Reproduction and Germination in Plants
Reproduction is the ability of living organisms to make new individuals of the same type.
Flowering plants reproduce sexually and many also have special methods of asexual reproduction.
Sexual Reproduction in Plants
Sexual reproduction is the formation of new individuals of the same type by using gametes, egg cells (female gametes) and sperm (male gametes).
A gamete is a specialised sex cell that fuses with another forming a zygote from which a new individual develops by cell division producing a multicellular organism.
Fertilisation is the fusion of a male gamete with a female gamete forming a single cell called a zygote.
Sexual reproduction gives rise to great genetic variation in the offspring.
The sex organs of a flowering plant are contained inside the flower.
Asexual Reproduction is the formation of new individuals without the use of gametes, not involving fertilisation. When these structures become separated from the parent plant each grows into a new plant. Examples: stem tubers of potato, bulbs of onion, runners of strawberry.
How the Potato Plant Reproduces Asexually
During the growing season the potato plant forms many underground stems. Excess food made in the leaves is transported to the tips of these stems for storage. The tips become greatly swollen with the reserve food and are known as tubers. At the end of the growing season the aerial parts die away and wither but the tubers live on in the soil where they survive the winter. In spring the tubers ‘germinate’ and each grows into a new potato plant.
Note: The potato plant also reproduces sexually forming flowers leading to seed formation and seed dispersal; if conditions are favourable a seed will germinate forming a new potato plant.
Sexual Reproduction in Flowering Plants The flower contains the sexual organs of a plant.
The Structure of a Flower
A flower is a modified terminal bud composed of four sets of modified leaves.
a) Carpels: the female structures, in the centre of the flower.
Stigma: collects the pollen from the pollinating agent.
Style: places the stigma in a suitable position to collect pollen.
Ovary: contains the ovules; each ovule has one female egg cell.
Fertilisation takes place in the ovule.
The ovules become seeds after fertilisation.
The ovary becomes the fruit protecing and dispersing the seeds.
b) Stamens: the male structures, surrounding the carpels.
Anther: produces the pollen – the male gametes are made in the pollen.
Filament: places the anther in a suitable position for the pollinating agent to collect the pollen.
c) Petals: attract the insect pollinators, surround the stamens.
d) Sepals: protect the flower bud and support the open flower, surround the petals at their base.
Pollination and Fertilisation
Pollination: the transfer of pollen from the anthers to the stigma.
Self-pollination: transfer of pollen from the anther to the stigma of the same flower or a flower of the same plant.
Cross-pollination: transfer of pollen from the anther of one plant to the stigma of a different plant.
Pollen carries the male gametes safely to the stigma of the carpel leading to fertilisation. Pollen can be transferred by wind or by insects. Insect pollinated flowers have colourful petals and nectar to attract insects. The stamens and carpels are inside the flower.
Wind pollinated flowers have feathery stigmas to collect pollen and stamens that hang outside the flower.
After pollination, a tube grows from the pollen grain to the ovule carrying the male gamete to the female gamete and fertilisation takes place. Fertilisation takes place in the ovule forming a zygote.
After fertilisation the zygote grows into a ‘baby plant’ and when it reaches a certain size it goes dormant.
The ovule also stores food for the growth of the ‘baby plant’ and extra to be used later in seed germination when the ‘baby plant’ restarts its growth in a mature plant.
The outer part of the ovule then forms a tough outer protective coat, called the testa, around the ‘baby plant’ and its food reserve. The fertilised ovule has now become a seed
A seed has three major components: a dormant plant embryo (baby plant), food reserve and a protective outer coat. The food reserve may be within the plant embryo or outside the plant embryo.
The plant embryo is composed of one or two leaves, a plumule which will later form the shoot system and the radicle which will form the root system.
Dispersal is the scattering of individuals away from each other.
Dispersal reduces competition and increases the chances of colonisation of new suitable habitats.
(a) Wind dispersal: fluffy style of dandelions, winged fruit of sycamore.
(b) Animal dispersal: animal food berries of the bramble (blackberry hedge).
(c) Water dispersal: air pockets in the seed coat of the water lily.
(d) Self-dispersal: seed pod of the pea flicks out the seeds.
Germination is the growth and development of a seed’s plant embryo into an independent plant able to supply all it own food by photosynthesis.
Conditions necessary for seed germination: water, oxygen and a suitable temperature.
MandPractivity 8 Investigate the Conditions Necessary for Germination
Set up four boiling tubes.
Tube A: pea seeds on wet cotton wool, exposed to the air and at room temperature; water, oxygen and a suitable temperature are all present.
Tube B: water control set up - dry pea seeds, on dry cotton wool, exposed to the air at room temperature; oxygen and a suitable temperature are present but water is absent.
Tube C: oxygen control – pea seeds on wet cotton wool, oxygen-free air at room temperature; water and a suitable temperature are present but oxygen is absent. Wet filter paper sprinkled with iron filings deoxygenates the air when the iron rusts.
Tube D: suitable temperature control - pea seeds on wet cotton wool at 4°C in a fridge, exposed to the air; water and oxygen are present but the temperature is very low.
Observe the seeds every day for a week.
Results: germination in A but not in B, C or D.
Conclusion: water, oxygen and a suitable temperature are together needed for seed germination.