Does self-incompatibility impose any restrictions on autogamy? Give reasons and suggest the method of pollination in such plants.
Self-incompatibility imposes restrictions on autogamy. The season far this may be as Majority of flowering plants produce hermaphrodite flowers and when pollens grains comes in contact with the stigma of the same flower to continue self-pollination.
Such type of continued self-pollination result in inbreeding depression. That's why flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. One of the major way to prevent self-pollination is self-sterility.
Self-sterility in some bisexual flowers, if the pollen grains fall on the stigma of the same flower, germination does not occur. But the same pollen grains germinate when they fall on the stigma of other flowers of the same species. It is a genetic mechanism to prevent self pollination.
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Embryos of monocotyledons possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis.
At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheat called coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl.
Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
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.