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. It also protects the embryo from mechanical injury. In gymnosperms, the endosperm is haploid (n) and formed before fertilisation. On the other hand, in angiosperms it is triploid (3n) and formed after fertilisation. Endosperm may be completely consumed by developing embryo before it matures or it may persist in the mature seed and used up during seed germination.
The endosperm is developed from primary endosperm nucleus (PEN) which is formed as a result of triple fusion sperm nucleus which is formed as a result of triple fusion. It is a nutritive tissue that supplies food material to the growing embryo and also the seedling. The primary endosperm nucleus is generally triploid (as it is formed by the fusion of one of the male gametes with two polar nuclei). It divides to form a large triploid endosperm. Endosperm is absent in members of families Orchidaceae, Podostarnonaceae and Trapaceae.
Types of Endosperm
There are three types of Endosperm:
1. Nuclear Endosperm
2. Cellular Endosperm
3. Helobial Endosperm
This type of endosperm is the most common in Angiosperm. In this type of endosperm, the primary endosperm nucleus divides to form many free nuclei. The division of the nucleus is not accompained by wall formation. These free nuclei formed in this way start arranging towards the periphery in the cytoplasm and after sometime wall formation start form periphery towards the centre of the sac. This type of endosperm mostly found in Dicotyledon (polypetalae). Nuclear endosperm is also present in Capsella. The liquid syncytium of coconut is in fact watery endosperm of nuclear type.
In this type division of the primary endosperm nucleus is immediately followed by the wall formation. The first division results in the formation of two equal sized chambers: chalazal and micropyler chambers. The subsequent divisions are followed by regular cell wall formation. So that endosperm is remains cellular from the biginning. Cellular endosperm is characteristic of gamopetalae. Example: Petunia, Datura.
This type of endosperm is intermediate between cellular and nuclear types. The first division of PEN results in the formation of large micropylar cell and a small chalazal cell. The nucleus of the micropylar cell divides freely. But that of the chalazal cell may remain undivided or divide only a few times. The endosperm so formed is called helobial. The chalazal endosperm cell is sometimes described as basal apparatus. Such a type of endosperm is characteristic of Helobieae (an order of monocotyledons).
All types of endosperm become cellular at a very late stage of development. It is generally believed that nuclear endosperm is primitive, helobial intermediate and cellular advanced.