Sexual Reproduction in Flowering Plants
Pre-Fertilisation Events and Structures
- Several hormonal and structural changes happen which lead to the differentiation and further devlopment of floral primordium before flower in seen on a plant.
- Inflorecences are formed which bears the floral bud and then the flowers.
- Structures of a flower
- It is the male reproductive organ of the flower.
- Androecium consists of stamens.
- The number and length of stamens are variable in different species.
- Stamen has two parts.
- anther → It is the bilobed structure at the terminal of the stamen.
- filament → It is the long and slender stalk.
- It is the female reproductive organ of the flower.
- It consists of pistil.
- If gynoecium consists of one pistil it is called monocarpellary. If it consists of more than one then it is called multicarpellary.
- Each pistil consists of three structures.
- Stigma → It is where the pollen grains are landed.
- Style → It is the long slender part beneath the stigma.
- Ovary → It is the basal bulged part.
- Inside the ovary there is ovarian cavity (locule).
- The placenta is located in the ovarian cavity.
- Megasporongia arise from the placenta. Which are commonly known as Ovules
- Number of ovules in an ovary can be one(wheat, paddy, mango) or multiple (papaya, water melon, orchids).
Stamen, Microsporongium and pollen Grain
- A typical angiosperm stamen is bilobed and dithecous.
- A longitudinal groove runs lengthwise seperating the theca.
- The anther is a tetragonial structure consisting four microsporongia in each four corners.
- It devlops further to make pollen sacs.
- They are packed with pollen grains.
- In transverse section a typical microsporongia appears to be near circular in outline.
- It has a four layer of walls srrounding it. From outside to inside it those are
- middle layer
- Outermost three layers (epidermis, endothecium, middle layer) helps in protection and dehiscence of anther to release the pollen.
- Tapetum nourishes the pollen grains.
- Tapetum has dense cytoplasm and usually have more than one neucleus.
- When anther is young compactly arranged homogenous tissues occupies the middle of the microsporongium, these cells are called sporogenous tissue.
- Each microsporongium has lots of microspore mother cells/pollen mother cell (MMC/PMC)
- Each cells of the sporongenous tissue undergo miotic division to form four cells which together are called Microspore Trtrad.
- As the anther dehydrates and matures they disassociate from each other and each make pollen grain or microspore.
- Each pollen grains are male gametophyte.
- It is the yellowish powdery thing that sticks on your finger when you touch the anther of hibbiscus.
- Pollen grains are generally spherical, and has a diameter of about 25-50 micrometers.
- It has has prominent two layered wall.
- It is the outermost layer of the pollen grain.
- It is made up of sporopollenin which is a fatty substance. It is one of the most resistant organic material known. It can withstand high temparature and acid and alkali. No engyme can degrade sporopollenin so far known.
3. It has prominent apertures which which are called germ pores, where sporopollenin is absent.
4. germ pores are required for fertilisation.
5. Pollen grains are well preserved as fossils because of this sporopollenin.
1. It is under the extine.
2. It is a thin continious layer made up of cellulose and pectin.
- When pollen grains mature it contains two cells.
- It is bigger in size.
- It has abundunt food reserve.
- Necleus is irregularly shaped.
- It is smaller in size
- Spindle shaped.
- Dense cytoplasm and a necleus.
- Pollen grains can cause allergic reactions, leading to chronic repiratory disorder like, asthma, bronchitis etc.
- Parthenium or carrot grass that came into india as a contaminent with imported wheat, is cause of pollen allergy.
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