Endosperm: Development types, haustoria, cytology and function of endosperm
Development of Endosperm:- The endosperm, a food tissue of varying degree of importance in different species of angiosperm plants, is formed in most cases as a result of fusion of the two polar nuclei and one male gamete. Since all three of die fusing nuclei are haploid, the endosperm is triploid. In the families Orchidaceae and Podostemonaceae, the endosperm formation is completely or partly suppressed.
Types of endosperm formation:- Three types of endosperm formation has been reported in the angiosperms:
1. Free nuclear type of endosperm:-
> The first and usually several of the following divisions of the primary endosperm nucleus are not accompanied by cell wall formation, The endosperm nuclei may either remain free or, in later stages, they may gel separated by cell-walls. The divisions of the endosperm nuclei are quite irregular and, in an endosperm, can be seen in different stages of divisions. As divisions progress, the nuclei are pushed more and more towards periphery that the centre is occupied by a large vacuole.
> Quite frequently, the endosperm nuclei in the chalazal part of the embryo sac have been observed to be larger than those in the micropylar region. The number of free nuclear divisions varies in different species of plants, for example in Primula, Mangifera, Malva, Mains Cannabis etc. several hundred endosperm nuclei may be seen lining the wall of the embryo sac.
> In Asclepias, Rafflesia and Calatropis etc., the cell wall formation starts at the very early stage when only 8 to 16 nuclei have been formed, and in Coffea cell wall formation occurs at the 4-nucleate stage. The cell wall formation starts from the periphery of the embryo sac. Formation of endosperm haustoria has also been reported in several plants e.g., members of family Proteaceae.
2. Cellular type of endosperm:- Here the first division of the primary endosperm nucleus results in the partition of embryo sac into two chambers. The first wall is usually transverse (e.g., Villarsia raniformis, Impatiens roylei, Ruellia etc.,) but sometimes vertical (e.g., Adoxa, Scabiosa, and Circaeastere etc.,) or oblique (e.g., Paperomia, Centranthus, and Helosise to.,), and in few cases the plane of division is variable (e.g., Senecio). Endosperm haustoria may also develop at the micropylar or chalazal ends. In the family Scrophulariaceae, both micropylar and chalazal haustoria are formed.
3. Helobial type of endosperm:- It is an intermediate type between the nuclear and the cellular. Here the first division of the primary endosperm nucleus results in the chambering of the embryosac. The micropylar part of the sac is usually larger than the chalazal part. The endosperm nucleus in the micropylar chamber undergoes several free nuclear divisions, while nucleus in the chalazal region either remains undivided or undergoes only a few divisions, (e.g., Eremurus).
Endosperm Haustoria:-
> Characteristic feature of cellular endosperm.
> One or more cells becomes specialized to function as haustoria.
> The haustoria are formed at micropylar or chalazal or at both the ends and penetratethe nucellar tissue to absorb nutrition.
> Some secondary haustoria are also formed in addition to micropylar and chalazal haustoria. Example- Alectra.
> Multinucleated chalazal endosperm haustorium occurs in Magnolia obovata.
Cytology of Endosperm:-
> The endosperm, as stated earlier, is a food tissue of varying degree of importance in different species of angiosperms. Its cells are mostly isodiametric in shape and store large amount of food materials. Although the cell-walls are usually thin and devoid of pits. In majority of the grasses and some other plants, an aleurone layer is present on the outside of the endosperm. Possibly, the aleurone layer secretes certain enzymes which transform the stored food into liquid form so that it may be consumed by the developing embryo.
> In fully ripe seeds, the endosperm represents a physiologically dead tissue because of the degeneration of the nuclei of the endosperm cells. Dying of the nuclei in the endosperm cells, however, promotes the filling of the grain, and embryo can secure the food material more easily from a dead rather than a living tissue.
Functions of Endosperms:-
i. An important nutrient medium for the successful development of the embryo.
ii. Rich in fat, carbohydrates and proteins which are used in the establishment of theseedling during seed germination.
iii. At the time of fertilization little nutrition available in the embryo sac. But with theformation of endosperm enough food becomes available for the developing embryo.
iv. The division of zygote usually begins after the endosperm is sufficiently grown. Even ifthe zygote and primary endosperm nucleus divide simultaneously, the endosperm growsmore rapidly.
v. Juice of immature endosperm of a plant is used as nutrition for the developing embryoof the other plant. For example coconut milk is used as nutrient medium in in vitroembryo culture.