STD-12 UNIT-6 CHA-2
REPRODUCTION IN FLOWERING PLANT
STRUCTURE OF ANTHER & MICROSPORENGIUM
Structure of Anther |
| STRUCTURE OF ANTHER |
- It is a bilobed tetragonal oblong knob like fertile part of stamen.
- Each anther lobe has two chambers .
- The two chambers of an anther lobe possess long and cylindrical pollen sacs or microsporangia . Thus a bilobed anther is tetrasporangiate .
- Rarely , an anther lobe has only one microsporangium , e.g. , Wolffia
- There is just one microsporangium per anther , e.g .. Arceuthobium .
- The four microsporangia of an anther lie at its four corners .
- They run parallel to one another .
- The two microsporangia of an anther lobe are separated from each other by a shallow groove on the outside and a strip of sterile parenchymatous tissue internally .
- All the microsporangia are covered on the outside by a well defined common epidermis of the anther .
- The cells of epidermis often become stretched and shrivel off at maturity
- The microsporangia develop hypodermally in anther from strips of archesporial eells ,
- microsporangial wall consisting of endothecium , middle layers and tapetum .
T.S. young anther , & . T.S. anther at the time of dehiscence
Structure of Microsporangium .
- A microsporangium or future pollen sac is a cylindrical sac which appears circular in transverse section .
- It consists of two parts , outer wall and central homogeneous sporogenous tissue .
- Microsporangial wall has four types of layers- epidermis ( common anther covering ) , endothecium , 1-3 middle layers and tapetum .
- The outer three perform the function of protection in the young anther and mechanism of dehiscence in the ripe anther.
- Both endothecium and tapetum consist of larger cells.
- In a typical anther the endothecial cells develop fibrous thickenings of a cellulose on the inner and radial walls and become dead. Because of the presence of rous thickenings, the endothecium is also called fibrous layer.
- In the shallow groove present between the two microsporangia of an anther lobe the hypodermal cells lying at the level of endothecium remain thin walled.
- They con statute the stomium or line of dehiscence.
- Cells of middle layers shrivel in the mature anther.
- The tapetal cells enlarge radially and become filled with dense protoplasmic contents as well as nutrients.
- They either become multinucleate or their nucleus becomes polyploid due to endoploidy.
- Tapetum is of types amoeboid (= invasive, periplasmodium) and secretory (= glandular, parietal) amoeboid type the tapetal cells fuse to form a plasmodium or periplasmodium because it passes in between the sporogenous cells to nourish them.
- The cells of the secretary tapetum pass out substances over the sporogenous cells for their growth and differentiation. Ultimately both the types of tapetum degenerate.
- Tapetum has a number of functions:
(i) Nourishment of the developing microspore mother cells and pollen grains.
(ii) It produces lipid rich Ubisch granules containing sporopol lenin for exine formation, pollenkitt (oily, sticky covering of lipids and carotenoids) in case of entomophilous pollen grains, special proteins for the pollen grains to recognize compatibility and hormones IAA.
(iii) It secretes enzymes like callase responsible for the degradation. of callose wall arround pollen tetrad
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MANISH MEVADA
M.Sc, M.Phil, B.Ed
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