CHAPTER 4 :REPRODUCTION & GROWTH4.5 – SEXUAL REPRODUCTION IN FLOWERING PLANTS
LEARNING OUTCOMES• Identify male & female structure in a flower;• Describe the formation of pollen grains;• Describe the formation of the embryo sac in the ovule;• Describe the formation of pollen tube;• Describe the formation of zygote;• Describe the formation of triploid nucleus;• Conceptualise double fertilisation;• Relate the structure of a fruit to the flower parts;• Explain the importance of double fertilisation for the survival of flowering plants.
TH E G E N E R A L S TR U C TU R E O F A F L O WE R• FLOWER = a modified shoot which is the reproductive structure of angiosperms.
Petal AntherStamen Stigma Filament Style Ovary Ovul Sepal Th e S tru c tu re s O f F lo w e r
• Each part of the flower serves a certain function : – the pedicel supports the flower in the best position for pollination; – The petals are usually big & brightly coloured to attract insects @ birds for pollination; – The sepals protect the flower when it is still a bud.
• The male structure = stamen (consists of a filament & an anther).• The anther functions in producing pollen grains & is supported by the filament.• The male gametes are found in the pollen grain.
• The female structure = pistil (consists of stigma, style & ovary).• Inside the ovary, one @ more ovules can be found.• The female gametes @ egg cell is found in the ovule.• Pollen grain are received by the stigma.
THE FORMATION OF POLLEN GRAINS• Are formed in the anther.• Each anther contains 4 chambers called pollen sacs in which pollen grains are formed.• The tapetum provides nourishment to the developing pollen grains.
• Each pollen sac contains many diploid pollen mother cells meiosis a tetrad of 4 haploid cell separate & become the pollen grain.• The haploid nucleus divides by mitosis to produce a generative nucleus & a tube nucleus.• Pollen grain is a microspore & not a gamete.• Male gamete are form when the generative nucleus divides.
THE FORMATION OF EMBRYO SAC• Embryo sac = a structure containing the egg cell found in the ovule.• Initially, the ovule contains similar diploid cells, nucellus one of the cells enlarges to become the embryo sac mother cell.• Embryo sac mother cell meiosis 4 haploid cells, 3 disintegrate.
• The remaining cell (which represents the young embryo sac) will undergo mitosis thrice to produce 8 nuclei.• 2 of the nuclei will move to the middle of the embryo sac polar nuclei.• Other 6 nuclei are enveloped with cytoplasm & become cells 3 antipodal cells, 2 synergids, 1 egg cell mature embryo (megaspore containing the female gamete)
• Only the egg cell & the 2 polar nuclei are important in fertilisation the other will disintegrate.• The ovule becomes enveloped by 2 layers of integuments, leaving a small opening (micropyle) connected to the ovary by funicle. megaspore (embryo sac containing the female gamete).
THE FORMATION OF POLLEN TUBE• POLLINATION = the transfer of pollen from the stamens to the stigma• Self-pollination / cross pollination• The stigma secretes a sugary liquid which stimulates germination.• The pollen germinates a pollen tube is produces through the tissues of the style into an ovule.• The generative nucleus divides by mitosis to produce 2 male nuclei.
FERTILISATION• When the pollen tube arrives at the embryo sac, the tube nucleus disintegrates.• 1 of the male nuclei fuses with egg cell to form a diploid zygote develops into embryo which consist of the radicle, plumule, & 1 @ 2 cotyledons.• The other male nucleus fuses with the 2 polar nuclei to form a triploid nucleus (3n) endosperm (store food for the developing embryo).
• The ovary becomes a fruit pericarp & the ovules become seeds.• The outer integument becomes the seed coat @ testa (to protect the seed)• The inner integument becomes tegmen, a thin membrane underneath the testa.• All parts of the flower wither away.
PERI CARP M ODE OF FRUI T DI SPERSAL F l e s h y, Eat e n b y an i mal s s uccul e nt ; s e e d s ar e t h e n ed i bl e d i s c ar d e d , f ar f r o m Ho o k / s p i n e s S t i tchkes ptar efnutr. o f o p as s i n g an i mal sW n g - s h ap e d ; d r y i Eas i l y c ar r i e d b y & l i gh t ; wi n d . f e at h e r y h ai r s Fi b r ou s t i s s u e Car r i e d b y wat e r . wi t h man y ai rUn e ve s p ac e s n g o f n d r yi Se e d s d i s pe rs e d by f r u i t wal l e x p l o s i ve me c h an i s m c au s i n g s u d d e n
D o u b le F e r t ilis a t io n In F lo w e r in g P la n t s
THE IMPORTANCE OF DOUBLE FERTILISATION FOR THE SURVIVAL OF FLOWERING PLANTS• Double fertilisation is important for two reasons : • By formation of the 3n nucleus endosperm, the parent plants provide an important nutrient store for the developing plant. • It is ensure that the nutritive tissue formed is not wasted because it will be used by the growing embryo the endosperm only develops if the ovum has been fertilised.
• The provision of food for the embryo is important for the survival of flowering plants.• The presence of food enables the embryo to live for a long period of time even when conditions are unfavourable such as dry & cold conditions.• When favourable the food in the endosperm will be broken down into simple molecules such as sugar, amino acids, glycerol & fatty acids absorbed by the embryo to build tissues for germination.
DON’T FORGET TO DOCHECKPOINT 4.5 (page 147).PLEASE MAKE A REVISION!!!