Download Note 8

South Tuen Mun Government Secondary School
Biology Revision Note 8
Asexual reproduction – only one parent is required, do not involve gametes, the offspring is identical genetically
as the parent and to each other
Sexual reproduction – normally two sexes : always involve fusion of male gamete and female (fertilization),
genetic variation in offspring is produced
Vegetative propagation (asexual reproduction in flowering plant) - by perennating organ of flowering plant
(annual plant) e.g. rhizome, corm, bulb, stem tuber
Principle of vegetative propagation :
In winter, parent plant dies
leaving a food storage organ
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In spring, the storage organ supplies
The leaves make food by photosynthesis, some food
food for the growth of root and
green leaves.
→→→→
are transported to make the new storage organ
Some examples of flowering plant
Food storage organ
Ginger, lotus
rhizome - horizontal underground stem
Gladiolus
corm - vertical underground stem
Onion, Daffodil
bulb - fleshy scale leaf
Potato
stem tuber - underground branch of a stem
Advantage of vegetative propagation:
(1) A very fast way of reproduction – many offspring can be produced at a short period of time
(2) Identical offspring - good characters of parents are certainly inherited by offspring
(3) It allows the organisms to survive very bad condition e.g. cold winter
(4) It does not need any external agent e.g. wind, insects
Disadvantage of vegetative propagation:
(1) Overcrowd – strong competition of space, water and light
(2) No genetic variation – any change in the environment or disease can destroy the whole population
(3) Inherited diseases of parents or unfavourable characters are transmitted to the offspring
Sexual reproduction in flowering plants
Structure and function of flower :
Anther – for the formation of pollen grains which carry the male gamete, male gametes are produced by meiotic
cell division [Note : **pollen grains is not the same as male gametes]
Ovary – for the formation of ovule which carry the female gamete / egg / ovum, the female gametes are produced
by meiotic cell division [Note : ovule is not the same as female gamete / egg / ovum]
The ovary develops into the fruit after fertilization
Pollination – the transfer of pollen grains from anther to stigma
Advantage of cross-pollination: more genetic variation, so the offspring can survive better in a changing
environment.
Insect-pollinated flowers
Wind-pollinated flowers
Petals large, colourful, scented to attract insects. Has of Petals small and white.
insect guide.
Anther and stigma are inside the petal.
Anther and stigma are out of the petal, anther exposed
so pollen grains can be blown away easier; stigma
exposed to catch pollen grain easier.
Stigma sticky to catch pollen grain.
Stigma feathery with a large surface area to catch
pollen grain.
Has nectar as food for insect.
No nectar.
Pollen grains have rough coat - easily stick to the body of Pollen grains are small and light - easily blown away,
insects.
larger in number.
Fertilization
When pollen grain falls on the stigma of the same species, the sugary solution
produced by the stigma stimulates the pollen grain to germinate and develop a
pollen tube.
The pollen tube carries the male gamete towards the ovary. The male gamete enters
the ovule and fuses with the egg (female gamete) to form the zygote (fertilized
egg).
*Significance of fertilization :
 to restore original diploid number of chromosomes of the parent plant

due to independent assortment in meiotic cell division and random fertilization, the genetic materials of the
offspring are genetically different, genetic variation increases the chance of survival of the species in a
changing environment
Fruit - developed from the flower after fertilization
Stamen, style, stigma – wither and fall off
Ovary wall - forms the juicy part of fruit
Ovule - develops into seeds
Integument of ovule - develops into protective seed coat
Fertilized egg (zygote)  embryo and endosperm,
Embryo  cotyledon (子葉), plumule (future shoot), radicle (future root); in some plants, cotyledon develops
into a food storage part of the seed, the food is used for germination.
The functions of fruit: (1) to protect the developing embryo and (2) to help seed dispersal
Seed
In germination, the radicle develops into root and plumule develops into shoot.
The functions of seeds :
(1) to protect the embryo before germination: to prevent excess loss of water; to prevent the entry of foreign
particles; and it can remain alive under very bad condition
(2) to develop into a new plant
(3) to help dispersal of the species
Dispersal - the distribution of offspring to other places
(a) Wind dispersal:
(1) Fruit is light so that it can easily blown away by wind.
(2) Fruit is dry / non juicy. Juicy part is not needed.
(3) Fruit has a large surface area so that it can be blown by wind easily.
(4) Seeds are tiny and light so they can be easily blown away by wind.
(5) Seeds have large surface area e.g. wings in pine (non-flowering plant), hair in dandelion so
that it can be blown away by wind easily.
(b) Animal dispersal
(1) Some fruits are colourful to attract animals.
(2) Some fruits are fleshy, juicy and sweet with good smell to attract animals as tasty food
(3) Some plants have small, non tasty food. These seeds have hooks or sharp hair for attachment onto the
body of animals.
(4) For plants with juicy fruit, some seeds have thick coat to resist against the digestive juice. Some fruits
develop stone with large seeds which is not eaten by animals.
Importance of fruit and seed dispersal:
prevents overcrowdedness and exploit more living area  less competition of space, water and light