Question

What is endosperm? Explain various types of endosperms.

Answer 1

In most flowering plants this tissue (endosperm) is produced inside the seeds that are followed by fertilization. In most species it is triploid. It surrounds the embryo and provides nutrition to the embryo in the form of starch.

Complete answer:
The endosperm is a part of the seed that acts as stored food available for the developing embryo of the plant because it is made up of starch and proteins and other nutrients. It is formed when the two sperm nuclei inside a pollen grain reach a female gametophyte or the embryo sac. One of the sperm nuclei fertilizes the ovum, to form a zygote, while the other sperm nuclei do not fertilize but fuse with the binucleate central cell, forming a primary endosperm cell. This primary endosperm cell created in the process of double fertilization develops into the endosperm. The aleurone is the outer layer of endosperm cells, present altogether in small grains, and retained in many dicots with transient endosperm. The cereal aleurone has got both the functions, the storage, and the digestion. It contains starch, oils, and proteins. This will make endosperms a source of nutrition in animal diets. During the process of germination, the endosperm secretes an enzyme amylase which is responsible for the breakdown of endosperm starch into sugar for nourishing the growing seedling. Some examples to show endosperm as stored food is; for the making of bread the wheat endosperm is ground into flour, the main source of sugars for beer production is barley endosperm, the coconut "meat" and coconut "water" and corn are other examples of endosperm that form the majority of the edible portion. Some plants, like orchids, lack endosperm in their seeds.
Based on the development pattern endosperms are divided into three main types:

1. Nuclear Endosperm: This is the most common type of endosperm in which the primary endosperm nucleus divides mitotically without cytokinesis, again and again, resulting in the formation of a large number of free nuclei in the cell.
   At this stage, the cell wall is formed from the periphery towards the center, and a multicellular endosperm is formed. Examples: maize, rice, wheat, cotton, sunflower.
   In coconut, the formation of the cell wall is incomplete resulting in the formation of outer multicellular solid endosperm and inner multinucleated with free nuclei, liquid endosperm (coconut milk).
2. Cellular Endosperm: It is not a very common endosperm and in its development the division of the primary endosperm nucleus i.e., karyokinesis is followed by cytokinesis, and two cells are formed due to transverse division. Further, a similar division occurs, which leads to the formation of the cellular endosperm. Examples: Coconut meat is a cellular endosperm. Acoraceae family has cellular endosperm development while other monocots are helobial. Petunia, Balsam, Datura.
3. Helobial endosperm: In monocotyledons, this type of endosperm development is commonly found. Here, the first division is similar to cellular endosperm resulting in the formation of a large micropylar cell and small chalazal cell. The chalazal cell does not divide further and it functions as a base cell. Whereas the micropylar cell divides further, similar to nuclear endosperm. So it is a combination of both nuclear and cellular endosperms. Examples: Eremurus.

Note:
-The Nuphar polysepala , one of the species of flowering plants, has been shown to have endosperm that is diploid, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei.
-In the dust-like seeds of orchids there is no endosperm.
-In angiosperms, the endosperm contains hormones such as cytokinins, which are responsible for the regulation of cellular differentiation and embryonic organ formation.
-During germination, the enzyme amylase is secreted by the aleurone that breaks down the endosperm.