9.4 REPRODUCTION IN PLANTS

Angiosperms: Any plant that has a flower

Animals are used to facilitate the transfer of male pollen to female reproductive parts for fertilisation and seed development within the ovaries

2 classes of Angiosperms:

Monocots

• One cotyledon
• Parallel veins
• Vascular bundles are complexly arranged
• Floral parts are usually in multiples of three

• Dicots

• Two cotyledons
• Netlike veins
• Vascular bundles arranged in rings and taproot usually present
• Floral parts in four or five

Female Gametes: Ovules in the Ovaries of the flower

Male Gametes: Pollen grains produced by Anters

Plant Reproduction depends on 3 different processes:

• Pollination

• Fertilzation

• Seed Dispersal

Pollination: Transfer of pollen containing male gametes from an anther to a stigma

• Self pollination: Pollen from the anther from the same plant falls upon its own stigma
  

This method decreases genetic variation

• Cross-pollination: Pollen is carried from the anther of one plant to the stigma of a different plant of the same species

This method is preffered as it increases genetic variation within the second generation

Flowering plants can rely on wind or water for pollenation but most flowers use animals (pollenators):

• Birds
•  Bats
• Insects such as Bees

Mutualism: Close association between two organisms where both orgamisms benefit from the relationship: animal rewarded with food (nectar), the plant with successful pollenation

Fertilization: Fusion of a male gamete with a female gamete inside the ovule forming a zygote

1. Pollen grains (with male gametes) germinate on the stigma of flower

2. Pollen tube containing male gametes grows down the style to the ovary

3. Pollen tube delievers male gametes to an ovule (containing female gametes) which they fertilize

• Pollen germinates on stigma to produce a pollen tube which grows down the style
• Within the growing of the pollen tube there is the nucleus which will produce the spem
• The pollen tube completes its growth by entering an opening at the bottom of an ovary
• The sterm moves from the tube to combine with the egg of an ovule to form a zygote

Seed Dispersal: Spreading of seeds away from the parent plant to sites where they germinate and grow without competition

• Fertilized ovules develop into seeds
  

• Ovaries containing fertilized ovules develop into fruits

• Function of fruits is seed dispersal 

Both species in a mutulistic relationship benefit:

• Plants - Flowers become pollenated
• Pollenator - Obtains nectar (source of energy) and pollen

Plants have a trend where they develop mutualistic relationships for pollenation with one type of insect species.

This shows that it is important to protect entire ecosystems not just individual species as all species depend on each other

Seeds will not germinate unless external conditions are suitable. Factors needed for germination are:

• Availability of water
• Availability of oxygen
• Warmth is needed

To test an hypothesis:

• A control treatment giving the seeds all the factors needed
• A treatment giving the seeds all the factors except one

If the seeds with all the factors are able to germinate and the seeds that are missing one factor do not germinate - concluded that the missing factor is needed for germination

Once seeds are formed they are dehydrated and become dormant. If conditions are favourable the seed will germinate

• Water - rehydrate the dried seed tissues                        Temperature - Enzyme Activity
• Oxygen - allow aerobic respiration to produce ATP 

• Apical meristem produces more stem and leaves till it recieves a stimulus which makes it change to producing flowers (floral meristem)
• This switch is a change in gene expression in the shoot apex cells

The switch to flowering is in response to a stimulus: the length of light and dark periods (photoperiods) in plants

• Short day plant: Chrystanthemums
  
• Long day plant: Iris

SIGNIFICANT FACTOR: LENGTH OF NIGHT 

Floral Initiation: Caused by the growth and differentiation of apical cells

• Cell in shoot apex change how they divide and differentiate - because of changes in gene expression
• The triggers for change in gene expression vary between plants but the most common is day length

Photoperiodism: Process through which the control of flowering is achieved

CRITICAL FACTOR: NIGHT LENGTH

Plants use photoreceptors called phytochromes to determine the length of night and day

Two interconvertible forms of phytochrome:

• Pfr
• Pr

Plants measure the time taken to convert from far-red form (Pfr) to its red form (Pr) as a mesure of "day" but in actuality this conversion takes place at night

Sunlight contains more red light than far red light - in the day most Pr is quickly converted to Pfr

During darkness Pfr is slowly converted back to Pr

Long day plants: A lot of Pr during day which allows for a lot of Pfr to trigger flowering

Short day plants: Pfr binds to inhbiting factor binds to an inhibiting factor

• Flowering is affected by light
• Phytochrome exists in two inconvertible forms Pr and Pfr
• Pr (red absorbing) is converted to Pfr in red or day light and the gradual reversion of Pfr to Pr occurs in darkness

• Pfr is the active form in long day plants and occurs when day is longer than a critical length
• Enough Pfr remains in long day plants at the end of short nights to stimulate flowering. Pfr acts as a promoter of flowing in long day plants

• Short day flowers are induced to flower when the days are shorter than a critical value. At the end of long nights enough Pfr has been converted to Pr to allow flowering to occur. Pfr acts as an inhibitor of flowering in short-day plants

Chrysanthemums can be produced out of flowing season by keeping in greenhouse with blinds