Reproduction in plants

Flowering plants

called angiosperms. Most flowering plants are diploid

Meiosis takes place within the reproductive tissues and produces haploid spores which contain the gametes

male spores are pollen grains produced in the Anther

female spores are the embryo sac produced in the Ovule, in the ovary

Hermaphrodic

flowers have 4 sets

Contains both male and female parts

flowers are four sets of modified leaves arising from the receptacle at the base of the flower

Flower diagram

calyx. corolla, stamen, carpel

• Calyx: comprises the sepals they are usually green and protect the flower in the bud

• Corolla: inside the sepals, “a ring of petals”. There may be a nectary at the base releasing nectar which is scented and attracts pollinators

• Stamen: each statemen consists of a filament supporting an anther which produces pollen grains. The filament contains vascular tissue which transports sucrose, mineral ions and water to the developing pollen grains. The anther usually contains four pollen sacs arranged in two pairs side-by-side. When mature the pollen sacks dehisce and they open to release its pollen

• Carpel: female parts, contains developing ovules, ovary and the stigma

Pollinators

Attracted to large-coloured petals by scent and nectar

they use their long tongues to reach the sugary nectar at the base of the petals. As the insect enters the flower the anthers brush against its thorax and legs leaving sticky pollen behind

when the insect enters another flower it brushes some of the pollen against the ripe stigma and cross-pollination takes place

Wind pollinated flowers

Do not need bright scented flowers that attract insects

the anthers hang outside the flower so that the wind can blow away the small, smooth and light pollen

The feathery stigmas hang outside the flower and provide a large surface area for catching pollen grains that are blown into their path

Dehiscence

The opening of the anther, releasing pollen grains

Development of the male gamete

in the pollen sac

meiosis

In the pollen sac of the anther, diploid pollen mother cells undergo meiosis. Each forms a tetrad containing four haploid cells which become four pollen grains.

The tapetum a layer of cells around the pollen sack provide nutrients and regulatory molecules to developing pollen grains. It has a significant role in the formation of a pollen cell wall which is tough and resistant to chemicals.

The cell wall resist desiccation so the pollen grains can be transferred from one flower to another without drying out. UV lights cannot penetrate the pollen cell wall so the DNA in pollen that is carried at high altitude is protected from mutation

Inside the pollen grain

The haploid nucleus undergoes mitosis to produce two nuclei: a generative nucleus and tube nucleus

The generative nucleus producers to male nuclei by mitosis

When the pollen is mature

outer layers

opening

The outer layers of the anthers dry out causing tension in lateral grooves

eventually occurs in which the tension pulls the walls of the anther apart and the edges of the pollen sacs curl away

The opening called stoneium exposes the pollen grains and they are carried away by insects or wind

Development of the female gamete

• the ovary contains one or more ovules

• in each ovule: a mega-spore mother cell undergoes meiosis making 4 haploid cells

• 3 disintegrate

• the remaining cell undergo 3x mitosis, producing 8 haploid nuclei (1 is female gamete)

• two of the haploid nuclei fuse to make a diploid nucleus, called polar nucleus

Purpose of the nuclei

surrounded by nucleus

The nuclei are in the embryo sac surrounded by the nuclleus, a layer of cells which provide nutrients

around the nucleus are two layer of cells integuments. a gap in the interguments is the micropyle

As with the formation of the male gamete that type of cell division that directly produces the female gametes mitosis

8 nuclei form

• One forms the haploid female gamete

• Two form the haploid synergids

• Two will fuse to form a polar nucleus that is diploid.

• Three form 3 antipodal nuclei.

ovule diagram

Pollination

of the same species

The transfer of pollen grains from the anther to the mature stigma of a plant of the same species

postandry

The stamens of the flower ripen before the stigmas

self and cross pollination

• self pollination: The pollen from the anther of the flower is transferred to the mature stigma of the same flower or another flower on the same plant

• cross pollination: pollen is transferred from the anther of one flower to the mature stigma of another flower on another plant of the same species

self pollination leads to

genomes of gametes

self fertilisation- resluts to inbreeding

• self fertilized species depend only on independent assortment and crossing over during meiosis and on mutation to bring about genetic variation in the genomes of the gametes. They display less genetic variation than cross fertilized species

• greater chance of two potentially harmful recessive alleles being brought together at fertilization

• the advantage of inbreeding is that it can preserve those successful genomes that are suited to a relatively stable environment

Cross pollination can lead to

Cross fertilization- results in outbreeding

• outbreeding combined gametes from two individuals, in addition to events in meiosis and mutation, and so it generates more genetic variation

• outbreeding reduces the chance of producing harmful allele combinations

• outbreeding is of great evolutionary significance, because in a particular environment some genomes are more successful than others. It may allow a species to survive in a changing environment as there are always likely to be some members of a population with suitable combination of alleles

Ensuring cross pollination

• Dichogamy: eg. Protandy. Or protogyny (stigma opens first)

• the anther is below the stigma so pollen cannot fall onto it eg. Pin eyed primrose

• genetic incompatibility eg. red clover. pollen cannot germinate on the stigma of the flower that produced it

• separate meal and female flowers on the same plant eg. Maizev

• separate male and female plants eg. Holly

Fertilisation

fusion

The Fusion of a female and male gamete

producing a zygote

After pollination

• When a compatible pollen grain lands on the stigma, it germinates in the sucrose solution secreted by the stigma and produces a pollen tube

• the pollen tube nucleus is at the tip of the tube with two male nuclei behind

• the pollen tube grows out of the pollen grain through a gap in the cell wall (Pit) and down the style up a gradient of chemoattractants

Tube nucleus

codes for production

pollen tn disentegrates

• The pollen tube nucleus codes for the production of hydrolases including cellulases and proteases, and it digests its way through the tissues of the style. The products of digestion are used by the growing pollen tube

• the pollen tube grows through the gap in the integument, the micropyle and passes into the embryo sac

• the pollen tube nucleus disintegrates, presumably having completed its function of controlling the growth of the pollen tube

Two male gametes released

tip of the pollen tube opens

• The tip of the pollen tube opens, releasing the two male gametes into the embryo sac

• the male and female gametes are haploid one of the male gametes fuse with the female (the oosphere), to form the zygote, which is diploid

• the other male gamete fuses with the diploid polar nucleus to form a triploid nucleus. This triploid nucleus is the endosperm nucleus. When it divides repeatedly by mitosis, it generates the endosperm tissue which takes over from the nucleus in providing nutrition for the developing embryo

Double fertilisation

Starts when the pollen tube grows through the gap

Fusions have occurred to form the zygote and to form the endosperm

Fruit

ovary wall

A structure developing from the ovary wall, containing one or more seeds

Seed

Structure developed from a fertilized ovule, containing an embryo and food store enclosed within a testa

Development of the seed

zygote endosperm integuments

• Diploid zygote divides by mitosis, becoming an embryo which consists of a plumule ( developing shoot) a radical ( developing root) and one or two cotyledons (seed leaves)

• Triploid endosperm nucleus develops into a food store, providing food for their developing embryo

• outer integuments to dry out, harden and become waterproof, with deposits of ligon they become the seed coat/testa

• the micropyle remains as a pore in the seed

Development of the seed

ovule funicle/stalk ovary

• Ovule comprising the embryo, enfosperm and testa becomes the seed

• funicle or stalk of the ovule becomes the funicle of the seed, it attaches to the seed at the hilum

• the ovary becomes the fruit [in cherries the ovary wall become sweet juicy and pigmented] [in almonds the ovary wall becomes dry and hard]

Structure of the fruit and seed

Dicotyledon

Eg. Broad beans

Its seeds have two seed leaves or cotyledons. The embryo lies between them

the endosperm (which was the food store for early embryo) is absorbed into their cotyledons, so the broad bean has a non endospermic seed

Monocotyledon

Eg. Maize

it's only has one cotyledon

the endosperm remains as the food store, so maize seeds are endospermic

the cotyledon remains small and does not develop further. The testa of a maize seed fuses with the ovary wall, so maize has one seeded fruit

seed becomes dormant, water content falls below 10% and reduces their metabolic rate

They can survive long periods and will not germinate until conditions are suitable

Seed dispersal

The movement of seeds away from the parent plant

these seeds produce plants which avoid competition

dispersal methods have been subject to natural selection

Seed dispersal method

wind

Ash and Sycamore fruits have sales to allow wind dispersal

Dandelion fruits have parachutes of stiff hair

the fruit of poppies has pores, through which the seeds are shaken out when the stem is blown in the wind

Seed dispersal method, scarification

transport

A birds eat seeds that pass through the digestive system and are dispersed in faeces

mammals, Reptiles and fish can also disperse seeds this way

Scarification - the digestive system weakens the testa by physical attack by acid and by enzymes and the seeds of some species can only germinate when this happens

seed dispersal method

rolling

When the fruit of a horse chestnut tree breaks open, the conker, falls to the ground and rolls away from the parent tree

Seed dispersal method

bursting

When Lagoon pods dry they split and the seeds scatter

in many species the pods rotate as they burst open sending the seeds in different directions

Seed dispersal method

water

Coconut palms grow by water

coconuts are seeds and when they fall into the water, they float, because the air cavities make them buoyant and the water carries them

Seed dispersal method

carrying

Hooked seeds attached to animals coats and are carried away

Seeds and survival

• Dormant seeds

• the testa is chemically resistant

• the water content

• the testa

• the endosperm of cotyledons

• seeds can be dispersed

• dispersal allows colonization

• inhibitors

• Dormant seeds have a low metabolic rate and so they survive cold weather

• the testa is chemically resistant, so seeds survive adverse chemical conditions

• the water content of a dormant seed is produced below 10% and so seeds can survive very dry conditions

• the testa can physically protect the embryo

• the endosperm of cotyledons provide a supply of nutrients which last until the emerging seedling can photosynthesize adequately

• seeds can be dispersed great distances from the parent plant and don't compete with it

• dispersal allows colonization of new habitats

• inhibitors may allow germination at suitable times of the year. They are broken down in cold weather ( vernalization) so that the seed can germinate in spring

Germination

The biochemical and physiological processes through which a seed becomes a photosynthesizing plant

Conditions for successful germination

• suitable temp: optimum temp for germination is the optimum for the enzymes in the process. usually between 5-30 degrees

• water: to mobilise enzymes, for transport in the xylem and phloem and to vacuolate cells, making them turgid

• oxygen: aerobic respiration releases energy, which fuels metabolism and growth

Mobilisation of food reserves and germination

water

1. When conditions are suitable, water is taken up or “inbribed” rapidly by the seed through the micropyle. Water causes the tissues to swell and provide suitable conditions for enzymes activity

Mobilisation of food reserves and germination

amylase and protease

2. Food reserves in seeds are insoluble in water and can't be transported to the embryo

• The reserves must be broken down into soluble molecules. amylase hydrolyes starch into maltose and protease hydrolyes proteins to amino acids

• Te soluble products are transported to the embryo and carried in the phloem to the apical meristems of the plumule and radicale, where rapid cell division occurs

• some of these sugars are converted to cellulose for cell wall synthesis

• aerobic respiration releases energy from sugars and amino acids are used for protein synthesis

Mobilisation of food reserves and germination

swollen tissues

3. The swollen tissues rupture the testa and the radicale emerges from the seed. It is positively geotropic and negatively phototropic and so it grows downwards

4. then the plumule emerges it is positively phototrophic and negatively geotropic so it grows upwards

Mobilization of food reserves and germination

cotyledons

5. During germination the cotyledons of the broad bean remain below ground

   • the part of the plumule above the join between the embryo and the cotyledons elongate rapidly, pushing the plumule upwards

   • the plumule is bent over in the shape of a hook as it's pushes its way up through the soil. This protects the tip from damage of soil abrasion.

Mobilisation of food reserves and germination

straightens

If the seed has been planted at the correct depth in the soil, when the plumule emerges, the hook straightens and the leaves unfurl and begin to photosynthesize

By now the food reserves in the cotyledons will have been depleted

The brewing industry

Uses germinating barley seeds to make beer

terms “malt” and “Malting” used in brewing refers to the maltose generated when the starch in barely is digested

Gibberellin process

1. Barley embryo secretes a plant growth regulator: gibberellin, which diffuses through the endosperm to the aleurone layer. a layer has a high protein content

2. the gibberellin switches on genes in the cells of the aleurone layer, resulting in transcription and translation producing enzymes including protease and amylase

3. the proteases hydrolysed protein in the aleurone layer to amino acids, which are used to make amylase and maltase

4. the carbohydrates's diffuse out of the aleurone layer and hydrolyze the starch stored in the endosperm cells

5. the sugars produce diffuse back through the endosperm to the plumule and radicale of the embryo

6. sugars are aspired for energy which fueled bio synthesis and cell division bringing the seed out of dormancy