The generative cell of a two celled pollen divides in the pollen tube, but not in a three-celled pollen. Give reasons.
In over $60 \%$ of angiosperms, pollen grains are shed at this 2-celled stage tube cell or vegetative cell generative cell. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed tubecell or vegetative cell two male gamete 3-celled stage.
In 3 celled stage, the pollen grains further germinate on the stigma to produce pollen tube through ane of the germ pores. The contents of the pollen grains move into the tube pollen tube grows through the tissues of the stigma and style and reaches to ovary.
Inplants, when pollen grain are shed at 2 celled stage the generative cell divides and form two male gametes during the growth of the pollen tube in the stigma.
In the figure given below label the following parts-male gametes, egg cell, polar nuclei, synergid and pollen tube
$$ \text { The following are the parts of this figure } $$
Starting with the zygote, draw the diagrams of the different stages of embryo development in a dicot.
What are the possible types of pollinations in chasmogamous flowers. Give reasons.
There are two types of pollinations (chasmogamy) in chasmogamous flowers, i.e., self--pollination and cross-pollination.
(a) Self-pollination(Autogamy) The transfer of pollen grains from anther to stigma of the same flower is called self-pollination. It is found in both cleistogamous and chasmogamous flowers.
(b) Cross-pollination (Allogamy) The transfer of pollen grains from anther to stigma of another flower is called a cross-pollination. It is of two types
(i) Geitonogamy It is the transfer of pollen grains from anther to the stigma of another flower of the same plant. It is functionally a type of cross-pollination involving a pollinating agent, genetically. It is similar to autogamy.
(ii) Xenogamy The transfer of pollen grains from the flower of one plant to the stigma of another plant. This is the only type of pollination, which brings genetically different types of pollen grains to the stigma.
With a neat, labelled diagram, describe the parts of a mature angiosperm embryo sac. Mention the role of synergids.
After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.
Six of the eight nuclei are surrounded by cell walls and organised into cells. Three cells present towards the micropylar end grouped together, constitute the egg apparatus. The egg apparatus, in turn consists of two synergids and one egg cell.
Three cells of the chalazal end are called the antipodals. The large central cell is formed by the fusion of 2-polar nuclei. Thus, a typical angiospermic embryo sac, at maturity consists of eight nuclei and seven cells. This embryo sac is formed from, the single megaspore, so it is called 'monosporic embryo sac.
Role of Synergids
The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid.