Microsporogenesis NEET Notes | EduRev

Biology Class 12

NEET : Microsporogenesis NEET Notes | EduRev

The document Microsporogenesis NEET Notes | EduRev is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET

MICROSPOROGENESIS:

  • The anther appears as outgrowth like structure in the initial stage which shows spherical or oval shaped structure.
  • At this stage, it is a mass of undifferentiated and homogenous meristematic cells which is surrounded by a single cell thick outer layer. This layer is known as epidermis.
    Microsporogenesis NEET Notes | EduRevFig: Scheme of MicrosporogenesisFirst of all vascular tissue are formed in middle region. Simultaneously group of  cells located just below the epidermis in vertical rows in the region of hypodermis at the four corners are become large has visible nucleus with dense cytoplasm. Due to this reason they are different from the rest of the cells. These cells are called archesporial cells .
  • These cells divide periclinally to form primary parietal cells below the epidermis and primary sporogenous cells towards the centre. Both of the cells usually undergo further divisions to form complete structure of anther except epidermis.
  • Primary parietal cells undergo further periclinal and anticlinal division to form a series of 3-5 layers making the walls of the anther.
  • Out of them outer most layer of anther is formed just below the epidermis by primary parietal cells is called endothecium or fibrous layer. The endothecium is followed by 1-3 celled thick layer is termed middle layer. The innermost layer of the anther which surrounds pollen sacs, is called tapetum. Later the tapetal cells play a significant role during the meiotic cell division in microsporogenous cells and in pollen development.
  • The primary sporogenous cells divide twice or more than two by mitotic division to form sporogenous cells and later sporogenous differentiated into microspore mother cells during the formation of wall of pollen sac.
  • Each microspore mother cell divide to form four haploid microspore or pollen grain by meiotic division or reduction division.
  • During this period spherical bodies are formed inside the tapetal cells before their disintegration. These spherical bodies are known as Ubisch-body. Ubisch body is made up of a complex substance called sporopollenin. It is the polymer of carotenoids.
  • After the formation of ubisch body, the tapetum layer degenerates. Ubisch bodies participate in the formation of exine of the microspores inside the pollen sacs. Now thick walled microspores are called pollen grains.
  • At the initial stage all four microspores are attached together with the help of callose layer. This group of microspores is called tetrad. After some time, this callose layer dissolve by callase enzyme. Which is secreted by tapetum.

Note:- Normally each microspore mother cell can form tetrad by meiotic division. But in some plant like Zostera, some microspore mother cells become sterile and provide nutrition to rest of microspore mother cells.

Note: Similarly, tapetum is not well developed in Gentianaceae family so some cells of sporogenous tissue become sterile and provide nutrition to remaining sporogenous cells.

Types of tetrads:- 

The arrangment of the microspores in tetrad condition as follows:-

(i)  Tetrahedral tetrad:- Four haploid microspores arranged in tetrahedral form

Example:- Dicotyledons- (Capsella).
Microsporogenesis NEET Notes | EduRevFig: Tetrahedral Tetrad(ii)  Isobilateral tetrad:- This condition is found in monocotyledons. Microspores are arranged at the lateral side of each other.

(iii) Decussate Tetrad:- In this two microspores lies at the right angle of other microspores.
Example:- Magnolia

(iv) T-Shaped tetrad:- Two microspores lie longitudinally and two microspores lie at transversely in this type of tetrad. Example :- Aristolochia & Butomopsis.

(v)  Linear tetrad- In this tetrad all four pollens arranged in linear order. e.g., Halophylla, Halophia.
All the above type of tetrads are found in Aristolochia elegans.
Most common type of tetrads is Tetrahedral.

Solved Question:
Question 1: Explain triple fusion in angiosperm. 
Explanation: 
(i) In the pollen tube, two male gametes are present. In embryo sac one of the male gametes fuses with egg cell to form zygote. This is called syngamy.
(ii) Second male gamete fuses with two polar nuclei located in the central cell to produce a primary endosperm nucleus.
(iii) In this fusion two polar haploid nuclei and one male gamete fuses. As it involves the fusion of three haploid nuclei, it is known as triple fusion.
(iv) Since two types of fusions, syngamy and triple fusion take place in an embryo sac so it is called double fertilization.

Question 2: Write the fate of the products of this fusion in the mature fruit of coconut. 
Explanation: Primary endosperm nucleus gives rise to endosperm by its repeated mitotic division. Coconut water is free-nuclear endosperm (made up of thousands of nuclei) and the white kernel is the cellular endosperm.

Question 3: How do seed bearing plants tide over dry and hot weather conditions ?
Explanation: Seed bearing plants produce seeds before onset of dry and hot weather conditions. Seeds enter into a stage of 'dormancy' which help them to survive unfavourable conditions.

Question 4: Why are some seeds of citrus referred to as polyembryonic ? How are they formed ?
Explanation: 
(i) Many embryos are present in citrus seeds so these are referred to as polyembryonic seeds.
(ii) Some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embryos.

Question 5: How do plants produce seeds through apomixis? Explain with the help of an example.
Explanation: 
The method in which seeds are produced without fertilization is known as Apomixis. The diploid egg cell is formed without reduction division and develops into the embryo sac and develop into the embryo, e.g., Asteraceae and grasses. 

Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!

Complete Syllabus of NEET

Dynamic Test

Content Category

Related Searches

video lectures

,

Microsporogenesis NEET Notes | EduRev

,

Free

,

mock tests for examination

,

Microsporogenesis NEET Notes | EduRev

,

Sample Paper

,

ppt

,

Important questions

,

Previous Year Questions with Solutions

,

Microsporogenesis NEET Notes | EduRev

,

Semester Notes

,

Objective type Questions

,

study material

,

pdf

,

Summary

,

shortcuts and tricks

,

practice quizzes

,

Viva Questions

,

MCQs

,

Exam

,

Extra Questions

,

past year papers

;