Sexual Reproduction in plants (WIP)

what is reproduction

-production of new individuals of the same species

-genetic information passed from parent to offspring

general features of sexual reproduction

-more than one parent

formation and fusion and of gametes

-not genetically identical offspring

-meiotic division

angiosperm structure

How are insect pollinated plants adapted?

bright colours and scents to attract insects to feed on them and exploit their mobility to transfer pollen from flower to flower

How are wind pollinated plants adapted?

different arrangement of sexual organs so there is an increased chance of fertilisation

how does insect pollination happen

the anther produces the pollen. As the insect reaches into the flower for nectar, pollen is brushed over its body, pollen is spiky and sticks to the insect. When the insect gets to the next flwoer pollen is transfered to the sticky stigma.

What is nectar

a sweet viscous secretion from the nectaries, a water solution of sugars fructose, glucose and sucrose.

How does wind pollination happen?

-dull flowers without scent

-petals usually absent leaving anthers and stigmas exposed

-hard bracts to give protection to sexual organs

-anthers hang outside the flower so wind can blow away the smooth, light pollen

-feathery sticky stigmas hang outside for a large surface area for catching pollen grains

cell division in plants

flowering plants are diploid and meiosis takes place in reproductive tissues to produce haploid reproductive structures or spores

-meiosis takes palce in anthers to produce male spores which contrain haploid gametes

-female spores are ovules, made in the ovary. female gametes develop inside the ovule

Formation of Pollen

-there are a large number of Microsporocytes (mother cells) in the 4 pollen sacs which are diploid. These divide by meiosis to provide 4 haploid microspores called a tetrad. An asymmetric division occurs by mitosis resulting in a generative nucleus (gamete) and a pollen tube nucleus.

pollen sac

What is the use of the fibrous layer

thickened inner walls which help release pollen grain when anthers split

what is the use of the tapetum

provides nutrients to developing pollen grains and microspore mother cells .

What is dehiscence

when the pollen is ripe, the outer layers of the anthers dry out and tensions are set up in lateral grooves. Dehiscence, or splitting open, occurs and the edges of the pollen sacs curl away

mature ovule

formation of female gamete

a megasporocyte (diploid) in the nucellus undergoes meiosis to form 4 haploid megaspores.

three of the haploid cells degenerate.

the last undergoes mitotic division three times:

• a female gamete (egg cell nucleus)

• 2 synergids (flanking the egg cell at tye micropyle end

• 2 polar nuclei

• 3 antipodal cells

2 polar nuclei are considered as one cells

what is pollination

the transfer of pollen from an anther to a stigme

what is fertilisation

the fusion of gametes

cross pollination

pollen is transferred from tge anthers of one flower to the stigma of another on another plant of the same species

self pollination

pollen from the anthers of a flower transferred to the stigma of the same flower or a different flower from the same plant.

advantages of self pollination/ in-breeding

-if there are no plants of the same species nearby

-can preserve "‘good’ genomes suited to a stable environment

disadvantages of self-pollination/ inbreeding

-reduces genetic variation

-

how do self pollinatiing plants bring about variation in gametes

-depend on random assortment and crossing over during meiosis

-mutation

strategies for promoting cross-pollination

-chemical self-incompatibility>protein interactions lead to inhibition of germination

-anthers and stigmans at different levels in flower/ in different position

-anthers adns stimas mature at different times

-seperate male and female flowers

heterotyly

a botanical trait in flowering plants where different plants of the same species produce flowers with different physical lengths of their reproductive organs

heterostyly example

Primrose thrum-eyed flowers have stamens higher then stigma and pin-eyed have stigma higher then stamen

how does pollination and fertilisation occur

pollen grains land on the stigma. If it is compatible it will absorb water and germinate, producing a pollen tube controlled by the pollen nucleus. Pollen tube grows down the style, secreting hydrolase enzymes and digest through the tissue of the style. As it penetrates the embryo sac via the microple the tip opens and 2 male gametes are released into the sac which contain the egg cell nucleus

double fertilisation

when moving down the pollen tube, the male gamete splits. One male gamete fuses with the female gamete and forms a diploid zygote which will form the embryo plant

the second male gamete fuses with the 2 polar nuclei to form a triploid primary endosperm nucleus, this will form the nutritive tissue inside the seed

what is the testa

the ovule develops into a seed, with a resistant coat so the seed can withstand hard conditions called the testa. This occurs through the toughening and thickening process from laying down waterproof substances.

how does the seed develop?

the seed develops from the fertilised ovule and contains embryonic plant and food store until the leaves are produced

what is the plumule?

developing shoot

what is the radicle?

the developing root

endosperm tissue

the triploid endosperm nucleus divides by mitosis to form the endosperm tissue which surrounds the developing embryo. This provides nourishment. In dichotyledons this only happens for a few days but in some species like cereal grains and other monocotyledons the tissue remains as storage tissue

what happens to the ovary

the ovary becomes the fruit and the ovary wall becomes the pericap of the fruit

Label this broad bean diagram

where is the food stored in the broad bean

absorbed into the two cotyledons

label this maize diagram

where is the food stored in maize (monocotyledon)

the food store surrounds the seed leaf

what are monocotyledons

have one seed leaf

what are dicotyledons

have two seed leaves

what are the advantages of a seed staying dormant

-allows for time for dispersal- if too close to parent, they may have to compete for nutrients

-prevents seeds from being germinated at the same time- competition for nutrients

-prevent seed being damages by adverse weather

what are the main methods of dispersion

gravity, animals, force, wind and water

what are the conditons required for a seed to germinate

Temperature- optimum for enzymes

Water- for mobalisation on enzymes and formation of vacuoles

Oxygen- respiration

what is dormancy

A period of suspended growth from the withdrawal of water whilst the seed is still attached to the parent plant with hormones keeping it dormant

what is germination

involves the rapid onset of biochemical activity and growth of the seedling until the plant can carry out photosynthesis and become independant of the food stores

what are the stages of germination

Water is taken up rapidly by the seed (imbibed through the micropyle), causing the cotyledon to swell. This causes the Testa to rupture, allowing entry of more oxygen and the radicle pushes its way through first. the radicle grows downwards and the plumule upwards. Startch and proteins in the cotyledons are hydorlysed and transported to growing points for respiration

What is aleurone layer and what is it for?

layer found between the startchy endosperm and the pericarp. It accumulates large quantaties of lipids and oils whihc are needed for seed developement and has many strong protein granules

what is giberellin and what is it for

released by the embryo in the aleurone layer and causes the secretion of hydrolytic enzymes which convert startch into maltose. They activate the gene that codes for amylase