Endosperm exists in the seeds of most of the angiosperms. It is replaced by fleshy cotyledons in dicotyledons. Whereas in the case of monocotyledons, endosperm persists in the mature seeds too and stores the food. It provides nourishment to the growing embryo. Endosperm tissue is rich in carbohydrates and also contains proteins and lipids.
Endosperms also have a lot of economic importance as they form the base for cereals and pulses.
Basic Features of Endosperm:
Development of endosperm takes place through the triploid cell i.e 3n in the embryo sac. There is a fusion of secondary male gamete with the diploid secondary nucleus which is formed by two polar nuclei to form the primary endosperm nucleus (PEN) and triploid cell. Endospermic cells are mainly triploid in nature but, in some plants, they may be diploid (water lily) or polyploid (up to 15n) form.
The endosperm is completely absorbed in peas, beans, gram seed during development and food is stored in two cotyledons. They are known as exalbuminous seeds.
In some of the dicots, mature seeds store food in the endosperm, they are known as endospermic or albuminous seeds, e.g. castor.
Endosperm mainly contains starch as a food reserve, but in some cases, they also contain fats, e.g. castor.
Seeds during the dormancy phase get nutrients from the endosperm.
Endosperm also contains several hormones like cytokinins and they help in the cell differentiation process.
Endosperm acts as a main source of food in grains like wheat, maize, barley, corn, etc.
Coconut water is one of the common examples of a liquid endosperm.
The flour i.e white flour which is used to prepare bread only contains endosperm of wheat seeds.
The outer layer of the endosperm is known as the aleurone layer. This layer secretes the amylase enzyme, which helps in breaking starch present in endosperm to sugar for utilization by seedings.
Orchid plant seeds lack an endosperm.
Types of Endosperm:
Endosperms are of three types based on their developing pattern:
Before knowing the different types of endosperms, we should know about the formation of endosperms. It happens through a process called “double fertilization”. Every pollen grain has two male gametes, when one of those reaches the ovary, they fuse to form a zygote. The other male gamete forms triploid endosperm which further develops into endosperm and protects the seed from endogenous and exogenous threats. It could sense the difference in its surroundings and prepare the seed for fertilization.
Nuclear Endosperm: It is the most common type of endosperm found in plants. Here PEN divides repeatedly (mitotic division) without cytokinesis. It results in the formation of a large number of free nuclei in the cell. A large central vacuole is formed, and nuclei get arranged at the periphery. More nuclei are there at the chalazal and micropylar end when compared to the sides. At this stage, cell wall formation takes place from the periphery towards the multicellular endosperm. Examples: maize, rice, wheat, cotton, sunflower.
In the case of coconut, the cell wall formation is incomplete and they result in the formation of outer multicellular solid endosperm and inner multinucleated having free nuclei and a liquid endosperm commonly known as coconut milk.
Cellular Endosperm: This endosperm is not very common. In this development process, the division of PEN i.e karyokinesis is followed by cytokinesis and two cells are formed due to transverse division, which gives rise to the chalazal and micropylar chamber. Further division is similar, due to which the formation of the cellular endosperm takes place. For example Petunia, Balsam, Datura.
Helobial Endosperm: This endospermic development is most common in monocotyledons. The very first division is similar to cellular endosperm and results in a large micropylar cell along with a small chalazal cell. There is a further division of micropylar cells, similar to nuclear endosperm. So it is an intermediate type of endosperm, a combination of both nuclear and cellular endosperms. Examples: Eremurus
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Functions of Endosperm:
Endosperm stores the food reserve and is important for the growth of an embryo.
They provide protection to the developing embryo and supply nutrients.
They have the capability to regulate gene expression and seed germination.
Endosperm induces signals according to environmental conditions and regulates embryonic growth.
The endosperm contains cytokinin, which regulates cellular differentiation.
It may induce the abortion of seeds from the genetically mismatched cross.
Functions of Endosperms During the Seed Germination
Endosperms help in protecting the embryo. It helps by supplying all the required nutrients to the embryo. It acts as a barrier during seed germination and fertilization and protects it. These functions vary among different plant species. Recent studies have suggested that endosperms can sense the environmental conditions and regulate the seed germination according to it. This increased knowledge about seed germination is helping biotechnology and horticulture spheres.
Types of seeds Based on Endospermic Variation
Based on the utilization of endospermic activity, they are divided into the following types:
Albuminous seed: these type of seeds get nutrition from the endosperms and also remains the same during germination
Exalbuminous seed: these types of seeds completely utilize the functions of the endosperm. They are non - endospermic seeds in nature.
Development of Endosperm in Grass:
The origin of grass seed or caryopsis is from a monocarpellary ovary with a single ovule and contains the major storing tissue, the endosperm. For most grass crop species (i.e. cereals), the value of the crop is largely determined by the endosperm, both in quantitative and qualitative terms.
When two polar nuclei in the central cell of the embryo sac by one sperm cell nucleus undergo fertilization process they lead to the formation of the endosperm. The fertilization of the egg cell by the second sperm cell nucleus leads to the formation of a secondary embryo i.e 2n. The major function of the endosperm is to provide nutrients to the developing and, later, germinating embryo. In contrast to many species, including Arabidopsis, the grass endosperm is a persistent seed structure. It is the foremost source of calories for human and livestock nutrition and provides the raw material for countless manufactured foods, goods, and biofuels.