SEED development - Endosperm

SEED development

                

The zygote thus formed develops into embryo. When embryo is fully developed, the ovule is transformed into the seed.  During the formation of the seed, many changes take place in the ovule such as –

Integuments get dry and form seed coats. The outer covering is called testa and the inner covering is called tegmen. Insoluble food material is formed due to dehydration.Insoluble food material is stored in cotyledons and endosperm of the seed.The ovary wall becomes hard and fleshy and forms fruit wall. The overall changes involve many processes such as -

Endosperm        

The PEN which forms as a result of triple fusion so the resultant is generally triploid but may vary from diploid to polyploidy. In some families like Orchidaceae, Podostemataceae and Trapaceae the endosperm formation does not takes place. The endosperm produced may be consumed (exalbuminous) or may remain in bulk (albuminous) in mature seed. Endosperm is the chief nutritive tissue present in angiosperms, as in these plants; the embryo sac itself is extremely reduced and is incapable of supporting the embryo.

          

Types of Endosperm-

Based on the behavior of the PEN, the endosperm is divided into three main types-

 

Nuclear endosperm- 

The PEN divides by free nuclear divisions, without any wall formation. So, a central cavity with a few to several thousand nuclei at periphery is formed. After several free nuclear divisions, the endosperm enters into the cellular phase. The wall formation starts    mostly from centripetal end near proembryo. Either the central or the chalazal end may remain free nuclear. Examples of this type of endosperm are coconut  (Cocos nucifera), Cassia, etc.

 Cellular endosperm- 

 This is also called ab initio cellular  endosperm. In this type there is no free nuclear stage in the endosperm. Here the PEN divides and the first division is always followed by wall formation, forming two endosperm cells. Further divisions are also cellular and as a result the entire embryo sac gets filled up by a cellular tissue.

                                

The most peculiar feature of cellular endosperm is that in several plants one or more cells become highly specialized and perform a haustorial function. Examples of this type of endosperm are Acanthaceae, Santalaceae.

 Helobial endosperm- 

This is seen mostly in monocots. This is an intermediate type between the nuclear and cellular types. The PEN migrates to the chalazal region of the embryo-sac where it divides to form a large micropylar chamber and a small chalazal chamber. In the micropylar chamber, divisions are always free nuclear and cell wall formation, if exists, starts very late. In the chalazal chamber, the nucleus either remains undivided or divides only a few times. The chalazal chamber usually remains in the coenocytic condition. Examples of this type of endosperm are Juncus, Asphodelus, and Enalus.

Ruminate endosperm- 

 

Any irregularity or unevenness in the outline of the mature endosperm resulting in convolutions forms a ruminate endosperm. This is not a fourth type of endosperm. Any one of the endosperm types may become ruminate. Ruminate endosperm is seen in 32 families of Angiosperms.

            

 Rumination of the endosperm is caused by the activity of the seed coat or by the endosperm itself. When the seed coat is responsible for rumination, it may be due to (a) unequal elongation of the wall layer, or (b) infolding of the seed coat. When the endosperm itself is responsible it is due to the unequal growth in the peripheral region of the endosperm. E.g. Annonaceae, Aristolochiaceae.

              

On morphological characters, seven types of rumination have been recognized as Passiflora. Verbascum, Annona, Myristica, Cocoloba, Spigelia and Elytraria.  (Periasamy, 1962).