Plant Reproduction

• Lifecycle of a flowering plant

Two main stages to the lifecycle
–Sporophyte
•Diploid stage
–The plant produces a flower with both male and female parts. The majority of a plants life is spent as the sporophyte
–Gametophyte
•Haploid stage
–Gametes (egg and male gametes). The plant only spends a brief amount of time in the gametophyte stage.

The Flower
The Reproductive Organs

The female parts of a flower consist of an ovary, which contains one or more ovules, a style and the stigma. The ovary is at the base of the flower.

 

From the ovary, extends a tubular structure called the style and on the top of the style is a surface receptive to pollen called the stigma.

 

The stigma can take many different forms, most of them designed to help trap pollen. There are many variations on this basic structural theme.

After fertilization the ovule becomes the seed and the ovary becomes the fruit.

The male parts of a flower consist of one or more stamens. Each stamen is made up of paired anthers (sacs containing pollen) on a filament or stalk.

 

The anthers are the orange/yellow structures often seen in the centre of a flower.

Pollen from the anthers of one flower is transferred to the stigma of another usually either by wind, or by animals, especially insects.



Pollination
Pollination is the transfer of pollen from one flower to the stigma of another flower of the same species
There are two types of pollination

1. –Wind (Grasses and cereals)


2. –Insect (Dandelions, daisies etc.).

• The most sophisticated relationships between plants and insects are generally those involving bees. Bees collect pollen and nectar not only for themselves but also to feed their young. For this reason bees have developed a number of adaptations that make them particularly good pollen carriers. Bees have special hairs that are arranged to form pollen 'baskets' on their hindlegs and the underside of their abdomen. These adaptations allow them to gather and carry large volumes of pollen. Bees are ideal pollinators because they visit many flowers while carrying lots of pollen, before returning to their nest. So the chance that a bee will transfer the pollen between flowers of the same species is very high.

• Many insects eat pollen. In the process of eating they become covered in it. Pollination happens when the pollen feeder transfers the pollen to the pollen receivers of the same plant, or another plant of the same species, as the insect looks for more pollen to eat.

Both wind and insect pollinated flowers have many adaptations for pollination.



Fertilisation

 Fertilisation is the union of the male and female gametes to form a zygote. Since the male and female gametes are haploid (n) when the two unite the zygote is diploid (2n).
The pollen lands on the stigma, the tube nucleus in the pollen grain forms a pollen tube down to the ovary
The male gametes travel down to the ovary
One nucleus will fuse with the egg to form the plant embryo.
The other nucleus will fuse with two other egg nuclei to form the endosperm which will be used as a food source for the embryo.

 

Seed Formation: The fertilized becomes the seed. The integuments become the wall of the seed called the testa. The micropyle closes. The endosperm nucleus leads to the formation of triploid endosperm, a food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing embryo draws nourishment from the endosperm. The embryo ceases development and goes dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and the protective coat called the testa.

•  In monocots, the   endosperm usually  remains

•  In dicots, the cotyledons usually absorb the endosperm.

Seed formation:

Monocots and Dicots

            Monocots have one cotyledon in the seed while dicots have two cotyledons. The cotyledons are food reserves for the young plant after it germinates from the soil. It uses these food reserves until it is capable of making its own food. In monocots the food is absorbed from the endosperm while in dicots the food is stored in the cotyledons.

The embryo consists of:
–Radicle: future root
–Plumule: future shoot
–Cotyledons: seed leaves
A seed coat (testa) develops around the embryo
In many cases, the ovary develops into a fruit.

Seed Structure
The outside of the seed is called the testa.
The embryo of a seed develops into a seedling and also provides a supply of food.
The embryo contains a radicle which forms a root and a plumule which forms a shoot.
The seed leaves are called cotyledons.
Seeds with one cotyledon are called monocotyledons and seeds with two cotyledons are called dicotyledons.

Seed dispersal

Seed dispersal is the scattering of offspring away from each other and from the parent plant. As a result of dispersal there is an improved chance of success by reducing competition and overcrowding. Dispersal also enables colonisation of new suitable habitats  and thus, there is an increased chance of species survival.
Seeds can be dispersed by
– Wind
– Animals
– Self
– Water

Dormancy:

Dormancy is a period of inactivity. There is very little cellular activity and no growth. One or many of the following reasons bring about dormancy:

 

Auxins that inhibit growth- Growth Inhibitors

The testa is impermeable to water and oxygen- The testa will eventually break down and allow water and oxygen into the seed.

The testa may be too hard for the embryo to germinate.

 

 An Auxin (Growth Regulator) may be absent until suitable environmental conditions develop.

 

Germination
The embryo will germinate from the seed if the proper environmental conditions are present. When this occurs the embryo resumes its growth. In order for germination to occur the following conditions must be present:

• Water must be present. This allows the seed to swell and enzymes to function.


• Oxygen must be present in the soil.


• The temperature must be suitable for the species of plant. Suitable temperatures are usually between 5-30 degrees Celsius depending on the species.


• The dormancy period must be complete.


• Some seeds need light and others need darkness.

Two types:
–Epigeal – cotyledons emerge above ground.
–Hypogeal – cotyledons do not emerge above ground.



Modified Organs and Vegetative Reproduction
Many plants have modified organs for food storage, while others use their modifications for asexual reproduction.
–Strawberries reproduce asexually by producing horizontal stems known as runners (stolons) above the ground
–Scutch grass reproduces asexually by producing underground stems known as rhizome.

Modified organs
The majority of modifications for food storage in plants are associated with the plants lifespan
–Annuals: complete their lifecycle in one year
–Biennials: complete their lifecycle in two years
–Perennials: is a plant that lives for more than two years
•Perennials flower and produce seeds many times during the course of their life time.

Modified roots
Modified roots:
–Tap root is modified to store food
•Examples include carrots and sugar beet
–If lateral roots are modified for food storage then it is called a tuberous root
• For example, dahlias.

Modified stems
The potato plant produces tubers which are modified underground stems for the storage of food.

Modified stems
Rhizomes are modified stems.
– If these stems are broken into pieces, each piece can produce a new plant
Corms are modified stems
–The crocus is an example

Modified Leaves
A bulb is an example of modified leaves used for food storage.
–An onion is a bulb

Plant Tropisms
A tropism is a plant’s growth response to an external stimulus
–Phototropism is a plant’s growth response to light
–Geotropism is a plant’s growth response to gravity
Growth responses to stimuli in plants are under the control of plant hormones.