Plant reproduction, seed dispersal & germination

The anther

The site of meiosis - 4 haploid cells “each is called a pollen grain”. Then, they undergo mitosis BUT THEY DO NOT SEPARATE. Only the nuclei undergo mitosis: haploid nuclei a → 2 haploid nuclei = 3 haploid nuclei in ONE pollen grain. 2 nuclei in each pollen grain will be male gametes, 1 will help with fertilization

The ovum

Site of meiosis - produces 4 haploid cells, but the unequal division of the cytoplasm/organelles leads to only 1 haploid cell and 3 polar nuclei. This egg goes through multiple rounds of mitosis to produce 8 haploid cells. 7 of these cells are helper cells, only 1 will be the haploid egg.

Pollination

Transfer of pollen from anther to stigma through wind/animals.

Fertilization

Pollination is the transfer of pollen grains from the anther to the stigma.

After pollination, a tube cell (also called the helper cell) within the pollen grain grows a pollen tube down the style and into the ovary.

The pollen tube carries two haploid sperm nuclei (male gametes) to the ovule.

One sperm nucleus fuses with the egg cell to form a diploid zygote (2n). This is fertilisation.

The zygote undergoes mitosis to form an embryo, which later develops into the seed.

The second sperm nucleus fuses with the two haploid polar nuclei in the central cell.

This process is called triple fusion because three haploid nuclei fuse together:

n+n+n=3n

Triple fusion produces a triploid endosperm (3n).

The endosperm surrounds and nourishes the developing embryo.

Because two fusion events occur, the process is called double fertilisation, a feature unique to flowering plants.

Petal

Attracts pollinators

Stamen

Anther + filament.

Filament

Holds up anther to position the pollinator so it’ll come in contact with the pollen

Stigma

Captures pollen, it’s sticky

Style

Long tube that positions stigma to allow this pollination to happen

Sepal

protects the developing flower

Ovule

meiosis and seed development

Cross pollination

Transfer of pollen to one stigma from a different anther. More genetic diversity than self-pollination.

Methods of promoting cross pollination

Using wind/animal pollinators to go from x plant to y plant

Have separate male & female flowers/plants

Male anthers and female stigmas developing at different times

Self incompatibility

A mechanism where the pollen grain fails to develop when it reaches the ovary of the same plant. If alleles from pollen grains match alleles from stigma, fertilization does not occur. Eg. pollen grains have S1S2 alleles , stigma has S1S3 alleles, S1 and S1 are the same so fertilization is not successful/does not occur, but S2 and S3 is successful and fertilization will occur.

Dispersal of seeds

Parents do not want their offspring planted near them, so instead they ensure their seeds are dispersed everywhere. They do this by:

Seeds being hidden by fruit (eg. apple) so animals eat them and the seeds are dispersed via stool being released

Seeds being hidden in a hook shell so it hooks onto an animal and will be put down elsewhere

Wings on the seeds so the wind will pick them up and put them elsewhere.

What does a seed consist of

An embryo, food stores (starch and studd from the endosperm tissue), and a protective seed coat called the testa. It is dormant until it becomes active & germinates.

What do seeds need to germinate

1) Water. When water enters the seed through the microscopic pore on the testa, which causes the seed to swell (imbibition). The seed coat softens and then splits, the addition of water also allows for metabolism where enzymes become active and the seed can respire.

2) Oxygen. The embryo needs to do aerobic cellular respiration to make ATP to grow and allow for cell division.

3) Warm temperature. Respiration is directed by enzymes, which are directly affected by temperature. Too cold = enzymes don’t work efficiently, too warm = enzymes denature and respiration cannot occur = no growth = no germination

Steps of germination

1) Water enters, the seed absorbs the water and swells up, also becoming metabolically active.

2) Enzymes activate and start breaking down food stores from the endosperm, starch is broken down into glucose, proteins into amino acids, and these are transferred to the embryo

3) The embryo receives the glucose, enzymes, etc. and uses oxygen to respire and thus make ATP and grow

4) The radicle emerges, it grows downward and becomes the root, which anchors the cell and allows for absorption of water and minerals.

5) The shoot (pumule) grows upward and pushes through the soil.

6) The cotyledons grow leaves since the food stores are used up and when leaves form, photosynthesis begins.