds cant be digested by digestive system cuz of the coating

  • the seeds are passed through faeces

animals (like chipmunks & squirrels) may hoard seeds & store them in different places

seeds can hv hooks or spines that attach to skin or fur of passing animals

*check kognity for produce of germination in seeds