endosperm development in angiosperm

POST FERTILIZATION EVENTS

Following double fertilisation, events of endosperm and embryo development, maturation of ovule(s) into seed(s) and ovary into fruit, are collectively termed post-fertilisation events.

 

ENDOSPERM

                                  >> IS a nutritative , Food-ladden,non-vascular,undifferentiated tissue formed after  triple fusion in angiosperm.It

Is generally triploid in angiosperm and haploid in gymnosperm. The endosperm in gymnosperms is haploid because it is formed from the haploid megaspore by repeated divisions. This endosperm consequently develops into female

gametophyte with haploid chromosomes.

J Endosperm development precedes embryo development.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 the nutrition of the developing embryo

 Depending upon mode of development endosperm is of three types –

.

 (i) Nuclear type:

J In the most common type( about 60 % endospermic seeded families )  of endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei. This stage of endosperm development is called free-nuclear endosperm. Subsequently cell wall formation occurs Centripetally ( From periphery to centre) and the endosperm becomes cellular. The number of free nuclei formed before cellularisation varies greatly. The coconut water/milk from tender coconut that you are familiar with, is nothing but free-nuclear endosperm/Liquid syncytium (made up of thousands of nuclei)  . another example is Capsella-bursa-pestoris.

(ii) 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. (Fig.2.29C).  surrounding oily,white kernel of coconut called coconut meat is the cellular endosperm.

(iii) 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. Ex- Vallisneria,Eremurus,Limnophyton

JNon-endospermic/Exalbuminous seeds.

Endosperm may  be completely consumed by the developing embryo before seed maturation  and cotyledons become fleshy by absorbing food . (e.g., pea, groundnut, beans) . such seeds are known as Non-endospermic/Exalbuminous seeds. It is happened in most of the dicots.

J Endospermic or albuminous

Endosperm may persist in the mature seed and be used up during seed germination.  Become a permanent part of the seed(e.g. castor and coconut,wheat.maize.rice and other cereals )

 

*** For determination the number of chromosome/Ploidy level-

i) In EMBRYO=

      Ploidylevel /chromosome number in male plat/2  +

        Ploidylevel/chromosome number in female plant/2

ii) In Endosperm=

 Toatal Ploidylevel/chromosome number of female plant +

   Ploidylevel/chromosome number of male plant/2

***  If genotype of female is tt and of male is TT then

Genotype of endosperm is ttT.

*** One meiosis in total produces 8 male gametes

**** Number of meiosis required to produce 100 fruits

        = Number of fruits asked x 1.25

   But for cyperus  no. of fruit askedx2

 Xenia= effect of pollen grains on visible characters(Colour of endosperm/fruit)

Metaxenia= effect of pollen grains on  the seed coat/fruit wall

        In citrus colour and flavor of fruit is due to metaxenia.