What is polyembryony and how can it be commercially exploited?
Polyembryony is the occurrence of more than one embryo in a seed. In many citrus and mango varieties, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embryos. In such species, each ovule contains many embryos.
Polyembryony plays a main role in plant breeding and horticulture. The plantlets obtained from these embryos are virus free and has more vigour. Hybrid varieties of several food and vegetable crops are being extensively cultivated and these hybrid cultivars possess high productiviy.
Yes, parthenocarpy and apomixis are different phenomenon.
Importance of Parthenocarpy
(i) The fruit production without fertilisation of the ovary is called parthenocarpy. This phenomenon is applied for the commercial production of seedless fruits. e.g., banana, grapes.
(ii) This is more useful far the juice industries.
Importance of Apomixis
(i) During apomixis, chromosomal segregation and recombination does not occur. So, characters are stable for several generations.
(ii) It simplifies commercial hybridised production because isolation is not necessary to produce $F_1$ or maintain parental generation.
(iii) Adventive embryony is being used in the production of uniform root-stock and virus free varieties.
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
$$ \text { The following are the parts of this figure } $$
