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).
Fig: 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.
(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.
Question 1: Explain triple fusion in angiosperm.
(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 ?
(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.
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.