What is the Function of Endospermic Tissues?
Endosperm or endospermic tissue is a tissue produced inside the seed of most flowering plants. The formation of endosperm is double fertilization that starts when two sperm nuclei inside a pollen grain reach the interior of an embryo sac or female gametophyte. One of the sperm nuclei fertilizes the egg forming a zygote while the other nucleus fuses with a binucleate central cell to form a primary endosperm nucleus. Binucleate central cell plays an important role in guiding the pollen grain through pollen tube and endosperm initiation after fertilization. The primary endosperm nucleus is produced by the fusion of monoploid second male gamete and monoploid polar nuclei, thus making it a triploid. However, endosperm in some cases may be pentaploid or pepromia. The endosperm and zygote are two separate parts of the organisms. On the basis of the mode of development, endosperms are of three types:
- Nuclear Endosperm
The primary endosperm nucleus divides by free nuclear divisions with no walls forming between them. A vacoule appears and grows in the center of the embryo sac. Eventually, the nuclei are pushed onto the periphery along the embryo sac wall. Later, walls develop between the nuclei, extend inwards and cellular tissues are formed. In coconut (CocusNucifera) the milky endosperm, also known as liquid syncytium that is filling the large embryo sac contains numerous free nuclei and several multinucleated cells which eventually accumulate along the periphery and from coconut meat.
- Cellular Endosperm
There is no free nuclear division but the primary endosperm nucleus divides and in between daughter nuclei, walls are formed transversely or longitudinally. The walls divide the embryo sac into two cells and later the tissues form by repeated divisions of these cells. The tissue formed is irregularly arranged cells. The cellular endosperm is further divided based upon the orientation of the walls during the first two or three divisions:
- Species Examples: Adoxa and Cetranthus, where the orientation of the wall after first nuclear division is vertical (or longitudinal to embryo sac) and the second wall is also vertical but at the right angle to the first. Wall formation is restricted to the micropylar end.
- Species Examples: Scutellaria and Verbascum, where the orientation of the first wall is transverse and then one or both cells divide vertically.
- Species Examples: Ericaceae and Annonaceae, where the orientation of walls during the first two or three divisions is transverse.
- Species Examples: Myosytisarvensis, where the orientation of the first wall is oblique and two cells formed may be of equal or unequal sizes.
- Species Examples: Senecia and Gunnera, where first wall orientation is indefinite
- Helobial Endosperm
The primary endosperm nucleus divides followed by a transverse wall which divides the embryo sac into the small chalazal chamber and a large micropylar chamber. The nuclei of each chamber then divide by free nuclear divisions with fewer divisions in the chalazal chamber. The endosperm in the chalazal chamber degenerates and cellular endosperm develops among the nuclei within the walls in the micropylar chamber.
Endosperm develops into two patterns; the Mosaic and RuminateEndosperm. Mosaic Endosperm in maize is the endosperm lacking uniformity, containing two patches of colors forming an irregular mosaic pattern and developing two taste textures, starchy and sugary. In Ruminate endosperm, the tissue surface is irregular due to the in-growth of the seed coat.
The embryo sac has little or no nutrition available in it. But following double fertilization, endospermic tissue is formed, surrounds the embryo, and feeding it nutrients, a combination of starch, which also contains protein and oils. The aleurone layer, the endosperm outer layer secretes the amylase enzyme that breaks starch to sugar for seeding utilization. Endosperm also contains cytokinin hormone that helps during the cell differentiation process. The zygote begins division upon sufficient growth of the endosperm. Even during the development of both, the endospermic tissue grows rapidly than the zygote. In some seeds, the endosperm is completely absorbed in maturity (non-endospermous or exalbuminous seeds like peas, gram seed, and beans) while in some seeds (endospermous or albuminous seeds like wheat and castor bean), it remains until germination.
Developing and matured seeds are also consumed by organisms of the higher-order. Upon consumption by animals, endosperm creates a nutrition source for the animal diet. Coconut water is an example of the liquid endosperm of the coconut. wheat seeds contain the only endosperm when grounded to flour. Other endospermic tissues that are nutritious for human consumption are barley, maize, millet, rice, coconut, etc. Some seeds with endospermic tissue containing oils that are economically valuable are palm, coconut, castor, etc.
Learn more from concepts from Class 12 Biology.
Functions of Endosperm:
The main functions include:
- It stores the reserved food which is important for the embryo’s growth.
- It protects the forming embryo and supplies the needed nutrients.
- Helps in regulating the gene expression and the germination of seed.
- It can also induce the abortion of seeds.