ENDOSPERM

 

Endosperm is the most common nutritive tissue for the developing embryos in angiosperms. Functionally it is comparable to the female gametophyte in gymnosperms but has a unique origin. Whereas the female gametophyte in gymnosperms differentiated before fertilisation and is haploid, the endosperm is the product of fertilisation​ is usually triploid.

After double fertilisation the egg is called zygote and the fusion product of Polar nuclei and the second male gamete is termed primary endosperm nucleus. The former develops into are organised embryo whereas the latter gives rise to a almost formless tissue the endosperm. The only angiosperms that do not form endosperm are the members of the families Orchiadaceae, Podostamaceae, Trapaceae.

  When present the endosperm may either be consumed by the developing embryo so that non- endospermous seeds are formed, e.g., Pea, Beans. Or it may persist in nature seeds continue to support the growth of embryo during seed germination, so that endospermous seeds are formed.

  Common examples of endospermou seeds are cereals, castor-bean, coconut. Endosperm forms the edible part of cereals, coconut, it is the source of commercial castor oil in castor bean.

 DEVELOPMENT OF ENDOSPERM:

The primary endosperm nucleus is normally located directly below the egg cell undergoes divisions almost immediately after its formation.

   During triple fusion only the sperm nucleus fuses with the polar nuclei while the male cytoplasm doesn't take part in this process. The membrane of the primary endosperm nucleus in contributed by both the secondary nucleus and the male nucleus.

Following fertilisation several changes occur in the fine structure of the central cell that are indicative metabolic activity and organisation of the protein synthesis machinery for the differentiation of the primary endosperm cell.  Depending upon its mode of development three types of endosperm have been recognised. These are nuclear Cellular and Helobial.

 NUCLEAR ENDOSPERM:

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

This results in a condition where the central cell of the embryo sac has formed a few to several thousand nuclei freely suspended in its sap. Such a condition of endosperm may persist until it is consumed by the developing embryo or it may become cellular at a later stage.

  When latter is the case which is more common the wall formation is mostly centripetal i.e., from the periphery toward the center. The degree of cellularisation varies a great deal. Mostly the endosperm becomes completely cellular but in Phaseolus cellularisation occurs only around the embryo.

  Using the technique of dissections kaushik (1941) for the first time reported the presence of a  vermiform appendage at the chalazal end of the endosperm in Grevillea robusta. Since then endosperm haustoria have been reported in several members of the Cucurbitaceae, Leguminosae, Protoceae.

 An interesting feature is the formation of endosperm nodules or cytoplasmic vesicles. These originate from the peripheral lining of the embryosac. The nodules may be nucleate as in Salix, Cineria,Carica, Capsella or may be enuceleate as in Stackhousia, Pennisetum and some cucurbits

 CELLULAR ENDOSPERM:

The cellular endosperm is characterized by the absence of free nuclear stage. The division of the primary endosperm nucleus and a few subsequent nuclear divisions are followed regularly by wall formation.

  The occurrence of haustoria is a common feature of this type of endosperm; it is more varied than that in the nuclear endosperm. The haustoria may be micropylar or chalazal. Occassionally both types of haustoria are present in the same plant.

   The most peculiar feature of cellular endosperm is that in several plants one or more cells become highly specialised and perform haustorial function. This is exhibited by their hypertrophied nature and densely staining protoplasmic contents. The haustoria may penetrate for a considerable distance into the adjacent tissue of the ovule are believed to contribute significantly to the movement of food materials into the growing endosperm.

   The development of endosperm in the Loranthaceae is unique. There being no true ovule, all the embryo sac in the ovary lie close to each other after fertilisation the primary endosperm nucleus moves to the basal part of the embryo sac where it divides. During their development, the endosperm of all the embryo sac in an ovary fuse to form a composite endosperm.

 HELOBIAL ENDOSPERM:

This type of endosperm is restricted largely to the monocotyledons.The primary endosperm nucleus moves to the chalazal end of the embryosac where it divides forming a large micropylar chamber and a small chalazal chamber

    In the micropylar chamber, as a rule, free nuclear divisions and cell formation, if any, start at a much later stage. In the chalazal chamber the nucleus either remain undivided or divides only a few times. If latter is the situation, the divisions are usually free nuclear. However, sometimes as in Phylidrum lanerginosum, it may become cellular.