Sexual & Asexual Reproduction In Plants

• The sepal protects the flower and prevents it from drying out. Petals attract insects to the flower for pollination.
• Stamen produce the pollen grains in the anthers. The filament holds the anther in place.
• The carpel produces the ovules. Each ovule contains an egg cell inside an embryo sac. The pollen lands on the stigma after pollination and travels down the style.
• Cross-pollination is the transfer of pollen from the anther to the stigma of a different plant of the same species.
• For insect pollination, petals are brightly coloured and scented with nectaries and contain small amount of sticky pollen. In wind pollination, the petals are small and not coloured brightly. The anthers are outside the petals.
• Hay fever is an allergic reaction to allergens such as pollen grains. Causes sneezing and a blocked or runny nose.
• The pollen grain produces the male gametes and the embryo sac produces an egg cell and polar nuclei.
• in fertilization, a pollen grain lands on a stigma and a pollen tube grows down the style toward the ovule. the Ovule releases chemicals to cause the pollen tube to grow towards the micropyle. The nucleus divides in two.
• In Double fertilization, one sperm nucleus fuses with the egg nucleus to form a diploid zygote. The second sperm nucleus fuses with the two polar nuclei to form a triploid endosperm.
• In seed formation, the zygote grows repeatedly to form an embryo. This consists of a plumule, a radical and cotyledons.
• In seed formation the endosperm nucleus divides to form the endosperm in endospermic seeds. This acts as a food store for the developing seed. E.g. maize
• In non-endospermic seeds the endosperm is used up in the early stages of seed development so the food Is stored in cotyledons. E.g bean
• In fruit formation, the ovule becomes the seed while the ovary becomes the fruit.
• Seedless fruits can be formed in two ways​. Genetically​, either naturally or by special breeding programs​ e.g. seedless oranges, or with growth regulators e.g. auxins e.g. seedless grapes.
• Fruit and seed dispersal minimizes the competition for water, light etc. and avoids overcrowding.
• Sycamore and ash trees produce fruit with wings to aid with wind dispersal.
• Light, air filled fruits e.g. lilies float away on water. This is water dispersal.
• Fruits eaten by animals then disperse seeds. This is known as animal dispersal. e.g. strawberries
• Some fruits explode open when they dry out and flick the seed away. This is known as self-dispersal. ​E.g. peas and beans
• Dormancy is a resting period when seeds undergo no growth and have reduced cell activity or metabolism.
• Dormancy allows for plants to avoid harsh winter conditions and provides time for dispersal.
• Germination is the re-growth of the embryo after a period of dormancy, if the environmental conditions are suitable
• Germination requires water to activate the enzymes. Oxygen is needed for aerobic respiration. A suitable temperature ensures maximum enzyme activity.
• The two events in germination are digestion ​Of stored food in endosperm and cotyledon​ and respiration​ to produce ATP to drive cell division.
• An anther consists of four chambers called pollen sacs. Each pollen sac is protected by a fibrous layer. Inside the fibrous layer is the tapetum – a food store that supplies energy for cell divisions in the pollen sac.
• Microspore mother cells (2n) are located in the pollen sacs. They are diploid​. They divide by meiosis to produce a group of four haploid cells called a tetrad​. Each tetrad breaks up to form four separate haploid pollen grains .
• Pollen grains divide by mitosis producing two haploid nuclei:​ Tube nucleus – forms the pollen tube ​and Generative nucleus – form the male gametes​
• When pollen grains have matured the walls of the anther dry and split. Pollen grains are then exposed and are ready for pollination
• After pollination the generative nucleus divides by mitosis to form two sperm nuclei. These will each play a role in double fertilizations.
• Each ovary contains one or more ovules​. An ovary has two walls called integuments​. Between the integuments is a small opening (micropyle) through which a pollen tube can enter.​ The nucellus provides nutrients for growth in the ovule​
• One cell in the ovule, the megaspore mother cell divides by meiosis to form four haploid cells​. Three of these cells disintegrate. The remaining cell is called the embryosac
• The nucleus of the embryo sac (n) divides by mitosis three times forming eight haploid nuclei​. These are still contained within the embryo sac​. Five of these nuclei will later disintegrate​. The remaining three are the gametes​
• Structure of the flower

  A) petal

  B) anther

  C) filament

  D) sepal

  E) stalk

  F) nectary

  G) receptacle

  H) ovary

  I) style

  J) stigma