Embryo and endosperm development

—EMBRYO AND ENDOSPERM DEVELOPMENT

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— True seed - fertilized mature ovule - embryo, stored food material and protective coats.

—Events

   

1. Pollination 

2. Fertilization 

3. Development of the fertilized ovule by 

    

a. Cell division 

 b. accumulation of reserve food       material 

 c. Dehydration.

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—Pollination?

—Pollination: The transfer of pollen from the male anther to the female stigma

—Fertilization

— One sperm nucleus (1n) fertilizes the egg       zygote (2n) à which becomes the plant embryo inside the seed.

—Another sperm nucleus fuses with the polar nuclei, resulting in a triploid endosperm (3n)

—Endosperm is a source of food for the young embryo.

—ENDOSPERM 

—Endosperm is the nutritive tissue formed as a result of triple fusion in the angiosperms.

 Endosperm is  meant for nourishing the embryo.

—The formation of endosperm is initiated by mitotic divisions of the primary endosperm nucleus (3N)

—Endosperm formation starts prior to embryo formation.

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—The formation of endosperm starts with degeneration of the nuclear tissue.

—Based on the mode of development there are three types of endosperms :

q (i) Nuclear type

q(ii) Cellular type

q(iii) Helobial type

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—Nuclear type:

—In nuclear type of endosperm the first division of primary endosperm nucleus and few subsequent nuclear divisions are not accompanied by wall formation.

— The nuclei produced are free in the cytoplasm of the embryo sac and they may remain free indefinitely or wall formation takes place later.

— In the coconut, cell wall formation of endosperm is never found complete.

—In Areca and Phoenix the endosperm becomes very hard

—Cellular type:

—In this case, there is cytokinesis after each nuclear division of endosperm nucleus.

— The endosperm, thus, has a cellular form, from the very beginning because first and subsequent divisions are all accompanied by wall formation.

—e.g. Petunia, Datura, Adoxa, etc.

—Helobial type:

—It is an intermediate type between the nuclear and cellular types.

—The first division is accompanied by cytokinesis but the subsequent ones are free nuclear.

—The chamber towards micropylar end of embryo sac is usually much larger than the chamber towards chalazal end.

—A large number of nuclei are formed in the micropylar chamber by free nuclear divisions while the nucleus of the chamber towards chalazal end divides to form a fewer free nuclei or may not divide at all.

—Endosperms formed may remain in the seeds or it may be consumed by the developing embryos- the food is generally stored in the cotyledons.

—Exalbuminous / nonendospermic seeds: Endosperm is used up during the development of embryo and food is preserved only in the cotyledons.

— Eg: Groundnut, Mustard, Sunflower.

Albuminous or endospermicseeds: Endosperm persists with the seed along with embryo. Cotyedons thin papery and do not store food.eg cotton papaya

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