Why does the zygote begin to divide only after the division of Primary Endosperm Cell (PEC)?
The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with reserve food materials and are used for nutrition of the developing embryo.
Embryo develops at the micropylar end of the embryo sac where zygote is situated. Most zygotes divide only after certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.
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.
